Pituitary receptors for GnRH and estradiol, and pituitary content of gonadotropins in beef cows. II. Changes during the postpartum period

Impact of 17β-estradiol administration at the moment of timed-AI in Nelore cows with small dominant follicle or not showing estrus

We aimed to evaluate the effects of administering estradiol (E-17β) at the moment of timed-AI (TAI) on uterine gene expression, estrous expression rate (EER), and pregnancy rate (P/TAI) in Nelore cows with a small dominant follicle (DF) or not showing estrus at TAI. In Experiments 1 and 2 (Exp1, Exp2) cows were submitted to a P4/E-17β-based protocol (day 0) for synchronization of ovulation. On day 7, devices were removed, cows received 1 mg E-17β cypionate and 12.5 mg dinoprost. On day 9, cows with DF < 11.5 mm in diameter were split into different groups. In Exp1 (n = 16/group): Control (no treatment), E-2 (2 mg E-17β) and E-4 (4 mg E-17β). In Exp2: Control (n = 12); E-2 (n = 14); GnRH (0.1 mg gonadorelin acetate, n = 13); and E-2+GnRH (association of GnRH and E-17β, n = 13). Between days 9 and 11, endometrial thickness (ET), time of ovulation detection, and EER were recorded. In Exp1, a uterine cytological sample was collected 4 h after treatment to evaluate the transcript expression of receptors for E-17β (ESR1 and ESR2), oxytocin (OXTR), and P4 (PGR). In Experiment 3 (Exp3), 3829 suckled cows were submitted to a P4/E-17β-based protocol for TAI. On day 9, devices were removed and cows received 1 mg E-17β cypionate and 0.4 mg sodium cloprostenol. On day 11, TAI was performed and cows that did not demonstrate estrus received 0.1 mg gonadorelin acetate, and were allocated into two groups: GnRH (n = 368) and E-2+GnRH (2 mg E-17β; n = 363). In Exp1, plasma E-17β concentrations increased at 4 h after treatment in a dose-dependent manner but reduced at 12 h. The E-17β-treated cows had greater transcript abundance for OXTR and lesser for ESR1 and ESR2, and the ET was reduced 12 h after treatment (P < 0.05). No significant difference (P > 0.1) was observed between the E-17β doses in estrus or ovulation rate. In Exp2, the interval from treatment to ovulation was longer (P < 0.05) in the E-17β group. GnRH-treated cows showed higher ovulation rates (89 vs. 35 %) compared to cows not treated with GnRH, as E-17β-treated cows (P < 0.01) had a lower ovulation rate compared to those not receiving E-17β (44 vs. 78 %). In Exp3, P/TAI was 55 % for cows in estrus. For those not showing estrus, no difference (P > 0.1) in P/TAI was observed between GnRH (34 %) and E-2+GnRH (31 %) groups. Cows with a DF ≥ 11 mm (n = 192) had a greater (P < 0.05) P/TAI (49 %) than those with DF < 11 mm (n = 377; 29 %). In conclusion, E-17β administration in the moment of TAI modulates the mRNA expression of uterine receptors in cows with a small DF but does not impact the P/TAI compared with GnRH treatment in suckled Nelore not showing estrus previous to TAI.

Uterine Involution and Ovarian Activity in Postpartum Holstein Dairy Cows. A Review

Following parturition in the cow, there is a significant period of sexual quiescence of variable length. This period of reproductive quiescence was found to be longer in suckling or intensively milked animals.This acyclic period is generally considered as the postpartum anoestrous period. The postpartum period constitutes an important period in the reproductive life of dairy cows because of its enormous influence upon subsequent fertility. The entire postpartum period, puerperium, is defined as the period from parturition until the genital organs return to its normal physiological and histological condition, as in normal non-gravid state. They added that any extension of the puerperium in cows might have a detrimental effect on the reproductive performance of the individual animal. Thus, the main determinant of this period is essentially dependent on the resumption of normal ovarian cycles, the manifestation of estrus behaviour and conception following insemination.

