Developmental profiles of progesterone receptor transcripts and molecular responses to gestagen exposure during Silurana tropicalis early development

Environmental gestagens are an emerging class of contaminants that have been recently measured in surface water and can interfere with reproduction in aquatic vertebrates. Gestagens include endogenous progestogens, such as progesterone (P4), which bind P4-receptors and have critically important roles in vertebrate physiology and reproduction. Gestagens also include synthetic progestins, which are components of human and veterinary drugs, such as melengestrol acetate (MGA). Endogenous progestogens are essential in the regulation of reproduction in mammalian species, but the role of P4 in amphibian larval development remains unclear. This project aims to understand the roles and the regulatory mechanisms of P4 in amphibians and to assess the consequences of exposures to environmental gestagens on the P4-receptor signaling pathways in frogs. Here, we established the developmental profiles of the P4 receptors: the intracellular progesterone receptor (ipgr), the membrane progesterone receptor β (mpgrβ), and the progesterone receptor membrane component 1 (pgrmc1) in Western clawed frog (Silurana tropicalis) embryos using real-time qPCR. P4-receptor mRNAs were detected throughout embryogenesis. Transcripts for ipgr and pgrmc1 were detected in embryos at Nieuwkoop and Faber (NF) stage 2 and 7, indicative of maternal transfer of mRNA. We also assessed the effects of P4 and MGA exposure in embryonic and early larval development. Endocrine responses were evaluated through transcript analysis of a suite of gene targets of interest, including: ipgr, mpgrβ, pgrmc1, androgen receptor (ar), estrogen receptor α (erα), follicle stimulating hormone β (fshβ), prolactin (prl), and the steroid 5-alpha reductase family (srd5α1, 2, and 3). Acute exposure (NF 12-46) to P4 caused a 2- to 5-fold change increase of ipgr, mpgrβ, pgrmc1, and ar mRNA levels at the environmentally relevant concentration of 195 ng/L P4. Acute exposure to MGA induced a 56% decrease of srd5α3 at 1140 ng/L MGA. We conclude that environmental exposure to P4 induced multiple endocrine-related transcript responses in amphibians; however, the differential responses of MGA suggest that the effects of MGA are not mediated through the classical P4 signaling pathway in S. tropicalis.

Impact of puberty status and melengestrol acetate supplementation before the breeding period on reproductive efficiency of Bos indicus beef heifers

Two experiments were designed to evaluate the impact of puberty status and the administration of melengestrol acetate (MGA) before onset of the breeding period on ovulatory responses (Exp. 1) and conception rate after AI performed on estrus detection during 10 d and the pregnancy rate through 80 d of breeding period (Exp. 2) of pasture-grazed beef heifers. In Exp. 1, heifers (15 pubertal and 15 prepubertal) received 0.5 mg per heifer/d -1 of MGA over 14 d. No differences in the ovulatory responses were found 10 d after the MGA administration (pubertal = 46.7% vs. prepubertal P = 53.3%; P = 0.72). In Exp. 2, 368 heifers were randomly assigned to groups according to pubertal status and the MGA treatment. All heifers were inseminated on estrus detection for up 10 d after MGA administration and following exposure to bulls between 20 and 80 d. The MGA-treated heifers exhibited a greater AI service rate than control heifers (72.1 vs. 41.6%;P < 0.01); however, heifers receiving MGA had lower conception results following AI (51.6 vs. 71.4%; P = 0.01). In addition, MGA-treated heifers were more likely to have a corpus luteum in the middle of the breeding period (95.3 vs. 87.5%;P < 0.01), although the Cox proportional hazard of pregnancy rate was similar (P = 0.29) at the end of the breeding period. At onset of the breeding period, pubertal heifers presented a greater pregnancy rate following AI (pubertal P = 42.2% vs. prepubertal P = 24.9%; P = 0.01). Therefore, pubertal heifers seem to have greater overall reproductive efficiency than prepubertal heifers, particularly at the beginning of the breeding period. Interestingly, administration of MGA before the onset of the breeding period increased AI service rate but did not alter the rate of pregnancy throughout the breeding period of pasture-grazed beef heifers.

Fungal transformation and T-cell proliferation inhibitory activity of melengestrol acetate and its metabolite

Biotransformation of melengestrol acetate (MGA, 17α-acetoxy-6-methyl-16-methylenepregna-4,6-diene-3,20-dione) (1) was investigated for the first time by using fungal cultures. Incubation of compound 1 with Cunninghamella blakesleeana yielded a new major metabolite, 17α-acetoxy-11β-hydroxy-6-methyl-16-methylenepregna-4,6-diene-3,20-dione (2). The metabolite 2 was purified by using HPLC, followed by characterization through (1)H- and (13)C-NMR and other spectroscopic techniques. Single crystal X-ray diffraction analysis was used to deduce the three dimensional structures of melengestrol acetate (1) and metabolite 2 for the first time. T-cell proliferation assay was employed to evaluate the immunosuppressant effect of compounds 1 and 2 with IC50=0.5±0.07 and 0.6±0.08μg/mL, respectively. The results indicated that these compounds possess sixfold potent T-cell proliferation inhibitory activity as compared to the standard prednisolone (IC50<3.1μg/mL). Both compounds were found to be non-toxic in a 3T3 (mouse fibroblast) cell-based cytotoxicity assay. This discovery of potent anti-inflammatory activity of compounds 1 and 2 can lead the way to develop new immunosuppressant compounds for clinical application.

Utilization of soybeans or corn milling by-products in beef heifer development diets

Whole raw soybeans (SB), wet corn gluten feed (WCGF), and corn dried distillers grains (DDG) are sources of protein in heifer development rations. The objectives of this study were to compare puberty status before synchronization of estrus, response to synchronization, and AI and final pregnancy rates in heifers developed on diets containing SB, WCGF, or DDG that were formulated to be similar in energy and CP. These ingredients vary substantially in fat content, which may affect reproductive performance. Rate of gain during the feeding period and post-AI performance were also compared. In a preliminary experiment, 104 crossbred heifers were fed diets containing either 1.25 kg of SB/d or 2.0 kg of WCGF/d for 110 d (DM basis), beginning at 10 mo of age. In Exp. 1, 100 crossbred heifers received either 1.25 kg of SB/d or 2.5 kg of WCGF/d from approximately 7 to 10 mo of age (91 d; 4 pens/diet), and then were fed 1.25 kg of SB/d for an additional 114 d (4 pens/diet). In Exp. 2, 1.25 kg of SB/d or 1.25 kg of DDG/d was fed to 100 crossbred heifers for 226 d, beginning at 6 mo of age (4 pens/diet). At approximately 13 mo of age, heifers were fed melengestrol acetate (0.5 mg/d) for 14 d, followed by an i.m. injection of PGF(2 alpha) (25 mg) 19 d later to synchronize estrus. Heifers (14 mo of age) received AI for 5 d after PGF(2 alpha), at which time the dietary treatments were ended. Heifers were commingled while grazing on native pasture and were exposed to bulls for approximately 60 d beginning 10 d after the last day of AI. Pregnancy to AI was determined by ultrasound 45 d after the last day of AI. Heifers fed SB in the preliminary experiment had a lower (P < 0.05) synchronization rate (81 vs. 96%) and longer interval (P = 0.05) from PGF(2 alpha) to estrus (76.6 vs. 69.2 h) compared with heifers fed WCGF. In Exp. 1, the age at which the heifers were begun on SB diets did not alter (P > 0.10) the synchronization rate (79%) or timing of estrus after PGF(2 alpha) (77.8 h). In Exp. 2, the synchronization rate (86%) and timing of estrus after PGF(2 alpha) (69.3 h) did not differ (P > 0.10) because of diet. No differences (P > 0.10) were due to diet for AI conception rates (overall mean for each experiment: 76.5, 60, and 68.5%), percentage of all heifers becoming pregnant to AI (67, 46, and 59%), or final pregnancy rates (92, 90, and 90%) in the preliminary experiment, Exp. 1, or Exp. 2, respectively. In summary, SB, DDG, and WCGF can be used as sources of protein in heifer development diets at the inclusion rates used in these studies.

Effects of melengestrol acetate on the inflammatory response in heifers challenged with Mannheimia haemolytica1

Previous research from our laboratory has indicated that melengestrol acetate (MGA) added to the diet during the first 35 d after arrival in the feedlot improves growth rates and tends to reduce chronic respiratory disease in heifers naturally challenged with bovine respiratory disease. The current study was conducted to provide further insight into the possible immunomodulatory effects of MGA. Crossbred heifers (n = 48; 232 +/- 5.5 kg of BW) were used in a randomized complete block design to determine the effects of MGA on lung pathology and markers of inflammation after Mannheimia haemolytica challenge. On d 0, cattle were blocked by BW and randomly assigned, within block, to diets (54% concentrate) that provided 0 or 0.5 mg of MGA per heifer daily for the duration of the experiment. Inoculum containing from 1.3 x 10(9) to 1.7 x 10(9) cfu of M. haemolytica (20 mL) was instilled at the bifurcation of the trachea on d 14. Blood samples were collected, clinical observations were made, and rectal temperatures were recorded for each animal at 0, 12, 24, 48, 72, 96, 120, and 138 h after inoculation. Heifers fed MGA had greater circulating concentrations of eosinophils and postchallenge concentrations of segmented neutrophils and white blood cells (P < 0.01) than controls, as well as elevated plasma protein, serum haptoglobin, and fibrinogen after M. haemolytica challenge (P < 0.01). Heifers fed MGA had lower plasma glucose (P < 0.01), greater plasma urea N (P = 0.02), and elevated respiratory indices (P < 0.01) compared with controls. Necropsies performed on d 6 after inoculation suggested that M. haemolytica challenge was relatively mild, because lesions were confined to a small portion of the lungs. On a 0 to 100 scale, average lung lesion scores were 3 and 1 for MGA-fed and control groups, respectively (P < 0.06). Heifers fed MGA before mild M. haemolytica challenge were more susceptible to infection, as evidenced by a greater number of heifers fed MGA exhibiting pulmonary lesions 138 h after inoculation than controls (14 out of 23 vs. 6 out of 24 for MGA and controls, respectively; P < 0.02).

Comparison of controlled internal drug release device and melengesterol acetate as progestin sources in an estrous synchronization protocol for beef heifers

The objectives of this experiment were to compare estrous synchronization responses and AI pregnancy rates of beef heifers using protocols that included either CIDR or MGA as the progestin source. The hypotheses tested were that: (1) estrous synchronization responses after (a) progestin removal, and (b) PGF(2alpha); and, (2) AI pregnancy rates, do not differ between heifers synchronized with either progestin source. At the start of the experiment (Day 0) in both years, heifers were assigned randomly to receive, MGA supplement for 14 days (MGA-treated; n=79) or CIDR for 14 days (CIDR-treated; n=77). On Day 14 progestin was removed and heifers were observed for estrus up to and after PGF(2alpha) on Days 31 and 33 for CIDR-treated and MGA-treated heifers, respectively. Heifers that exhibited estrus within 60h after PGF(2alpha) were inseminated by AI 12h later; the remaining heifers were inseminated at 72h after PGF(2alpha) and given GnRH (100mug). More (P<0.05) CIDR-treated heifers exhibited estrus within 120h after progestin removal than MGA-treated heifers. Intervals to estrus after progestin removal were shorter (P<0.05) for CIDR-treated heifers than MGA-treated heifers. More (P<0.05) CIDR-treated heifers exhibited estrus and were inseminated within 60h after PGF(2alpha) than MGA-treated heifers. Pregnancy rates did not differ (P>0.10) between MGA-treated (66%) and CIDR-treated (62%) heifers. In conclusion, the use of CIDR as a progestin source in a 14-day progestin, PGF(2alpha), and timed AI and GnRH estrous synchronization protocol was as effective as the use of MGA to synchronize estrus and generate AI pregnancies in beef heifers.