Effect of age at puberty/conception date on cow longevity

Age at puberty is a critical trait, because pregnancy success during the breeding season is correlated with the percentage of heifers that reach puberty before or early in the breeding season. A negative genetic correlation between age at puberty and heifer pregnancy rate indicate that selection to decrease age at puberty would increase heifer pregnancy rates. Calving late has been reported to increase the chance of calving late or not calving the following year, and heifers need to wean 3 to 5 calves to pay for development costs. Therefore, puberty is important to the sustainability and profitability of beef operations.

Exposure to estrogen mimicking the level of late pregnancy suppresses estrus subsequently induced by estrogen at the level of the follicular phase in ovariectomized shiba goats

A high-estrogen environment during late pregnancy is suspected to cause postpartum silent ovulation, and progesterone (P₄) is suggested to recover estrus. However, few attempts have been undertaken to elucidate the influence of these steroids on estrus by analyzing hormonal profiles. We investigated estrus and luteinizing hormone (LH) surges in ovariectomized goats (n=6) assigned to three treatments in a cross-over design. In groups 1 and 2, 200 µg/kg body weight/day estradiol benzoate (Dose-200 E₂B) was administered for 14 days concurrent with P₄ for 11 days, while in the control, saline solution and P₄ were administered likewise. Ten days after the final administration of Dose-200 E₂B, group 2 was treated with P₄ for 8 days, and all groups were treated with 2 µg/kg body weight E₂B (Dose-2 E₂B) 20 days after the final administration of Dose-200 E₂B (or saline solution). The proportion of cases expressing estrus after the administration of Dose-2 E₂B was smaller (P<0.01) in group 1 than in the control (1/6, 3/6 and 6/6; groups 1 and 2 and the control, respectively). The proportions of cases generating LH surges did not differ (P>0.1) among the groups (5/6, 5/6 and 6/6; groups 1 and 2 and the control, respectively), but the peak concentrations in groups 1 and 2 (26.2 ± 14.7 and 11.3 ± 6.7 ng/ml) were lower (P<0.01) than those in the control (67.8 ± 19.4 ng/ml). These results demonstrated that elevation of plasma estrogen mimicking late pregnancy inhibits the subsequent estrus induced by estrogen simulating the follicular phase.

Non-genomic actions of progesterone and estrogens in regulating reproductive events in domestic animals

It has been established that nuclear receptors mediate the action of estrogens and progestins in regulating gene expression in the hypothalamic-hypophyseal-gonadal axis of domestic animals during various reproductive states. Results of recent in vitro studies suggest that estradiol-17beta and progesterone can act non-genomically to affect signal transduction responses in target cells by binding to receptors in the plasma membrane. The genomic action of steroids is generally detectable in hours to days whereas non-genomic responses of cells occur in seconds to minutes. The nature of the plasma membrane receptors for estrogens and progesterone has been explored but has not been conclusively established for all cell types studied. In the ewe, estradiol-17beta or estradiol-bovine serum albumin conjugate has been shown by in vitro and in vivo approaches to act non-genomically to suppress luteinizing hormone secretion by gonadotropes and stimulate production of nitric oxide by uterine arterial endothelial cells. Progesterone has been shown to inhibit oxytocin (OT) binding to its receptor in isolated ovine endometrial plasma membranes. This non-genomic action of progesterone blocks OT activation of the phosphoinositide cascade and production of prostaglandin F(2alpha) by ovine and bovine endometrium. The acrosome reaction of caprine and porcine spermatozoa is activated by the non-genomic action of progesterone. Further research is required to define the biological significances of the non-genomic actions of estrogens and progestins.

LH and IGF-1 release during oestrus and early luteal phase in lactating and non-lactating horse mares

The aim of the present study was to determine effects of lactation on basal LH and IGF-1 concentrations and on the LH response to a GnRH-analogue at different stages of the oestrous cycle in mares. A total of 17 cyclic Haflinger mares were included in the study. Experiments were performed on lactating mares in first postpartum oestrus, the subsequent early luteal phase, and second postpartum oestrus. Non-lactating mares were used in oestrus and early luteal phase. Blood samples were taken for 1 h at 15 min intervals. Mares were then injected with the GnRH-analogue buserelin (GnRHa; 5 microg i.v.) and blood samples were drawn every 15 min for further 2 h. LH in all samples and basal IGF-1-concentrations were determined by RIA. In lactating mares, basal LH concentrations during the early luteal phase tended to be lower (p = 0.07) and the LH response to GnRHa, calculated as area under the curve, was significantly less pronounced compared to non-lactating mares (p < 0.01). As well in lactating mares, the basal LH concentration between first early luteal phase and second oestrus differed significantly (p < 0.05) and the net response to GnRHa was significantly lower between first oestrus as well as second oestrus and first early luteal phase (p < 0.05) but not between first and second oestrous postpartum. Within the group of non-lactating mares, the LH response to GnRHa was as well significantly lower during oestrus than during early luteal phase (p < 0.01). IGF-1 concentrations differed neither between groups nor stages of the cycle within groups. In conclusion, basal and GnRHa-stimulated LH release in lactating mares is lower than in non-lactating mares. This difference, however, occurs only in the early luteal phase. In lactating mares, concentrations of LH appear adequate to allow ovulation to occur.

Restoration patterns for luteinising hormone and ovarian function following treatment with GnRH agonist implants (deslorelin) for 7, 14 or 21 days in cycling dairy cows

Continuous GnRH agonist treatment of cows results in downregulation of GnRH responsiveness and a state of induced anoestrus. Inducing anoestrus in a precisely controlled manner could have several potential applications in dairy herd management. However, relatively little is known regarding the processes involved in restoring reproductive normality following an induced anoestrus. This study describes an experiment that was conducted to examine patterns of recovery of LH release and follicle growth in non-lactating Holstein cows immediately following cessation of treatment for 7, 14 or 21 days with a deslorelin implant. Oestrus cycles were synchronized at 7 days intervals and a deslorelin implant inserted in every cow 13 days after detected oestrus so that a group had implants for either 21 days (n = 9), 14 days (n = 10) or 7 days (n = 9). On the day of implant removal every ovarian follicle greater than 4 mm in diameter was ablated using ultrasound guided vacuum needle aspiration in an attempt to standardize follicle sizes. Daily ovarian ultrasound examinations were performed on each cow until 35 days after implant removal and again at 45, 59 and 74 days. A subgroup of four cows randomly selected from each treatment group had frequent serial blood samples collected over 8 h at 4 and 10 days after implant removal for LH profiling. There was no significant effect of treatment duration on any LH parameter and results were pooled. Mean LH pulse amplitude increased by 67% between 4 and 10 days after implant removal (0.34 ng/ml versus 0.57 ng/ml; 4 days versus 10 days post-implant, P < 0.001). Mean pulse frequency remained unchanged between the two samplings (5.9 pulses versus 6.9 pulses per 8 h; 4 days versus 10 days post-implant, P > 0.1). Smoothed mean LH concentrations were unaffected by treatment duration or time (0.36 ng/ml versus 0.41 ng/ml; 4 days versus 10 days post-implant, P > 0.1). The pattern of follicle growth and ovulation did not differ significantly between treatment durations and pooled means were used for comparative descriptions. The emergence of a new follicle wave could be detected beginning at 4 days after implant removal (mean 7.9 +/- 0.8 days). After emergence, a period of rapid follicle growth generally ensued with signs of oestrus occurring when the follicle reached 12.3 +/- 0.5 mm and ovulation when mean follicle diameter was 13.1 +/- 0.7 mm at 13.6 +/- 1.5 days after implant removal. Oestrus preceded ovulation in all cases where ovulation subsequently occurred. The mean interovulatory interval after implant removal was similar amongst groups (18.2 +/- 1.3 days). Follicle growth could be categorized into three groups based on the time to emergence and fate of the first wave DF. Spontaneous recovery was characterised by ovulation of the newly emerged DF. Failure to ovulate the first DF was associated with the formation of a persistent follicle by 35 days after implant removal with some 25% (7/28) of cows showing persistent follicles. Delayed emergence (>14 days after implant removal) was detected in 11% (3/28) of cows, but when a follicle did eventually emerge it was seen to ovulate normally. The correlation between number of days to first oestrus expression and LH pulse amplitude on 4 days post-implant removal was significant (R2 = 44%, P < 0.05). A model was then proposed for the restoration of reproductive function following GnRH agonist removal. In conclusion, duration of deslorelin treatment was associated with only small changes in LH and follicle parameters. The formation of persistent follicles delayed the recovery of more cows than delayed emergence of a new follicle wave after implant removal.