Motivation of Hens to Obtain Feed During a Molt Induced by Feed Withdrawal, Wheat Middlings, or Melengestrol Acetate

Traditionally, molting was initiated by withdrawing feed. However, public criticism of feed deprivation, based on the perception that it inhumanely increases hunger, has led the poultry industry to ban the practice. Thus far, alternatives have not been demonstrated to ameliorate the increase in hunger that led to the ban on inducing molting by feed deprivation. Incorporating melengestrol acetate (MGA), an orally active progestin, into a balanced layer diet induces molting and increases postmolt egg quality. Hy-Line W-98 hens (n = 60) were randomly assigned to a balanced layer ration (control), a balanced layer ration containing MGA, or a 94% wheat middlings diet (wheat) for 20 d, or were feed deprived for 8 d. Hens were trained to peck a switch to receive a feed reward based on a progressive ratio reinforcement schedule. Motivation of hens to acquire feed was measured as the total number of pecks recorded in 15 min on d 0, 4, 8, 12, 16, and 20. On d 20, abdominal fat pad and digesta-free gizzards were weighed. The number of pecks in the feed-deprived group was greater than controls by d 4 and remained greater at d 8, when these hens were removed from the experiment. Hens in the wheat group that were rewarded with a layer diet pecked more than controls from d 8 to 20. Hens in the MGA group pecked for a reward at the same rate as control hens throughout the experiment. Hens fed the wheat diet had heavier gizzards compared with control and MGA-fed hens. Hens fed MGA had greater abdominal fat pad compared with wheat and control hens. Hens molted using a diet containing MGA have a similar motivation to obtain feed as control hens; therefore, this alternative does not appear to increase hunger. However, hens molted with a wheat middling diet appear to be as motivated to obtain feed as did the feed-deprived hens.

Resynchronization of estrus in beef cattle: ovarian function, estrus and fertility following progestin treatment and treatments to synchronize ovarian follicular development and estrus

The objective was to optimize rebreeding of nonpregnant, previously inseminated beef cattle. In Experiment 1, 43 cows received a used intravaginal progesterone-releasing insert (IVPRI; Days 0-7) 12.3 d after ovulation and received concurrently no treatment, 100 microg gonadotropin releasing hormone (GnRH), 1 mg estradiol cypionate (ECP), or 150 mg progesterone. Emergence of a new ovarian follicular wave was most synchronous (P < 0.0001) in the GnRH group. In Experiment 2, 675 heifers were given GnRH or no treatment on Day 0, fed melengestrol acetate (MGA; 0.5 mg/head/d) from Days 0-5 (Day 0 = 13-14 d after timed insemination; TAI), given 0.5 mg ECP or nothing on Day 7, and reinseminated 6-12 h after onset of estrus. Estrus was more synchronous (P < 0.05) in heifers given GnRH versus no treatment on Day 0. In Experiment 3, 317 TAI heifers were resynchronized with either MGA or a used IVPRI with or without ECP on Day 7; estrus was more synchronous (P < 0.05) and pregnancy rates were higher (54.1% versus 39.2%, P < 0.05) in heifers given a used IVPRI than those fed MGA. For resynchronization of heifers, pregnancy rates were not significantly improved with GnRH treatment, but were higher with a used IVPRI than with MGA.

Effect of melengestrol acetate as an alternative to induce molting in hens on the expression of yolk proteins and turnover of oviductal epithelium

Inducing hens to molt increases egg quality, egg production and extends the productive life of hens. It has been previously demonstrated that melengestrol acetate (MGA), an orally active progestin, decreased gonadotropic support for the ovary, which decreased the steroidogenic support for the oviduct and resulted in the cessation of lay. Estradiol produced by the theca cells of small follicles stimulates the production of the yolk proteins vitellogenin II and apolipoprotein II by the liver and supports the oviductal epithelial cells. The objective of the present experiment was to determine gene expression for yolk proteins and oviductal epithelial cell turn-over in response to a MGA-induced molt. Hy-Line W-36 laying hens were fed either 0 or 8mg MGA per day for 28 days in a balanced diet and then returned to a standard layer ration until day 36. Four birds per treatment on days 1, 8, 16, 28 and 36 were euthanized and the liver was removed and snap frozen in liquid nitrogen until RNA was extracted. Expression of vitellogenin II and apolipoprotein II genes was determined using real-time RT-PCR. A portion of the magnum was removed to determine proliferation and programmed cell death for secretory and ciliated luminal epithelium. Vitellogenin II and apolipoprotein II gene expression was reduced in hens fed 8mg MGA compared to those fed 0mg MGA. There was no effect of day on gene expression of either yolk protein. Cell proliferation was increased in the ciliated epithelial cells of the oviduct in hens receiving 8mg MGA compared to those receiving 0mg. However, programmed cell death of the ciliated epithelial cells was not different between controls and MGA treatment. Programmed cell death and proliferation increased in the secretory epithelial cells in hens receiving 8mg MGA compared to controls. Therefore, utilizing MGA as an alternative method to induce molt results in some, but not all, of the physiological changes previously described for hens molted by feed withdrawal. These findings lead us to suggest that some of the observed physiological changes resulting from feed withdrawal are required to increase egg quality and egg production following molt and other changes are not necessary, but are just a result of nutrient deprivation.

Melengestrol acetate alters muscle cell proliferation in heifers and steers1

In vitro experiments were performed to investigate the effects of melengestrol acetate (MGA) or progesterone (P4) on bovine muscle satellite cells and C2C12 myoblasts. Addition of MGA at physiological and supraphysiological concentrations resulted in a dose-dependent decrease (P < 0.05) in DNA synthesis as measured by [3H]-thymidine incorporation (TI). Similarly, P4 addition (0.01 nM) reduced (P < 0.05) TI. Addition of MGA (10 nM) increased (P < 0.05) IGF-I mRNA abundance but did not affect myogenin mRNA. Progesterone addition (10 nM) increased myogenin mRNA abundance (P < 0.05). In C2C12 cultures, P4 addition resulted in a dose-dependent decrease in TI. The antiprogestin RU486, in combination with MGA or P4, also resulted in reduced (P < 0.05) TI. Treatment with RU486 alone had a negative effect (P < 0.05) on TI that was similar to the progestins. Treatment of C2C12 myoblasts with MGA (100 nM) resulted in an increase (P < 0.05) in myogenin mRNA. These studies suggest that progestins may reduce satellite cell proliferation, ultimately affecting carcass composition.

The effects of cattle sex on carcass characteristics and longissimus muscle palatability

Two experiments were conducted to determine the effects of sex on carcass traits and cooked beef steak palatability. In Exp. 1, steers (n = 99), heifers (n = 51), and intravaginally spayed heifers (n = 46) were fed a high-energy diet for 161 d. No implants were administered, and heifers were not fed melengestrol acetate to suppress estrus. In Exp. 2, 60 steers and 60 intact heifers from the same ranch source used in Exp. 1 were fed in 2 locations (sites 1 and 2). All management factors were equal across experiments except that intact heifers were fed melengestrol acetate to suppress estrus in Exp. 2. Steers in Exp. 1 were 25 kg heavier (P < 0.01) in HCW than heifers at comparable (P = 0.39) carcass fat thickness. Spayed heifers (Exp. 1) had a 5.7% smaller (P < 0.05) LM area compared with steers and intact heifers, which were similar. In Exp. 2, there was no difference (P = 0.2) in carcass weight, and heifers had greater (P < 0.01) 12th rib fat thickness compared with steers. Calculated yield grades were similar (P = 0.21) among treatments in Exp. 1 and tended (P = 0.08) to be greater for heifers compared with steers in Exp. 2. In Exp. 1, USDA quality grades and marbling scores were lower (P < 0.01) for steers compared with intact and spayed heifers, which were similar. The effects of sex on tenderness were examined at a common level of fat-thickness and marbling by covariate analysis. Steaks from steers, compared with those from nonimplanted, intact heifers, in the 2 experiments combined were: (a) superior (P < 0.05) in 2 of 9 palatability assessments when subcutaneous fat thickness (at the 12th rib) was adjusted to a common level, and (b) superior (P < 0.05) in 6 of 9 palatability assessments when marbling score was adjusted to a common level. In Exp. 1, steaks from nonimplanted steers compared with those from nonimplanted spayed heifers were: (a) superior (P < 0.05) in 0 of 8 palatability assessments when subcutaneous fat thickness (at the 12th rib) was adjusted to a common level, and (b) superior (P < 0.05) in 3 of 8 palatability assessments when marbling score was adjusted to a common level. These findings suggest that sex should be added to the list of antemortem factors contributing to variation in cooked beef steak tenderness. However, more research is needed to precisely identify those factors contributing to the lower tenderness observed for steaks from heifer carcasses.

Effect of dietary melengestrol acetate on the incidence of acute interstitial pneumonia in feedlot heifers

Over a 3-y period, 906,000 cattle were monitored in 23 feedlots in southern Alberta for symptoms of acute interstitial pneumonia (AIP). Plasma, urine, and lung tissue were collected at slaughter from 299 animals clinically diagnosed with AIP and from 156 healthy penmates and analyzed for 3-methylindole (3MI) derivatives and reduced glutathione concentration. From each animal, the left lung was subsampled for histologic examination. Concentrations of glutathione in lung tissue were reduced (P < 0.001) in animals showing clinical symptoms of AIP as compared with their asymptomatic penmates. Animals histologically confirmed as having AIP had higher levels of 3MI protein adducts in blood and lung tissue (P < 0.05) than did emergency-slaughtered animals without AIP. Within feedlots, where pens of heifers were fed either a standard dosage of melengestrol acetate (MGA) or none, the rate of death attributable to AIP was similar between treatment groups, but emergency slaughter after clinical diagnosis of AIP was done 3.2 times more often (P < 0.001) in the MGA-fed heifers than in the group not fed MGA. Use of MGA did not influence glutathione concentration. As growth performance of heifers given steroidal implants may not be improved by feeding MGA, the most cost-effective method of reducing the incidence of AIP-related emergency slaughter in feedlot heifers may be to eliminate MGA from the diet.

Effects of growth-promoting agents and season on blood metabolites and body temperature in heifers1,2

To assess the efficacy of growth-promoting agents among seasons, triiodothyronine (T3), thyroxine (T4), plasma urea nitrogen (PUN), IGF-I, and tympanic temperature (TT) were measured in summer and winter studies. Heifers (n = 9/pen) were allotted to 12 pens in both December and June. Pens were assigned to 1 of 6 growth promotant treatments: control (no growth promotant), estrogenic implant (E), trenbolone acetate implant (TBA), E + TBA (ET), melengestrol acetate (MGA), and ET + MGA (ETM). Blood samples were collected from 4 heifers per pen per study on d 0, 28, 56, and 84 via jugular puncture. Near the midpoint of both studies, TT were obtained from the heifers. There was a season by sample day interaction for all blood metabolites (P < 0.05). During the winter, IGF-I levels peaked on d 28, whereas T3, T4, and PUN peaked on d 56. In the summer, IGF-I levels increased from d 0 to 28 and remained elevated throughout the study. Season by growth promotant interactions (P < 0.05) indicated that in the winter ET increased T3, whereas TBA alone decreased both T3 and T4, compared with control, or ET, and ETM treatment groups. Across seasons, treatments ET and ETM increased (P < 0.05) IGF-I and decreased (P < 0.05) PUN. However, E, TBA, and MGA alone had no effect on IGF-I or PUN concentrations. The maximum TT was greater (P < 0.01) in the summer than in the winter, whereas the minimum TT was lower (P < 0.01) in the summer. Mean TT did not differ among growth-promoting treatments. However, in the summer and over both seasons, the maximum TT was lower (P < 0.05) in E-, MGA-, and ETM-treated heifers. Although limited growth promotant by season interactions existed, changes in blood metabolite levels resulting from the use of growth promotants do not appear to influence seasonal changes in body temperature as measured by TT.

Melengestrol acetate in experimental diets as an effective alternative to induce a decline in egg production and reversible regression of the reproductive tract in laying hens I. Determining an effective concentration of melengestrol acetate

Induced molting increases egg quality and egg production and extends the productive life of hens. Molting is accomplished by feed withdrawal, which has been criticized for not addressing hen well-being, and current alternatives have resulted in poor postmolt performance and inadequate well-being. Molting leads to regression of follicles on the ovary and causes loss of steroidogenic support for the oviduct, leading to cessation of lay. Melengestrol acetate (MGA), an orally active progestin, may decrease support for the ovary, resulting in loss of support for the oviduct, while hens are fed a balanced diet. In this experiment, a dose response study, Hy-Line W-36 hens were fed 0, 0.1, 1, 4, or 8 mg of MGA per hen/d in a balanced diet for 28 d and then returned to a normal diet. Four birds on d 0 and 4 birds per treatment on d 1, 2, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, and 44 were euthanized. The weight of the ovary with follicles, magnum, shell gland, and oviduct were determined. A decrease in egg production was observed in those groups receiving 4 and 8 mg of MGA, until removal of MGA from the diet. After d 28, egg production increased to the production level of hens fed 0, 0.1, or 1 mg of MGA. The weight of the ovary with follicles, oviduct, magnum, and shell gland were unchanged throughout in groups fed 0, 0.1, or 1 mg of MGA. However, groups fed 4 or 8 mg of MGA exhibited a decrease (P < 0.05) in the weight of the ovary with follicles, oviduct, magnum, and shell gland until d 28. Recrudescence of the large yellow follicles as well as rejuvenation of the oviduct and its components, the magnum and shell gland, in the 4 and 8 mg MGA groups occurred by d 44. Melengestrol acetate, fed to hens on a balanced layer diet, caused reversible regression of follicles and, therefore, removal and return of support for the oviduct.

Melengestrol acetate as an effective alternative to induce a decline in egg production and reversible regression of the reproductive tract in laying hens. II. Effects on postmolt egg quality

Induced molting increases egg quality and egg production and extends the productive life of hens. Molting is accomplished by feed withdrawal, which has received criticism, and alternatives described thus far result in poor postmolt performance. Melengestrol acetate at a dosage of 4 or 8 mg/d, in a balanced diet, leads to reversible regression of the reproductive tract. However, this alternative must also increase egg quality after rest to be considered an adequate method by the industry. Hy-Line W-36 (n = 497) laying hens were assigned randomly to a diet containing 0 mg of melengestrol acetate (MGA; control) throughout the experiment or 4 or 8 mg of MGA/d for 2, 4, or 6 wk. Upon reaching 50 and 70% lay, after MGA removal, eggs were collected for measurements of egg quality, including Haugh units (i.e., internal egg quality), shell thickness, and breaking strength (i.e., external egg quality). Haugh units were greater (P < 0.05) for eggs laid by hens molted with a diet containing 8 mg of MGA for all durations compared with controls. Shell thickness was greater (P < 0.05) when hens were treated with 4 mg of MGA for 6 wk and 8 mg of MGA for 4 and 6 wk compared with control. Egg breaking strength was greater (P < 0.05) than controls for all hens fed MGA, regardless of dosage or duration of feeding. A subset of hens was fed 8 mg of MGA per hen/d for 2 wk, and eggs were collected for 3 wk. Seven days after MGA was removed from the diet, the amount of MGA in the yolk was below the level of detection of the assay, and the concentration found in the eggs at all time points was 3 orders of magnitude below the Food and Drug Administration's tolerance for MGA in edible tissue. When used as an alterative method to induce a rest, MGA leads to an increase in the internal and external egg quality of hens compared with nonmolted hens.

Basis of melengestrol acetate action as a progestin

To provide insight into potential mechanisms contributing to the various biological responses of cattle to treatment with progesterone, norgestomet, and melengestrol acetate (MGA), MCF-7 cells were utilized to determine the relative binding affinity of the progesterone receptor for MGA, norgestomet, progesterone, and a progesterone agonist (R5020), and to determine if progesterone, MGA, or norgestomet have estrogenic and/or anti-estrogenic activities. The progesterone receptor had greater affinity (P<0.05) for MGA, R5020, and norgestomet than for progesterone; and the affinity for norgestomet exceeded (P<0.05) that of MGA and R5020. Estrogen stimulates proliferation of MCF-7 cells; therefore these cells have been utilized as a bioassay to detect estrogenic and anti-estrogenic activity. Progesterone (10(-11) to 10(-5)M) did not promote cellular proliferation. However, MGA (10(-8), 10(-7), and 10(-6)M) increased (P<0.05) cell proliferation compared to the control group (10(-11) to 10(-9) and 10(-5)M MGA did not stimulate cell proliferation), and MGA-induced cell proliferation (10(-8)M) was reduced (P=0.095) by an estradiol antagonist (ICI 182,780; ICI). Cellular proliferation increased (P<0.05) with norgestomet (10(-5)M) compared to the control group (10(-11) to 10(-6)M norgestomet did not stimulate cell proliferation) and the increased proliferation was decreased (P<0.05) by ICI. Neither progesterone nor MGA demonstrated anti-estrogenic activity. Norgestomet (10(-10) to 10(-6)M) did reduce (P<0.05) maximal estrogen-stimulated cell proliferation, but not to basal levels. In summary, the affinities of the progesterone receptor for norgestomet, MGA, and progesterone are consistent with their effective dose to inhibit ovulation in vivo, but their progestin and their estrogenic/anti-estrogenic activities cannot fully explain why progesterone and norgestomet are more capable of reprogramming the reproductive axis in anestrous postpartum cows compared to MGA.

A comparison of progestin-based protocols to synchronize ovulation and facilitate fixed-time artificial insemination in postpartum beef cows1

The experimental objective was to compare pregnancy rates after fixed-time AI in postpartum suckled beef cows following administration of two progestin-based protocols to synchronize ovulation. Cows (n = 424) at three locations (n = 208, 122, and 92 per location) were stratified by age, BCS, and days postpartum (DPP) and assigned randomly to one of the two treatment protocols. The MGA Select-treated cows (MGA Select; n = 213) were fed melengestrol acetate (MGA, 0.5 mg x cow(-1) x d(-1)) for 14 d and carrier for 8 d, and then GnRH (100 microg i.m. Cystorelin; d 26) was injected 12 d after MGA withdrawal, and PG (25 mg i.m. Lutalyse) was administered 7 d after GnRH. Cows assigned to the 7-11 Synch protocol (7-11 Synch; n = 209) were fed carrier for 15 d and MGA for 7 d, and then injected with PG on d 22 (d 7 of MGA), GnRH on d 26, and PG again on d 33. Artificial insemination was performed at fixed times for cows in both treatments at 60 or 72 h after d 33 PG for 7-11 Synch and MGA Select groups, respectively. All cows were injected with GnRH (100 microg of i.m. Cystorelin) at AI. There was no treatment x location interaction for age (P = 0.90), BCS (P = 0.64), or DPP (P = 0.93), and the results were therefore pooled for the respective treatments (age [7-11 Synch, 5.5 +/- 0.2; MGA Select, 5.5 +/- 0.2], BCS [7-11 Synch, 5.7 +/- 0.1; MGA Select, 5.6 +/- 0.1], and DPP [7-11 Synch, 41.1 +/- 1.1; MGA Select, 42.1 +/- 1.1]). Blood samples were collected 8 and 1 d before MGA or carrier to determine pretreatment estrous cyclicity (progesterone >or=1 ng/mL; 7-11 Synch, 59/209 [28%]; MGA Select, 54/213 [25%]; P = 0.50) and again on d 33 PG to evaluate treatment response as a percentage of cows with progesterone concentrations in serum >or=1ng/mL (7-11 Synch, 184/209 [88%]; MGA Select, 177/213 [83%]; P = 0.15). Pregnancy rates resulting from fixed-time AI did not differ (P = 0.25) between treatments (7-11 Synch, 128/209 [61%]; MGA Select, 142/213 [67%]), nor did pregnancy rates (P = 0.77) at the end of the breeding season (7-11 Synch, 198/208 [95%]; MGA Select, 204/213 [96%]). These data indicate that pregnancy rates were comparable after fixed-time AI, following administration of the 7-11 Synch and MGA Select protocols. Both protocols provide opportunities for beef producers to use AI and eliminate the need to detect estrus.

Methods to reduce or eliminate detection of estrus in a melengestrol acetate-PGF2α protocol for synchronization of estrus in beef heifers1

Three experiments were conducted to evaluate methods to decrease or eliminate the detection of estrus inherent to a melengestrol acetate (MGA)-PGF2alpha (PGF) protocol for synchronization of estrus in heifers. In each experiment, all heifers received 0.5 mg of MGA x animal(-1) x d(-1) for 14 d (d -32 to -19) and PGF (25 mg, i.m.; d 0, 0 h) 19 d after the last feeding of MGA (MGA-PGF protocol). In Exp. 1, heifers (n = 709) were assigned to each of the following protocols: 1) the MGA-PGF protocol with AI 6 to 12 h after detection of estrus (estrus AI; MGA-PGF); 2) MGA-PGF plus 100 microg, i.m. of GnRH on d -7 (1x GnRH) and estrus AI; or 3) MGA-PGF, GnRH on d -7, and GnRH (100 microg, i.m.) at 48 h after PGF, coincident with insemination (2x GnRH-TB48). In Exp. 2, heifers (n = 559) received the MGA-PGF protocol and were inseminated by either estrus AI or fixed-time AI (TAI) at 60 h, coincident with an injection of GnRH (GnRH-TB60). In Exp. 3, all heifers (n = 460) received the MGA-PGF protocol and were inseminated by estrus AI when detected up to 73 h. Heifers not observed in estrus by 73 h received TAI between 76 and 80 h. Half the heifers inseminated by TAI received no further treatment (TB80), and the remaining half was injected with GnRH at insemination (GnRH-TB80). Variance associated with the interval to estrus and the proportion in estrus from d 0 to 5 was similar for 1x GnRH and MGA-PGF treatments in Exp. 1. Pregnancy rate (d 0 to 5) did not differ for the MGA-PGF and 1x GnRH treatments (62.5 and 60.4%, respectively), and both were greater (P < 0.05) than TAI pregnancy rate in the 2x GnRH-TB48 treatment (42.3%). In Exp. 2, the peak estrous response occurred 60 h after PGF. Pregnancy rate during the synchrony period was greater (P < 0.05) for the MGA-PGF (255/401; 63.6%) than the GnRH-TB60 (74/158; 46.6%) treatment. In Exp. 3, 75.7% of heifers (348/460) were detected in estrus by 73 h and were inseminated, with a conception rate of 74.4%. Pregnancy rates after TAI did not differ between TB80 and GnRH-TB80 (14/56 = 25% and 19/ 56 = 33.9%, respectively). Total pregnancy rate was 63.5% for heifers inseminated after detected estrus and by TAI. Collectively, these data indicate that the exclusive use of TAI for heifers treated with the MGA-PGF protocol resulted in lower pregnancy rates than when AI was performed after detection of estrus. However, estrus AI for 3 d and TAI at the end of d 3 could result in pregnancy rates similar to those achieved after a 5-d period of detecting estrus.