Body condition and suckling as factors influencing the duration of postpartum anestrus in cattle: a review

Prolonged postpartum anestrus is a main factor limiting reproductive efficiency in cattle, particularly in Bos indicus and Bos taurus/Bos indicus cows from tropical regions, because it prevents achievement of a 12 month calving interval. During anestrus, ovulation does not occur despite ovarian follicular development, because growing follicles do not mature. Although many factors affect postpartum anestrus, nutrition and suckling are the major factors influencing the resumption of postpartum ovarian cycles, as they affect hypothalamic, pituitary and ovarian activity and thus inhibit follicular development. Under-nutrition contributes to prolonged postpartum anestrus, particularly among cows dependent upon forages to meet their feed requirements and it apparently interacts with genetic, environmental or management factors to influence the duration of anestrus. The nutritional status or balance of an animal is evaluated through body condition score (BCS), as it reflects the body energy reserves available for metabolism, growth, lactation and activity. There is a converse relationship between energy balance and time to resumption of postpartum ovarian activity; inadequate nutrient intake results in loss of weight and BCS and finally cessation of estrous cycles. Suckling interferes with hypothalamic release of GnRH, provoking a marked suppression in pulsatile LH release, resulting in extended postpartum anestrus. The effects of suckling on regulation of tonic LH release are determined by the ability of the cow to identify a calf as her own or as unrelated. Vision and olfaction play critical roles in the development of the maternal-offspring bond, allowing the cow to identify her own calf, and abolition of both senses attenuates the negative effects of suckling on LH secretion. Thus, the maternal-offspring bond is essential for prolonged postpartum suckling-induced anovulation, and the suppressive influence of suckling is independent of neurosensory pathways within the teat or udder.

Reproductive responses of early postpartum dairy cattle to continuous treatment with a GnRH agonist (deslorelin) for 28 days to delay the resumption of ovulation

An experiment was conducted to investigate the potential of chronic delivery of a potent GnRH agonist (deslorelin) via subcutaneous implants to delay the resumption of ovulatory cycles in postpartum dairy cattle. Cows received either a single deslorelin implant (n=40; DES) within 7 days of calving or were untreated (n=24; CON). Blood samples were collected thrice weekly during the period the implants were in place. Plasma concentrations of progesterone (P4) and 17beta-oestradiol (E2) were measured along with selected serum metabolites. Implants were removed after 28 days and cattle monitored daily for behavioral oestrus. Serial weekly blood samples were collected to detect the occurrence of ovulation. Cows were artificially inseminated as they were detected in oestrus from 30 days after implant removal. Pregnancy status was subsequently determined by manual palpation of uterine contents at strategic intervals. Insertion of implants induced ovulation in 3/40 cows as determined by a rise in progesterone 7 days later. Deslorelin implants delayed the onset of ovulatory cycles compared with untreated herdmates (mean 43.4+/-4.2 versus 57.3+/-1.6 days postpartum; P<0.001). A noticeable delay of at least 12 days was observed between implant removal and the first animals ovulating. Mean plasma E2 concentrations during the period the implants were in place were similar for DES and CON cows that experienced a prolonged spontaneous postpartum anoestrus (low P4 >60 days), although both groups had concentrations only 20% of CON cows that had ovulated prior to 30 days postpartum. The patterns of recovery following implant removal were highly variable. A number of DES cows showed a low and transient rise in plasma progesterone around 21 days after implant removal. Some cows displayed oestrus but did not appear to form a fully functional corpus luteum with this phenomenon being more prevalent among DES cows (7 of 37 versus 1 of 21; P<0.05). Overall, significantly more DES cows were detected in oestrus without ovulating compared to CON cows. Final pregnancy rates did not differ between DES and CON groups. The mean time to conception for DES cows was longer (21.2+/-5.6 versus 41.1+/-7.4 days, CON versus DES; P<0.01). This difference was not present if the time from first ovulation to conception was compared (50.5+/-5.3 versus 43.5+/-9.3 days, CON versus DES; P>0.05). Deslorelin implants provided a reliable method of inducing anoestrus when treatment was initiated prior to 3 days postpartum. A variable pattern of recovery was observed which delayed conception but did not ultimately reduce the final proportion pregnant at the completion of mating. The study demonstrates the potential of GnRH agonists to control postpartum reproductive function to manipulate the fertility of dairy cows.