Ability of intravaginal progesterone inserts and melengestrol acetate to induce estrous cycles in postpartum beef cows

Postpartum anestrous interval in beef cows is a major factor contributing to reproductive failure during a defined breeding season. Our objectives were to determine the ability of a controlled internal drug-releasing device (CIDR, 1.9 g of progesterone), a normal dose of melengestrol acetate (MGA, 0.5 mg x cow(-1) x d(-1)), or a high dose of MGA (4.0 mg x cow(-1) x d(-1)) to induce ovulation and to eliminate short estrous cycles. Multiparous beef cows (n = 100) were equally assigned to one of four treatments: CIDR, normal MGA, high MGA, or control by age, days postpartum, body condition, and body weight. All cows were fed carrier (0.9072 kg x cow(-1) x d(-1)) with (normal MGA, 0.55 mg/kg; high MGA, 4.41 mg/kg) or without MGA for 7 d (d -6 to 0). On d -6, CIDR were inserted and then removed on d 0. Estrous behavior was monitored continuously from d -6 until 29 using HeatWatch electronic mount detectors. Blood was collected on d -13, and three times weekly from d -6 to 29. Treatment influenced (P = 0.03) the percentage of cows that were detected in standing estrus. Beginning on d 2, more CIDR-treated cows had exhibited standing estrus compared with high MGA-treated or control cows, but CIDR- and normal MGA-treated cows did not differ. The percentage of CIDR-treated cows that had ovulated was greater (P < 0.05) than the percentage of normal MGA-treated, high MGA-treated, or control cows beginning on d 4. The percentage of cows that exhibited standing estrus before the first postpartum ovulation (CIDR = 65%, normal MGA = 57%, high MGA = 35%, control = 30%) did not differ (P = 0.09) among treatments. Luteal life span following the first ovulation postpartum and the percentage of cows with a normal luteal life span (i.e., progesterone > 1 ng/mL for > or = 10 d) was greater (P < 0.01) in CIDR-treated cows (14.0 +/- 0.8 d; 20/20, 100%) compared with normal MGA-treated (6.2 +/- 1.0 d; 3/13, 23%), high MGA-treated (9.6 +/- 1.0 d; 8/14, 57%), or control cows (6.1 +/- 0.9 d; 4/17, 24%), and greater (P < 0.03) in high MGA-treated cows than in normal MGA-treated or control cows. In the present study, treatment of early postpartum suckled beef cows with CIDR induced ovulation and initiated estrous cycles with a normal luteal life span in more cows than did treatment with MGA. Treatment with MGA (normal or high dose) did not induce ovulation earlier than in control cows, but a high dose of MGA increased the percentage of cows with normal luteal life spans following the first ovulation postpartum.

Effects of growth-promoting agents and season on yearling feedlot heifer performance1

Angus x crossbred heifers (270 per trial) were used in an experiment conducted over one 105-d summer and one 104-d winter feeding period. Treatments were identical for each trial and included: 1) control, 2) estrogenic implant (E), 3) trenbolone acetate implant (TBA), 4) E + TBA (ET), 5) melengestrol acetate (MGA) in the feed, and 6) ET + MGA (ETM). Each treatment was replicated in five pens, with nine heifers per pen in each season. Initial weights (mean = 384 kg, SE = 57) were the same for each season. There were no treatment x season interactions for final BW, ADG, G:F, water intake, or carcass characteristics. Heifers receiving a growth-promoting agent were 11.6 kg (SE = 4.08) heavier and gained 0.108 kg/d (SE = 0.04) more (P < 0.05) than control heifers. Heifers receiving ET gained 0.09 kg/d (SE = 0.032) more (P = 0.05) than heifers not receiving ET. Heifers receiving ET (with and without MGA) had greater G:F (P < 0.05) than control, E, and TBA heifers. Carcass weights of ET-treated heifers were greater (P < 0.05) than carcass weights for unimplanted heifers, those fed MGA only, and heifers receiving either E or TBA implants. Marbling scores were increased (P < 0.05) by feeding MGA to ET-treated heifers. Water intake was greater (P < 0.01) in the summer (31 L/d) than in the winter (18 L/ d), with no difference among implant treatments. Heifers fed in the winter had heavier carcasses, less 12th-rib fat, greater marbling scores, larger LM area, and a greater incidence of liver abscesses than heifers finished in the summer (P < 0.01). A treatment x season interaction (P = 0.07) was evident for DMI during the 35-d coldest and hottest portions of the year. Heifers fed MGA and implanted with ET tended (P = 0.07) to have greater DMI in the summer but lesser DMI in the winter. In general, differences among growth-promotant programs were relatively similar over the entire summer and in winter.

A comparison of progestin-based protocols to synchronize estrus in postpartum beef cows1

Two progestin-based protocols for estrus synchronization in postpartum beef cows were compared following treatment administration on the basis of estrous response, interval to and synchrony of estrus, and pregnancy. Cows were assigned to one of the two treatment protocols by age, body condition score (BCS), and days postpartum (DPP). The MGA Select-treated cows (MGA Select; n = 109) were fed melengestrol acetate (MGA; 0.5mg x cow-1 x d(-1)) for 14 d, fed carrier for 8 d, GnRH (100 microg of Cystorelin) was injected i.m. 12 d after MGA withdrawal, and PG (25 mg of Lutalyse) was administered i.m. 7 d after GnRH. Cows assigned to the 7-11 Synch protocol (7-11 Synch; n = 111) were fed carrier for 15 d, fed MGA for 7 d, injected with PG on d 22 (d 7 of MGA), injected with GnRH on d 26, and injected with PG on d 33. Mean BCS (4.8 +/- 0.1, MGA Select; 4.7 +/- 0.1, 7-11 Synch) and DPP (40 +/- 1, MGA Select; 40 +/- 1, 7-11 Synch) did not differ between treatments. Blood samples were collected 8 d and 1 d before feeding of MGA or carrier to determine the pretreatment estrous cyclicity (progesterone > or = 1 ng/mL; 10/109 [9%], MGA Select; 12/111 [11%], 7-11 Synch), and again at PG on d 33 to evaluate treatment response (81/109 [74%], MGA Select; 84/111 (76%), 7-11 Synch). Serum concentrations of progesterone at PG on d 33 differed (P < 0.01) between treatments (3.3 +/- 0.3 ng/mL [MGA Select] vs. 1.7 +/- 0.1 ng/mL [7-11 Synch]). HeatWatch was used for 6 d after PG on d 33 to detect estrus, and AI was performed 12 h after the onset of estrus. Estrous response did not differ between treatments (100/109 [92%], MGA Select; 101/111 [91%], 7-11 Synch). Mean interval to estrus (65 +/- 2.7 h, MGA Select; 52 +/- 1.8 h, 7-11 Synch) and synchrony of estrus differed (P < 0.01) between treatments. Synchronized conception and pregnancy rates (61/100 [61%], 61/109 [56%], MGA Select; 71/101 [70%], 71/111 [64%], 7-11 Synch), and final pregnancy rates (94/109 [86%], MGA Select; 99/110 [90%], 7-11 Synch) did not differ between treatments. In summary, estrous response and fertility did not differ among cows assigned to the MGA Select or 7-11 Synch protocols. Synchrony of estrus, defined as the variance in the interval to estrus from PG, however, was improved following treatment with the 7-11 Synch protocol.

Follicular dynamics and steroid profiles in cows during and after treatment with progestin-based protocols for synchronization of estrus12

Two progestin-based protocols for the synchronization of estrus in beef cows were compared. Cyclic, nonlactating, crossbred, beef cows were assigned by age and body condition score to one of two treatments. Cows assigned to the MGA Select protocol were fed melengestrol acetate (MGA; 0.5 mg x cow(-1) x (-1)) for 14 d, GnRH was administered (100 microg i.m. of Cystorelin) 12 d after MGA withdrawal, and PGF2alpha (25 mg of i.m. Lutalyse) was administered 7 d after GnRH. Cows assigned to the 7-11 Synch protocol were fed MGA for 7 d and were injected with PG on d 7 of MGA, GnRH on d 11, and PG on d 18. Transrectal ultrasonography was performed daily to monitor follicular dynamics from the beginning of MGA feeding through ovulation after the synchronized estrus. All cows exhibited estrus in response to PG. Mean interval to estrus was shorter (P < 0.01) for 7-11 Synch-treated cows (56 +/- 1.5 h) than for cows assigned to the MGA Select protocol (73 +/- 4.7 h). Mean interval from estrus to ovulation did not differ between treatments (P > 0.10). Variances for interval to estrus differed (P < 0.01) between treatments. Mean follicular diameter at GnRH injection, PG injection, and estrus did not differ (P > 0.10) between treatments. Relative to MGA Select, serum estradiol-17beta concentrations were higher (P < 0.01) for 7-11 Synch 2 d and 1 d before, on the day of GnRH injection, in addition to 4 d after GnRH, and 24 h after PG. Mean progesterone concentrations were greater (P < 0.01) for MGA Select cows from 4 d before to 7 d after GnRH. Forty-four percent of the variation in interval to estrus between treatments was explained by differences in estradiol-17beta concentrations 24 h after PG. This study suggests that follicular competence is likely related to steroidogenic capacity of the follicle and the endocrine environment under which growth and subsequent ovulation of the dominant follicle occurs.

Synchronization of estrus in postpartum beef cows and virgin heifers using combinations of melengestrol acetate, GnRH, and PGF2α1

The efficacy of various combinations of melengestrol acetate (MGA), GnRH, and PGF2alpha for the synchronization of estrus in Angus-based beef cattle was compared. Hormones were administered as follows: MGA, 0.5 mg x animal(-1) x d(-1) mixed in a grain carrier; GnRH, 100 microg i.m.; PGF2alpha, 25 mg i.m. In Exp. 1, 2, and 3, cows were randomly assigned to treatments by parity and interval postpartum. The detection of estrus and AI were conducted from d -2 until 72 to 96 h after PGF2alpha, at which time cows not detected to be in estrus received GnRH and fixed-time AI (TAI). Data were analyzed separately for primiparous and multiparous cows. In Exp. 1, cows (n = 799) at three locations received GnRH on d -7 and PGF2alpha on d 0 and either no further treatment (GnRH-PGF) or short-term MGA from d -6 through d -1 (STMGA). Among multiparous cows, conception rate at TAI was greater (P < 0.05) for STMGA (41%, 47/115) than for GnRH-PGF treated cows (26%, 24/92). Across herds and parity, synchronized AI pregnancy rate (SPR) was not affected (P > 0.10) by treatment (GnRH-PGF vs. STMGA; 54%, 210/389 vs. 57%, 228/402). In Exp. 2, cows (n = 484) at three locations received either STMGA or long-term MGA from d -32 through d -19, GnRH on d -7, and PGF2alpha on d 0 (LTMGA). Among primiparous cows, SPR was greater (P < 0.01) in LTMGA (65%, 55/85) than STMGA-treated cows (46%, 40/87). Treatment had no effect (P > 0.10) on SPR among multiparous cows (STMGA vs. LTMGA; 59%, 92/155 vs. 64%, 101/157). In Exp. 3, cows (n = 838) at four locations received the LTMGA treatment and either no further treatment or an additional period of MGA exposure from d -6 through d -1 (L&STMGA). Among primiparous cows, SPR tended to be influenced (P < 0.10) by the herd x treatment interaction and was greater (P < 0.01) among L&STMGA (86%, 19/22) than LTMGA-treated cows (56%, 14/25) at a single location. Among multiparous cows, SPR was lower (P < 0.05) in L&STMGA (46%, 165/358) than LTMGA-treated cows (55%, 184/336). In Exp. 4, Angus heifers (n = 155) received either STMGA or 14 d of MGA (d -32 through d -19) and PGF2alpha on d 0 (MGA-PGF). The detection of estrus and AI were conducted from d -2 to d 6. Interval to estrus was greater (P < 0.05) and estrous response was lower (P < 0.05) in STMGA than MGA-PGF-treated heifers. In conclusion, primiparous cows responded more favorably to longer-duration MGA treatments than did multiparous cows. All protocols achieved sufficient SPR to justify their use for improved reproductive management of postpartum beef cows.

Synchronization of Bos indicus x Bos taurus cows for timed artificial insemination using gonadotropin-releasing hormone plus prostaglandin F2alpha in combination with melengestrol acetate

Nonlactating Bos indicus x Bos taurus cows were used in three herds to determine the efficacy of different PGF2alpha treatments in combination with GnRH and melengestrol acetate (MGA) for a timed artificial insemination protocol. The start of the experiment was designated as d 0, at which time cows were assigned a body condition score and received 100 microg of GnRH. Cows were fed MGA (0.5 x mg x cow(-1) x d(-1)) on d 1 to 7. On d 7, cows received either a single injection of PGF2alpha (Lutalyse sterile solution; 25 mg; n = 297), a single injection of cloprostenol sodium (Estrumate; 500 microg; n = 297), or half the recommended dose of PGF2alpha (12.5 mg; n = 275) on d 7 and 8. On d 10, all cows were artificially inseminated and received 100 microg of GnRH. Pregnancy rates to the timed artificial insemination (39%) were not affected by treatment, herd, or treatment x herd. There was an effect (P < 0.01) of artificial insemination sire on timed artificial insemination pregnancy rate for one herd, but not the other two herds. Herd influenced (P < 0.05) 30-d pregnancy rates, but there were no treatment or treatment x herd effects as 72.3% of the cows became pregnant during the first 30 d of the breeding season. Results indicate that the type of PGF2alpha treatment administered 7 d after GnRH did not influence timed artificial insemination pregnancy rates in nonlactating Bos indicus x Bos taurus cows.

Supplemental norgestomet, progesterone, or melengestrol acetate increases pregnancy rates in suckled beef cows after timed inseminations

In Exp. 1, 187 lactating beef cows were treated with injections of GnRH 7 d before and 48 h after prostaglandin F2alpha (PGF2alpha; Cosynch) or with Cosynch plus a 7-d treatment with an intravaginal progesterone (P4)-releasing insert (CIDR-B; Cosynch + CIDR). In Exp. 2, 183 lactating beef cows were treated with the Cosynch protocol or with Cosynch plus a 7-d treatment with norgestomet (Cosynch + NORG). In Exp. 1 and 2, blood samples for later P4 analyses were collected on d -17, -7 (first GnRH injection), 0 (PGF2alpha injection), and at timed artificial insemination (TAI; 48 h after PGF2alpha). In Exp. 3, 609 lactating beef cows were treated with the Cosynch + CIDR protocol or were fed 0.5 mg of melengestrol acetate (MGA) per day for 14 d before initiating the Cosynch protocol 12 d after the 14th d of MGA feeding (MGA + Cosynch). Blood samples were collected as in Exp. 1 and 2, plus additional samples on d -33 and -19 before PGF2alpha. In Exp. 4, 360 lactating beef cows were treated with a Cosynch + CIDR protocol, with TAI occurring at either 48 or 60 h after PGF2alpha, while receiving either GnRH or saline to form four treatments. Blood samples were collected as in Exp. 1 and 2. In Exp. 1, addition of P4 reduced the ability of the first GnRH injection to induce ovulation in anestrous cows with low P4 before PGF2alpha but improved (P = 0.06) pregnancy rates (61 vs 66%). In Exp. 2, the addition of NORG mimicked P4 by likewise increasing (P < 0.01) pregnancy rates (31 vs 51%) beyond those after Cosynch. In Exp. 3, the Cosynch + CIDR protocol increased (P < 0.001) pregnancy rates from 46 to 55% compared to the MGA + Cosynch protocol. In Exp. 4, administration of GnRH at TAI improved (P < 0.05) pregnancy outcomes (50 vs 42%), whereas timing of TAI had limited effects. We conclude that a progestin treatment concurrent with the Cosynch protocol improved pregnancy outcomes in all experiments, but pretreatment of cows with MGA was not as effective as the CIDR insert or NORG implants in this Cosynch-TAI model. Most of the improvement in pregnancy rates was associated with the increase in pregnancy rates of anestrous cows, regardless of whether ovulation was successfully induced in response to GnRH 7 d before PGF2alpha. Injection of GnRH at TAI following the Cosynch + CIDR protocol increased pregnancy rates in cycling cows with high P4 before the PGF2alpha injection and in anestrous cows with low P4 before PGF2alpha injection.

Evaluation of a fixed-time artificial insemination protocol for postpartum suckled beef cows

Treatment with melengestrol acetate (MGA), an oral progestin, prior to administration of gonadotropin-releasing hormone (GnRH) and prostaglandin F2alpha (PG) effectively synchronizes estrus and maintains high fertility in postpartum beef cows. The objective of this experiment was to determine whether treatment with MGA prior to a GnRH-PG-GnRH protocol would improve pregnancy rates resulting from fixed-time artificial insemination (AI). Multiparous crossbred beef cows at two University of Missouri-Columbia farms (n = 90 and n = 137) were assigned by age and days postpartum to one of two treatments. Cows were fed carrier (1.8 kg x animal(-1) x d(-1)) with or without MGA (0.278 mg x kg(-1)) for 14 d. All cows were administered GnRH (100 microg; intramuscularly) on d 12 after MGA or carrier withdrawal and 7 d before PG (25 mg; intramuscularly). All cows received a second injection of GnRH and AI 72 h after PG. Mean days postpartum for MGA and control cows at the initiation of treatment were 39.6 and 38.9 d for herd 1; and 51.9 and 50.9 d for herd 2, respectively (P > 0.70 within herds). Blood samples were collected from all cows at 10 and 1 d before the feeding of MGA or carrier began and at the times GnRH and PG were administered. Concentrations of progesterone in serum at the initiation of treatment were elevated (>1 ng/mL) in 0% of MGA and 7% of control cows in herd 1, and 54% of MGA and 49% of control cows in herd 2 (P > 0.05 within herds). Pregnancy rates to fixed-time AI were determined by transrectal ultrasonography 50 d after AI. Pregnancy rates in herd 1 were 58% (26/45) and 51% (23/45) for MGA-treated and control cows, respectively (P = 0.52), and 63% (44/70) and 45% (30/67) for MGA-treated and control cows in herd 2, respectively (P = 0.03). Differences in pregnancy rates to fixed-time AI were significant (P = 0.04) when data from the two herds were combined (with MGA = 70/115 [61%]; control = 53/112 [47%]). There was no difference (P > 0.20) in final pregnancy rates (timed AI plus 45 d exposure to bulls) between treatments, within herds, or when herds were combined. In summary, pregnancy rates resulting from fixed-time AI may be improved with treatment of MGA prior to a GnRH-PG-GnRH protocol.

Effect of an orally active progestin on follicular dynamics in cycling and anestrous postpartum beef cows

Although treatment of cycling cows with low concentrations of melengesterol acetate (MGA) results in formation of persistent follicles, in the absence of corpora lutea, it is not known whether persistent follicles form in anestrous cows in response to a similar treatment. The objective of this experiment was to determine the effect of long-term MGA treatment (14 d) on follicular dynamics and the secretion of estradiol in anestrous postpartum beef cows. Treatment groups (replicated over 2 yr) included the following: anestrous control (AC; n = 11), anestrous MGA (AM; n = 16), and cycling MGA (CM; positive control; n = 16). Angus-crossbred cows were assigned to treatment by age, cow body condition, and days postpartum. Cows were fed carrier (AC group) or 0.5 mg MGA x animal(-1) x d(-1) (AM and CM groups) for 14 d beginning approximately 38 d postpartum. Cows allotted to the CM group were injected with PGF2alpha, on the first day of MGA treatment to induce luteolysis. The preceding treatment (CM) results in formation of persistent follicles and secretion of elevated concentrations of estradiol. Ovaries of each cow were examined daily by transrectal ultrasonography beginning 5 to 7 d preceding the initiation of feeding MGA or carrier and continued until ovulation or 7 d following MGA feeding. There was no difference among groups in the stage of follicular wave or diameter of the largest follicle at the start of carrier or MGA feeding. The length of the follicular wave present at the start of MGA feeding was greater (P < 0.01) for cows in the CM (14.5 d, yr 1; 18.3 d, yr 2) group compared to the AM (9.4 d, yr 1; 7.9 d, yr 2) or AC (9.7 d, yr 1; 10.7 d, yr 2) groups. Maximum follicular diameter over both years was greater (P < 0.01) for the CM (20.6 mm) group than the AM (15.1 mm) or AC (16.4 mm) groups. Circulating concentrations of estradiol were also increased (P < 0.05) in the CM group compared to the AM or AC groups. However, MGA appeared to have no effect (P > 0.05) on the number of follicles recruited, growth rate of the dominant follicle during the first 6 d oftreatment, or growth rate to the maximum follicular diameter. In summary, MGA treatment did not increase the duration ot the follicular wave, maximum follicular diameter, or secretion of estradiol in anestrous postpartum cows, nor did MGA affect the number of follicles recruited or growth rate of dominant follicles in cycling or anestrous animals.

Effect of zeranol or melengestrol acetate (MGA) on testicular and antler development and aggression in farmed fallow bucks

Fifteen yearling fallow bucks were randomly assigned by BW to one of three treatment groups: control (C; n = 5), melengestrol acetate (MGA; n = 5), and zeranol (Z; n = 5), to evaluate effects on testicular development, aggressive behavior, antler growth, sexual activity, ADG, and BW. Zeranol-treated bucks received zeranol ear implants (36 mg) at 90-d intervals, and MGA-treated bucks received MGA in the ration (100 microg x animal(-1) x d(-1)). Bucks grazed ryegrass/Coastal bermudagrass pasture and were supplemented with 3:1 corn/soybean meal at 0.45 kg x animal(-1) x d(-1). Body weights, body condition scores (BCS), blood samples, and testis measurements were obtained at d 0 and at 14-d intervals for 229 d. As bucks reached hard antler (7/15 to 8/25), antlers were harvested and weighed, and ejaculates were collected at 14-d intervals. Aggression was evaluated using 10-min video sessions scoring body blows, avoidance, head pushes, and head bunts. Scrotal circumference (SC) and paired testis volume were affected by a day x treatment interaction (P < 0.01); testes of zeranol-treated bucks were smaller than those of control or MGA-treated bucks. First sperm in the ejaculate tended to be delayed (P < 0.10) in zeranol-treated bucks compared with control and MGA-treated bucks. Melengestrol acetate-treated bucks had a maximum sperm concentration in the ejaculate that was three times (P < 0.05) that of control bucks and nine times (P < 0.05) that of zeranol-treated bucks. Antler weight was the least (P < 0.01) for bucks receiving zeranol and greatest (P < 0.10) for MGA-treated bucks; intermediate values were recorded for the control bucks. Aggressive behavior was delayed (P < 0.05) for zeranol-treated bucks until treatment effects were overcome. Melengestrol acetate-treated bucks had decreased (P < 0.01) aggressive behavior compared with control bucks. Melengestrol acetate-treated bucks had increased (P < 0.05) serum testosterone concentrations compared with control and zeranol-treated bucks. Human chorionic gonadotropin-stimulated peak serum testosterone concentrations for zeranol-treated bucks were delayed (P < 0.01) compared with control and MGA-treated bucks. Although zeranol-treated bucks overcame treatment effects, they were never able to reach testicular measurements or sperm concentrations equal to those of the control or MGA-treated bucks. Zeranol and MGA treatments may have both positive and negative effects that can be utilized when producing slaughter bucks.

Melengestrol acetate blocks the preovulatory surge of luteinizing hormone, the expression of behavioral estrus, and ovulation in beef heifers

We tested the hypothesis that melengestrol acetate (MGA), an orally active progestin, blocks estrus and the preovulatory surge of luteinizing hormone (LH) in beef heifers. Cycling yearling Angus heifers were divided randomly into two groups: MGA-treated (n = 6) and control (n = 5). All heifers received injections of prostaglandin F2alpha (PGF) on d -25, -11, and 0 to synchronize estrus. Following the last PGF injection on d 0, heifers were fed either 0.5 mg MGA in a carrier or the MGA carrier each day for 8 d. At 4-h intervals on d 1 through 6, all heifers were observed for expression of estrous behavior, and blood samples were collected and assayed for LH. Daily blood samples were collected at 0800 on d 1 through 10 and assayed for circulating progesterone concentrations. All control heifers exhibited estrus and a preovulatory surge of LH. In each case, this was followed by increases in circulating concentrations of progesterone indicative of ovulation and normal luteal function. In contrast, none of the MGA-treated heifers exhibited estrus, LH surges, or evidence of ovulation. The results of this experiment show that MGA prevents ovulation in cattle by inhibiting the preovulatory surge of LH.

Effects of melengestrol acetate on reproductive behavior and concentrations of LH and testosterone in bulls

We investigated the use of an orally active progestin (melengestrol acetate; MGA) to suppress reproductive activity in yearling beef bulls. Twenty-four crossbred bull calves were given a daily dose of 0, 0.5, 1.0, or 2.0 mg MGA for 99 d. Pulsatile patterns of LH and concentrations of testosterone and MGA were characterized on d 8, 36, 63, and 92 of the experiment. Numbers of aborted mounts, mounts with intromission, total mounts, and flehmen responses were assessed on d 15, 43, 71, and 99. Plasma concentrations of MGA were proportional to dose of MGA. Melengestrol acetate did not consistently affect mounting behavior in a dose-related manner, but, on d 99, number of total mounts for MGA-treated bulls was lower (P = 0.07) than that for control bulls. On d 15, MGA suppressed (P = 0.07) numbers of flehmen responses in a dose-dependent manner, but this effect was not sustained throughout the experiment. On d 8, concentrations of testosterone in control bulls were higher (P = 0.02) than in MGA-treated bulls, but this effect was not observed at other time periods. Overall, MGA caused slight decreases in mean concentrations of LH (P = 0.09) and LH pulse frequency (P = 0.06). Scrotal circumference was not affected by MGA. None of the behavioral traits was correlated with mean concentrations of LH or LH pulse frequency. Mounting activity was not correlated with testosterone concentrations, but number of flehmen responses was positively correlated with testosterone concentrations (P = 0.01). These results fail to support the hypothesis that progestins impair male sexual behavior or fertility in males.

Effects of synthetic progestagens on the mRNA expression of androgen receptor, progesterone receptor, oestrogen receptor alpha and beta, insulin-like growth factor-1 (IGF-1) and IGF-1 receptor in heifer tissues

Synthetic progestagens like melengestrol acetate (MGA) are widely used for oestrus synchronization and for growth promotion in cattle production. The metabolic effects exceed its primary potency as a progestagen. It is speculated that MGA stimulates follicle development and thereby endogenous oestrogen production, but inhibits ovulation. To investigate the dose-dependent effects on mRNA expression levels, six heifers were fed for 8 weeks with different levels of MGA (0.5, 1.5, 5 mg) daily and two heifers served as controls. The expression of steroid receptor mRNA [androgen receptor (AR), progesterone receptor (PR), oestrogen receptor (ER) ERalpha and ERbeta], insulin-like growth factor-1 (IGF-1) and its receptor were quantified in liver, neck (m. splenius) and shoulder muscularity (m. deltoideus). Plasma concentrations of IGF-1 were quantified by radioimmunoassay. In treated animals the MGA plasma levels were elevated over the complete treatment period, corresponding to the MGA treatment concentrations. IGF-1 concentrations of control animals were at constant levels. Plasma levels for oestradiol (E2) and IGF-1 were increased in the low MGA treatment group. Overdosed MGA decreased progesterone (P4) and E2 levels. To quantify the IGF-1 and all receptor mRNA transcripts, sensitive and reliable real-time reverse transcription-polymerase chain reaction (RT-PCR) quantification methods were developed and validated in the LightCycler. A dose-dependent relationship between increasing MGA concentration and mRNA expression was observed in liver for AR and IGF-1 receptor, and in neck muscularity for IGF-1. ERalpha in liver and neck muscle showed a trend of increasing expression.

Improved synchrony of estrus and ovulation with the addition of GnRH to a melengestrol acetate-prostaglandin F2alpha synchronization treatment in beef heifers

The objective of this experiment was to determine the effect of a GnRH injection within a melengestrol acetate (MGA)-PGF2alpha (PGF) estrus synchronization protocol on follicular dynamics and synchronization of estrus. Pubertal crossbred beef heifers (n = 34) were randomly assigned to one of two treatments. Both treatment groups were fed MGA (0.5 mg x hd(-1) x d(-1)) for 14 d and injected (i.m.) with PGF (25 mg of Lutalyse) 19 d after MGA withdrawal. Melengestrol acetate was delivered in a feed supplement of 1.8 kg x hd(-1) x d(-1). Seventeen heifers received an injection of GnRH (100 microg Cystorelin) 12 d after MGA withdrawal and 7 d before PGF. The control group (n = 17) received only MGA-PGF. Estrus was detected four times/d for 7 d beginning on the day PGF was injected. Transrectal ultrasonography was performed daily on eight heifers from each treatment to monitor ovarian activity and characterize changes in follicular dynamics after MGA withdrawal and until ovulation after PGF. Each of the GnRH-treated heifers either ovulated or had a luteinized dominant follicle following GnRH and subsequently initiated a new follicular wave (8/8, 100%). All GnRH-treated heifers (17/17, 100%) and 94% of controls (16/17) exhibited estrus after PGF. Estrus was exhibited over a 132-h period (12 to 144 h) for control heifers compared with 60 h (48 to 108 h) for GnRH-treated heifers. The peak synchronized period for both treatments was between 48 and 72 h after PGF, during which time 76% (13/17) of the GnRH-treated heifers exhibited estrus compared with 63% (10/16) for controls. Seventy-one percent (12/17) of the GnRH-treated heifers exhibited estrus from 48 to 60 h after PGF, compared with 38% (6/16) for controls (P < 0.05). In summary, injection of GnRH within a 14- to 19-d MGA-PGF protocol increased the synchrony of estrus during the synchronized period and concentrated the period of detected estrus. This protocol may offer potential for the fixed-time insemination of replacement beef heifers.

Assessment of oestrogenic potency of chemicals used as growth promoter by in-vitro methods

Three in-vitro bioassays were used to compare the oestrogenic potency of chemicals used as growth promoter in beef cattle in certain non-European Union countries (17beta-oestradiol, alpha-zearalanol, testosterone, trenbolone, trenbolone acetate, melengestrol acetate) or found as food contaminant such as the mycotoxin zearalenone and some of their metabolites (17alpha-oestradiol, oestrone, 17alpha-epitestosterone, 19-nortestosterone, androstendione, zearalanone, alpha-zearalanol, beta-zearalanol, alpha-zearalenol, beta-zearalenol). The strong oestrogens 17alpha-ethinyl oestradiol and diethylstilboestrol were used as standards. The first bioassay was based on the activation of a reporter gene by oestrogens in recombinant yeast expressing human or rainbow trout oestrogen receptor. In the second bioassay, the vitellogenin gene induction of rainbow trout hepatocyte cultures was used as a biomarker for the exposure to oestrogens. The third bioassay was based on the alkaline phosphatase gene induction by oestrogens in the human endometrial Ishikawa cell line. The assessment of oestrogenic potency of these chemicals clearly demonstrates the strong oestrogenicity of the mycotoxin zearalenone and its metabolites and particularly alpha-zearalenol which was as potent as ethinyl oestradiol and diethylstilboestrol in the human endometrial Ishikawa cell line.

Effects of immunization against GnRH, melengestrol acetate, and a trenbolene acetate/estradiol implant on growth and carcass characteristics of beef heifers

A 2 x 3 factorial experiment was conducted to determine the effects of an implant (trenbolene acetate/estradiol or no implant) and method of estrus suppression (immunization against GnRH, melengestrol acetate, or no suppression) on growth performance and carcass characteristics of heifers fed for slaughter. At the start of a 21-d feed adaption phase, crossbred beef heifers (n = 144, 390+/-2.8 kg) were given their first dose of an anti-GnRH vaccine or started on melengestrol acetate (MGA). Thereafter, heifers were fed a high-concentrate diet (78% barley grain) for 84 d (Days 0 to 83), received implants on Day 0, a second vaccination on Day 21, and were slaughtered on Days 84 or 85. Implanting increased average daily gain (1.72 vs 1.50 kg/d, P < 0.01), feed efficiency (6.02 vs 6.75 kg dry matter intake/kg gain, P < 0.01), preslaughter weight (532 vs 513 kg, P < 0.01), carcass weight (301 vs 289 kg, P < 0.01), and ribeye area (88.6 vs 85.9 cm2, P < 0.05), but had no affect (P > 0.05) on dry matter intake, grade fat thickness, marbling score, or lean yield. Compared to heifers fed MGA, those immunized against GnRH had a greater ribeye area (90.0 vs 84.6 cm2) and lean yield (63 vs 61%), and had thinner grade fat (7.5 vs 8.6 mm; P < 0.05 for each). Furthermore, immunized heifers had lower (P < 0.001) plasma progesterone concentrations than control heifers on Days 42, 63 and 83. Heifers fed MGA had less estrus mounting activity (P < 0.05) and lower plasma progesterone concentrations (P < 0.001) than the remaining heifers. Method of estrus suppression did not affect (P > 0.05) preslaughter weight, average daily gain, dry matter intake, feed efficiency, carcass weight, or marbling score. In conclusion, implanting significantly increased growth performance and preslaughter and carcass weights. Compared to heifers fed MGA, immunization against GnRH significantly increased ribeye area and lean yield, and reduced grade fat thickness

Biochemistry and physiology of anabolic hormones used for improvement of meat production

A number of hormones are involved in endocrine regulation of growth. In general, these hormones enhance body protein accretion and metabolise fat stores resulting in increased lean growth rates. Most practical importance was obtained by sex hormones (oestrogens and androgens), beta-agonists and growth hormone - whether legally or illegally. Efficiency of growth promotion ranges between 0% and +20% depending on the prerequisites such as species, breed, gender, age, reproductive status, body score or feeding of the animals. Oestrogens and androgens mediate their activity via intracellular receptors - directly in muscular tissue as well as indirectly via stimulation of growth hormone from the hypophysis and other growth factors from liver plus several further organs. In addition, mineral absorption in the gut is improved. The outstanding efficiency of trenbolone is based on its androgenic plus antiglucocorticoid activity. Melengestrolacetate is thought to act indirectly via stimulation of endogenous ovarian oestradiol in non-pregnant heifers. The necessary dosages and residue formations depend on the pharmacokinetic parameters of each substance and extrapolations between compounds are hardly possible. Growth hormone and beta2-agonists use independent pathways for growth promotion not related to steroid biochemistry.

Dose-dependent effects of melengestrol acetate (MGA) on plasma levels of estradiol, progesterone and luteinizing hormone in cycling heifers and influences on oestrogen residues in edible tissues

Melengestrol acetat (MGA) is widely used as a growth promoting feed additive in cattle breeding in the USA and several other non-European countries. To explore the physiological effects of MGA four heifers were fed during 8 weeks with 0.5 mg MGA daily as registered in the USA and two heifers each received 0, 1.5 or 5 mg/day, respectively. Plasma samples were collected twice a week and concentrations of MGA, progesterone (P4) and estradiol-17beta (E2-17beta) were quantified. The pulsatile secretion of luteinizing hormone (LH) was investigated in 6-hour profiles before and during treatment. After slaughter the reproductive organs were examined and oestrogen residues in edible tissues were measured. Four days after the beginning of MGA feeding MGA concentrations in plasma reached levels of 30 and 100-400 pg/mL depending on the dose received. Three weeks after the beginning of MGA feeding P4 plasma concentrations had dropped to base levels below 0.3 ng/mL in all three treatment groups. Mean plasma E2-17beta levels increased in physiological range from 1 to 5 pg/mL during 0.5 mg MGA/day feeding with many acyclic peaks. Overdosed MGA decreased E2 levels and suppressed cyclic peaks. Number and size of ovarian follicles were not altered by any treatment. Mean LH levels and pulse frequencies increased significantly during labelled treatment (0.5 mg/day), while higher doses had reducing effects. The development of corpus luteum was suppressed. E2-17beta residues in fat increased about 300% following labelled MGA treatment.

Growth-promoting systems for heifer calves and yearlings finished in the feedlot

In a 172-d finishing trial (Exp. 1), 210 recently weaned crossbred heifers were allotted to six growth promotant treatment groups, involving implanting initially with Synovex-C (C) or H (H) followed by reimplanting with Finaplix-H (F) or H and F. Melengestrol acetate (MGA) was provided in the diet to four of the treatment groups. Heifers fed MGA and administered only F as the terminal implant had the greatest (P = .01) number of mature ovaries with follicles but also had lower (P = .01) gain/DMI. In a 182-d finishing study (Exp. 2), 270 recently weaned crossbred heifers were allotted to the following six implant (d 0)/ reimplant (d 70) groups using no implant (N), Ralgro (R) or H: N/R, R/H, R/R, N/R, H/H and R/R for Treatments 1 through 6, respectively. On d 70, all heifers were implanted with F. Heifers were fed MGA from d 70 to 182 (Treatments 1, 2, and 3) or for the entire trial (Treatments 4, 5, and 6). Implanting on d 0 increased (P < .05) overall ADG. Differences (P > .05) in performance were not found between MGA treatment groups. Using an H implant/reimplant regimen decreased (P = .01) ovarian and(or) follicular development when compared with an R implant/reimplant regimen. In a 126-d finishing trial (Exp. 3), 360 crossbred yearling heifers were used to evaluate F and estrogen (Implus-H) implants when used in combination with an MGA feeding program. Heifers receiving only F in combination with MGA had greater (P < .05) ADG, whereas all heifers fed MGA had greater (P < .05) gain/DMI than heifers not fed MGA. These data suggest that feeding MGA was not beneficial for young heifers, particularly if they are provided an initial estrogenic implant followed by a second implant. In older (yearling) heifers, increased gains and gain/DMI were obtained by feeding MGA and implanting initially or 56 d later with F.

Development of an estrus synchronization protocol for beef cattle with short-term feeding of melengestrol acetate: 7-11 synch

An estrus synchronization protocol (7-11 Synch) was developed to synchronize the first follicular wave and timing of ovulation in postpartum beef cows. In Exp. 1, follicular development and timing of ovulation in response to the following protocol were evaluated. Beef heifers (n = 12) and cows (n = 6), at random stages of the estrous cycle, were fed melengestrol acetate (MGA; .5 mg x animal(-1) x d(-1)) for 7 d and injected with PGF2alpha (PG; 25 mg) on the last day of MGA. A second injection of PG was administered 11 d after cessation of MGA. After the second injection of PG, estrus was synchronized in 6/12 heifers and 3/6 cows. The interval to estrus in heifers and cows was 54 and 64 h, respectively (P > .10). All animals exhibiting estrus ovulated first-wave follicles. Animals that failed to respond to the second injection of PG were in estrus later than 6 d after cessation of MGA and had corpora lutea that were unresponsive to the injection of PG. Based on the variation in interval to estrus following the first PG injection on the last day of MGA feeding in Exp. 1, an injection of GnRH (100 microg) was added to the protocol 4 d after the cessation of MGA to ensure ovulation or luteinization of dominant follicles and synchronization of first-wave follicular development. This revised protocol was termed "7-11 Synch." In Exp. 2, two estrus synchronization protocols were compared. Multiparous beef cows were stratified by breed and postpartum interval and randomly assigned to the 7-11 Synch (n = 44) or Select Synch protocols (GnRH injection followed by PG injection 7 d later; n = 45). Timing of estrus after the last PG injection (0 h) ranged from 42 to 102 h in the 7-11 Synch group and -30 to 114 h in the Select Synch group. Eight cows (18%) in the Select Synch group exhibited estrus 30 h before to 18 h after PG. Synchronized estrus peaked between 42 and 66 h after the last PG injection, and a maximum number of cows were in estrus at 54 h for both treatment groups. Synchrony of estrus from 42 to 66 h was greater (P < .05) in 7-11 Synch (91%: 41/44) than in Select Synch cows (69%: 31/45). Artificial insemination pregnancy rate from 42 to 66 h was greater (P < .05) in the 7-11 Synch group (66%: 29/44) than in the Select Synch group (40%: 18/45). In summary, the 7-11 Synch protocol improved synchrony of estrus without reducing fertility. This protocol has potential future application for fixed-time AI in beef cattle production systems.

Evaluation of melengestrol acetate and equine chorionic gonadotropin for out-of-season breeding in sheep on Prince Edward Island

The purpose of this study was to compare the efficacy of a recommended protocol of oral melengestrol acetate (MGA) to intravaginal medroxyprogesterone acetate (MPA), with or without equine chorionic gonadotropin (eCG), for out-of-season breeding of sheep on Prince Edward Island. One hundred and twenty ewes were assigned to 1 of 4 groups in a factorial design and were treated with either an intravaginal MPA sponge or oral MGA. Ewes received either an intramuscular injection of eCG or a saline placebo at cessation of progestogen treatment. The reproductive performance was best for the MPA-eCG group, followed by the MGA-eCG, MGA-saline, and MPA-saline. Pregnancy rates of 66.7%, 43.3%, 31.0%, and 16.7%, respectively, and ratios of lambs born to ewes exposed to the ram of 1.17, 0.63, 0.52, and 0.23, respectively, were recorded for these groups. We concluded that, under the conditions of this study, the use of oral MGA resulted in fewer lambs than did the use of MPA sponges with eCG. Nonetheless, the use of MGA may be attractive to producers because it is less expensive and more convenient than the use of sponges.

Effect of timing of artificial insemination on gender ratio in beef cattle

It was recently reported that cows inseminated at approximately 10 or 20 h before an expected ovulation deliver predominately a bull or heifer calf, respectively. The objective of this study was to further investigate the effect of timing of insemination on the gender of offspring in cattle. Angus heifers (n = 41) and cows (n = 98) were used in the study. Heifers were synchronized with a 16-d treatment of melengestrol acetate followed 17 d later with an injection of PGF2alpha. Cows were synchronized with GnRH followed 7 d later with PGF2alpha. A HeatWatch electronic estrus detection system was used to determine the onset of estrus. Based on previous studies, it was assumed that ovulation occurs approximately 32 h after the onset of estrus. Therefore, animals were artificially inseminated at either 8 to 10 h (early; > or = 20 h before expected ovulation) or 20 to 25 h (late; < or = 10 h before expected ovulation) after the onset of estrus. Sixty to 80 d after insemination, ultrasonography was used to confirm pregnancy status and to determine the gender of fetuses. Gender of calves was subsequently confirmed at calving. Data were analyzed for effects of time of insemination and sire or semen batch on gender ratio, as well as any effect of length and/or intensity of estrus on conception rate and gender ratio. Twenty-nine of 41 heifers and 69 of 98 cows were detected in estrus after synchronization and were inseminated; 20 of 29 heifers and 48 of 69 cows were subsequently confirmed pregnant. Neither the length of estrus nor its intensity (number of mounts) had an effect on pregnancy rate or gender ratio (P > or = 0.418). Timing of insemination (early versus late) had no effect on gender ratio (P = 0.887). Semen from 13 sires representing 17 lots was used to inseminate the cows and heifers. No differences (P = 0.494) were detected in the gender ratios resulting from different sires or semen batches. In contrast to previous findings, our results indicate that inseminating beef cattle at approximately 20 or 10 h before an expected ovulation does not alter the gender ratio of the resultant calves.

Estrus synchronization of beef cattle with a combination of melengestrol acetate and an injection of progesterone and 17beta-estradiol

Our hypothesis was that estrus synchronization in beef cattle using melengestrol acetate (MGA) and an injection of progesterone (P4) and 17beta-estradiol (E2) to regress dominant ovarian follicles would improve pregnancy rate (number conceived/number in group) to AI compared with feeding only MGA or injecting PGF2alpha. During 2 yr, peripubertal heifers (n = 52) and cows (n = 327) received either 1) MGA for 18 d (d 0 = 1st d of MGA) plus an injection of P4 and E2 in sesame oil (vehicle) on d 11 to regress persistent ovarian follicles (MGA+P4), 2) MGA for 18 d plus vehicle on d 11 (MGA), or 3) two injections of PGF2alpha 10 d apart (d 7 and 17, PG). Concentration of P4 was assessed in blood samples obtained on d 0, 7, and 17 to indicate estrual status (anestrual or estrual) during treatment to induce estrus synchrony. Observations for detection of estrus occurred every 6 h for 180 h following treatment cessation. Females showing estrus were inseminated 6 to 12 h after estrus detection. Conception to AI was determined by ultrasonography 35 to 40 d later. Conception rate was greater (P < .05) in females in the PG than in those in the MGA group but did not differ from conception rate of females in the MGA+P4 group. Among anestrual females, estrus synchrony rates were greatest (P < .10) among females treated with MGA+P4. Among females that were estrual before treatment cessation, estrus synchrony rates were greater (P < .10) among females treated with MGA+P4 or PG than among those given MGA. Pregnancy rates were greater (P < .05) among females that were anestrual before treatment cessation and treated with MGA or MGA+P4 than among those treated with PG. Estrus synchronization using MGA+P4 and E2 differentially improves estrus synchronization and pregnancy rates among anestrual and estrual beef cattle while maintaining conception rates similar to those of PGF2alpha-treated females.

Effects of melengestrol acetate on onset of puberty, follicular growth, and patterns of luteinizing hormone secretion in beef heifers

Our objective was to test the hypothesis that short-term (8 days) treatment of prepubertal heifers with melengestrol acetate (MGA) and subsequent steroid withdrawal would stimulate LH secretion and follicular growth. Angus heifers were divided randomly into two groups; MGA-treated (n = 8) or control (CON; n = 9). Puberty was determined by monitoring circulating concentrations of progesterone and ovarian morphology during a 14-day period following MGA withdrawal. LH secretory patterns were assessed upon initiation of MGA (Day 0), during MGA (Day 7), and 1 day after withdrawal of MGA (Day 9). All MGA-treated heifers, versus four CON heifers, exhibited corpora lutea and luteal phase concentrations of progesterone within 10 days after treatment (p = 0.01). Mean LH and LH pulse frequency increased (p = 0.005 and 0.0001, respectively) between Days 0 and 9 in MGA-treated heifers. In CON heifers, mean LH concentrations and pulse frequencies did not change. During the same period, diameter of the largest follicle increased in MGA-treated animals (p = 0.003) but did not change in the CON heifers. On the basis of these results, we suggest that MGA withdrawal enhances onset of puberty by stimulating pulsatile LH secretion that accelerates follicle growth to the preovulatory stage.

Development of a progestin-based estrus synchronization program: II. Reproductive response of cows fed melengestrol acetate for 14 days with injections of progesterone and prostaglandin F2alpha

We tested the efficacy of an estrus control system designed to provide optimal control of follicular development. In Exp. 1, postpartum cows (n = 133) and yearling heifers (n = 57) were fed either .5 mg x female(-1) x d(-1) of melengestrol acetate (MGA) or the carrier for MGA from d -13 to d 0 (d 0 = last day of MGA feeding). All females received 25 mg of PGF2alpha (i.m.) on d -13 and 0. On d -6, cows and heifers fed MGA were administered an i.m. injection of progesterone (200 mg; MGA/P4), and those fed the corn carrier (2XPGF2alpha) received no progesterone. Beginning on d 1, females were bred by AI from d 1 to at least d 5. During the estrus synchronization period (d 1 to d 5), more (P < .05) postpartum cows were observed in estrus (70.1 vs 42.4%), the timing of estrus was more (P < .05) precise, conception rate was similar, and pregnancy rate was higher (P < .05) in the MGA/P4 than in the 2XPGF2alpha treatment. More (P < .05) cows that were anestrous at the beginning of the breeding season were in estrus during the synchronization period in the MGA/P4 (55.8%) than in the 2XPGF2alpha (28.6%) treatment. In heifers, estrus was synchronized in over 90% of females, and neither conception nor pregnancy rate during the synchronization period differed between treatments. In Exp. 2, postpartum cows (n = 122) and heifers (n = 84) received treatments (MGA/P4 or 2XPGF2alpha) as described for Exp. 1 with one exception. In the MGA/ P4 treatment, progesterone was administered on d -7 rather than d -6. Females were bred by AI from d 1 to 5. The estrus response and conception rate during the synchronization period did not differ between treatments for either cows or heifers. We conclude that the progestin-based estrous synchronization system used in this study effectively synchronized an estrus of normal fertility in cyclic cows and induced a majority of anestrous cows to reinitiate estrous cycles.

Development of a progestin-based estrus synchronization program: I. Reproductive response of cows fed melengestrol acetate for 20 days with an injection of progesterone

We designed two experiments to determine the efficacy of an estrus control system in cows that combined long-term progestin exposure (20 d) with an acute increase in progesterone concentration. In Exp. 1, cows (n = 30) were fed either melengestrol acetate (MGA; .5 mg x cow(-1) x d(-1)) or ground ear corn (MGA carrier) for 20 d. On d -15 (last day of MGA feeding = d 0), cows were administered 25 mg of PGF2alpha to regress the corpus luteum (CL) and establish an environment conducive to the development of persistent follicles. To synchronously regress persistent follicles, cows fed MGA (n = 15) were injected with 200 mg of progesterone on d -2 (MGA-P), and the cows fed the MGA carrier were not treated (CONT; n = 15). Cows in the CONT group were artificially inseminated 12 h after detection of spontaneous estrus from d -20 to d 8. Estrus was observed, and all cows in the MGA-P group were artificially inseminated during the period of estrus synchronization (SYNC; d 1 to 8). No difference in conception rate was observed between treatments. In Exp. 2, postpartum cows (n = 113) received either the MGA-P (n = 56) or CONT (n = 57) treatment. More (P < .05) cows were observed in estrus during SYNC in the MGA-P (50%) than in the CONT (28%) group. Of the cows in the MGA-P group that were not observed in estrus during SYNC, 50% were in estrus for the first time 23 to 29 d after MGA withdrawal (SYNC2), suggesting that these cows ovulated without observable estrus during SYNC. Estrus was observed for the first time during SYNC2 in more (P < .05) cows in the MGA-P (25%) than in the CONT (7%) group. Conception rate at the synchronized estrus, pregnancy rate, and interval to first service and pregnancy were similar between treatments. We conclude that administration of MGA-P results in the synchronization and(or) induction of a fertile estrus in cows.

The effects of melengestrol acetate on the ovaries of captive wild felids

Melengestrol acetate (MGA) is the most widely used contraceptive in zoo felids, but the mechanism of contraception and the pathologic effects have not been investigated. For this study, the effects of MGA on folliculogenesis were assessed, and the association of MGA with ovarian lesions was evaluated. Comparisons were made among the histopathologic findings in the ovaries from 88 captive wild felids (representing 15 species) divided into three groups: 37 currently contracepted with MGA, eight previously exposed to MGA, and 43 never contracepted. Ninety-one percent of the felids evaluated had tertiary follicles, and no differences were noted between contracepted and uncontracepted cats. Some MGA-contracepted cats also had corpora lutea indicating recent ovulation. These results indicate that folliculogenesis not suppressed by current doses of MGA and ovulation occurred in some cats. Therefore, the contraceptive actions of MGA do not occur by suppressing folliculogenesis, and MGA-contracepted felids likely have endogenous estrogens that may confound progestin effects on the uterus. Cystic rete ovarii was the most common pathologic finding, but they were not more prevalent in MGA-contracepted cats. These findings indicate that MGA is not associated with ovarian disease, including ovarian cancer, in contrast to the uterine lesions noted in MGA-treated cats.

Serum concentrations of trenbolone-17 beta and estradiol-17 beta and performance of heifers treated with trenbolone acetate, melengestrol acetate, or estradiol-17 beta

The effects of the growth-promoting steroids estradiol-17 beta (E2), trenbolone acetate (TBA), and melengestrol acetate (MGA) in heifers on serum concentrations of E2 and trenbolone-17 beta (TBOH) were examined. Feed intake and growth performance were also measured. Serum concentrations of E2 and TBOH were measured on d 0, 1, 3, 5, 7, 13, 21, 28, 42, 56, 84, 112, and 140 in finishing heifers administered the following treatments: 1) control; 2) MGA, .5 mg per heifer daily; 3) Revalor-H (140 mg TBA + 14 mg E2); 4) Revalor-H + MGA; 5) Finaplix-H (200 mg TBA); and 6) Finaplix-H + MGA. Revalor-H implantation (Treatments 3 and 4) increased (P < .05) serum E2 concentrations; peak concentrations (67.5 pg/mL) occurred between d 21 and 56. Feeding MGA (Treatment 4) had no effect (P > .05) on this increase in serum E2 concentrations (63.3 pg/mL). From d 84 until d 140, serum E2 was greater (P < .05) for the Revalor-H treatment (average of 19 pg/mL) than for the control (7 pg/mL) or Finaplix-H treatments (6.5 pg/mL). Serum E2 concentrations increased numerically two- to threefold from d 56 to 140 in controls fed MGA, compared with controls not fed MGA. There was the expected increase in serum TBOH concentrations after TBA implantation in the Revalor-H and Finaplix-H treatments; concentrations were similar (P > .05) for Revalor-H (221 pg/mL) and Finaplix-H (280 pg/mL). After d 56, serum TBOH concentrations decreased in both treatments to 10 and 20% of these concentrations, respectively. Feeding MGA increased serum TBOH (P < .05). Dry matter intake by heifers did not differ among treatments. Feeding of MGA improved gains (P = .12) and efficiencies (P < .01) in nonimplanted heifers and had no effect (P > .4) on gains or efficiencies in Finaplix-implanted heifers.

[Progestational agents and bone metastasis in breast cancer]

Hormonotherapy in metastatic breast cancer is actually performed using 3 therapeutic classes: antiestrogen as tamoxifen (TAM), progestins, megestrol acetate (MA) and medroxyprogesterone acetate (MPA), aromatase-inhibitors as aminoglutethimide (AG). We investigated therapeutic efficacy of progestins compared to other hormonotherapies in case of breast cancer with bone metastasis. In a literature review, we found ten randomized trials comparing: MPA versus AG, MPA versus MA, MPA versus TAM, MA versus AG versus TAM. The results show an advantage on the response rate using MPA; the usual dose was 900-2,000 mg daily. MPA mode of action can explain these results since they combine gestagenic, androgenics and glucocorticoid effects.

Persistence of the dominant follicle during melengestrol acetate administration and its regression by exogenous estrogen treatment in beef cattle

Experiments were conducted to test the hypothesis that estrogen treatment will regress a persistent dominant follicle developed during melengestrol acetate (MGA) treatment in the absence of a functional corpus luteum (CL) with normal fertility following development and ovulation of a newly recruited follicle. In Exp. 1, nonlactating beef cows (n = 31) were administered .5 mg.cow-1.d-1 of MGA (d 0) for 14 d with 25 mg of prostaglandin F2 alpha (PGF2 alpha) administered on d 6 and 8 to regress the CL. On d 11 of treatment, approximately half the MGA-treated cows received 5 mg of estradiol valerate (EV) i.m. (MGAEV, n = 14) and the remainder were maintained on MGA (n = 17). Ovaries were checked with ultrasound on d 8, 10, 12, and 14 of MGA treatment and every day until ovulation. A persistent dominant follicle developed in 90% of the MGA-treated cows by d 10 of treatment. Most of the MGA-treated cows ovulated the persistent dominant follicle (n = 13/17), whereas EV treatment regressed the persistent dominant follicle (n = 10/14) with the recruitment of a new follicle that ovulated (n = 8/10). Diameter of the ovulatory follicle was larger (P < .05) for the MGA (19.8 +/- .6 mm) than for the control (15.1 +/- .8 mm) and MGAEV (14.8 +/- .7 mm) cows. In Exp. 2, nonlactating, multiparous beef cows (n = 97) and yearling heifers (n = 38) were equally allotted to either a control, MGA alone, or MGA + estradiol-17 beta (MGAE) group with the same dose of MGA as administered in Exp. 1. The 1st d of MGA feeding was the 1st d of treatment. On d 10 of treatment half the MGA-treated animals were injected i.m. with 5 mg of estradiol-17 beta. In controls, behavioral estrus was detected and animals were artificially inseminated (AI) for 5 d (d 10 to 14 of experiment). All controls not exhibiting estrus by d 15 of experiment were injected with 25 mg of PGF2 alpha. The remaining controls and all MGA cows were observed for behavioral estrus and AI commenced for 7 d following withdrawal of MGA (d 15 to 21 of experiment). More (P < .05) controls (90.3%) than MGA (84.8%) or MGAE (63.6%) cows showed estrus within 7 d after MGA withdrawal. The percentage of animals conceiving to the synchronized estrus did not differ (P > .05) among treatments. The data support our hypothesis that a persistent dominant follicle developed and can be regressed with exogenous estrogen treatment followed by the recruitment and ovulation of a new follicle after MGA withdrawal and fertility of that estrus does not seem to be significantly compromised.

Effect of interval from melengestrol acetate to prostaglandin F2 alpha on timed and synchronized pregnancy rates of beef heifers and cows

The objective of this experiment was to determine the optimal interval from the last day of melengestrol acetate (MGA) feeding to prostaglandin F2 alpha (PGF2 alpha) treatment on pregnancy rates of beef heifers and cows. All females (149 heifers and 399 postpartum cows) were fed MGA (.5 mg) daily for 14 d and then administered PGF2 alpha (25 mg Lutalyse) 13, 15, and 17 d (Groups 1, 2, and 3, respectively) after the last day of MGA feeding. Females not in estrus the first 52 h after PGF2 alpha treatment were artificially inseminated 72 h after PGF2 alpha treatment. Females in estrus 0 to 52 h and 78 h to 6 d after PGF2 alpha treatment were inseminated at estrus. Blood sera (collected immediately before and 3 d after PGF2 alpha treatment) were assayed for progesterone concentrations. Pregnancy was determined 44 to 47 d after the 72-h AI by rectal examination. The intervals from MGA feeding to PGF2 alpha that had the highest 72-h AI pregnancy rates were 17 d for heifers (43%) and 15 d for cows (43%). Heifers with a 17-d interval had a higher (P < .05) 72-h AI pregnancy rate than heifers with 13-d and 15-d intervals, and cows with a 15-d interval had a higher (P < .05) 72-h AI pregnancy rate than cows with a 17-d interval. The 4-d synchronized pregnancy rates (the 72-h AI and the succeeding 3 d inseminations) for both heifers (44%) and cows (53%) were not different (P > .05) among groups. Fewer (P < .05) cows with a 17-d interval from MGA to PGF2 alpha had corpora lutea regression (by 72 h) after PGF2 alpha treatment than cows with 13-d and 15-d intervals. We interpret the results to indicate that the interval from MGA to PGF2 alpha treatment may influence 72-h AI pregnancy rates, that optimal intervals are 17 d for heifers and 15 d for cows, and that pregnancy rates are improved by insemination for 3 d after the 72-h AI.

Use of melengestrol acetate-based treatments to induce and synchronize estrus in seasonally anestrous ewes

Ten experiments utilizing 1,659 ewes were conducted to determine the potential of melengestrol acetate (MGA)-based treatments in inducing and synchronizing estrous activity in ewes during the nonbreeding season. In May or June, ewes were given .25 mg of MGA.ewe-1.d-1 or control diet and(or) a subsequent i.m. injection of zeranol, estradiol-17 beta, or oil vehicle. Fertile rams were introduced, and estrous and(or) lambing responses were determined. In Exp. 1, more (P < .01) ewes given MGA for 14 d followed by 5 mg of zeranol (i.m.) were in estrus than controls. Melengestrol acetate and zeranol independently induced estrus in Exp. 2, with MGA-treated ewes exhibiting the greatest lambing response (P < .05). The optimum duration of feeding MGA was 8 d vs 11 or 14 d (Exp. 3). In Exp. 4, as the dose of zeranol increased, synchrony of estrus increased but lambing response decreased. A dose of 1.25 mg of zeranol represented a compromise in estrous and lambing response and was determined to be optimally effective when given 54 h after the last feeding of MGA (Exp. 5). Shortcomings in lambing responses were treated in Exp. 6 by supplementing ewes with MGA after breeding, which proved unsuccessful, and in Exp. 7 by substituting estradiol-17 beta for zeranol, which proved effective when given 54 h after the last feeding of MGA (Exp. 8). In Exp. 9, use of MGA + estradiol increased (P < .05) the estrous and subsequent lambing response of 30-d postpartum ewes temporarily weaned of their lambs. In contrast, lambing response of 90-d postpartum ewes treated with MGA + estradiol was not increased by temporary removal of lambs (Exp. 10). These data collectively support the hypothesis that treatment of ewes with MGA + estradiol is an effective and practical approach for inducing and synchronizing a fertile estrus in ewes during the non-breeding season.