Induction of ovulation in postpartum suckled beef cows: a review

Prolonged postpartum acyclicity in suckled beef cows reduces the calf crop, and causes economic loss to beef cattle producers. Once anterior pituitary LH stores have been replenished between Days 15 and 30 post partum in suckled beef cows, methods to initiate cyclicity include non-hormonal methods such as weaning of calves (either complete, temporary or partial), or exposure to bulls, and hormonal methods such as administration of GnRH (either single injection, intermittent injections, or continuous infusion), gonadotropins (eCG, FSH, hCG), and steroids (estrogens, anti-estrogens, and progestogens). Weaning is costly, reduces growth rate of weaned calves, and short cycles are common after weaning-induced ovulation. Exposure of cows to bulls is not practical and its effect is not predictable. Repeated injections of GnRH, or a single injection of hCG are not always effective; ovulation is always followed by a short cycle, and usually a return to acyclicity. Estrogens and anti-estrogens do not consistently shorten postpartum anestrus. Exogenous progestogens include intravaginal devices, such as controlled-internal drug release (CIDR) or progesterone-releasing intravaginal device (PRID), norgestomet implants, and the feed-additive melengestrol acetate (MGA). Administration of exogenous progestogens is more practical than, and offers more advantages over, other treatments to shorten postpartum acyclicity in suckled beef cows. Mimicking the short cycle after Week 3 post partum, by maintaining circulating progesterone at subluteal concentrations or circulating progestin at intermediate concentrations, extends the life-span and allows terminal maturation of the postpartum dominant follicle as in cyclic cows, by initiating endogenous GnRH and LH pulses. This is followed by an LH surge, ovulation and normal cycles. The benefit from using exogenous progestogens after Week 3 post partum in suckled beef cows is that ovulation is induced, cyclicity is initiated, the resulting CL has a normal life-span and function, and there is no need to change management, such as weaning of calves. We present a model for the induction of ovulation and initiation of cyclicity using exogenous progestogens after Week 3 post partum in suckled beef cows.

Postpartum acyclicity in suckled beef cows: a review

Prolonged postpartum acyclicity in suckled beef cows is a source of economic loss to beef cattle producers. Duration of postpartum acyclicity is influenced by suckling status, nutritional status, calving season, age, and several other factors. Although uterine involution begins and ovarian follicular waves resume soon after parturition, dominant follicles of these waves fail to ovulate, due to a failure to undergo terminal maturation. As a result, postpartum anovulatory dominant follicles are smaller than the ovulatory follicles in cyclic cows. Failure of postpartum dominant follicles to undergo terminal maturation is due to absence of appropriate LH pulses, a prerequisite for follicular terminal maturation prior to ovulation. Absence of LH pulses early post partum is primarily due to depletion of anterior pituitary LH stores, although GnRH pulses are also absent during this period due to suckling. Following replenishment of LH stores between Days 15 and 30 post partum, absence of LH pulses is due to continued sensitivity of the hypothalamic GnRH pulse-generator to the negative feedback effect of ovarian estradiol-17beta, which results in absence of GnRH pulses. This negative feedback effect of estradiol-17beta is modulated by suckling which stimulates release of endogenous opioid peptides from the hypothalamus. As the postpartum interval increases, sensitivity of the GnRH pulse-generator to the negative feedback effect of ovarian estradiol-17beta decreases. This is followed by an increasing frequency of GnRH discharges and LH pulses, terminal follicular maturation, ovulation, and continued cyclicity. The first ovulation post partum is usually followed by a short cycle due to premature luteolysis because of premature release of PGF2alpha from the uterine endometrium, which is possibly intensified by the suckling-induced oxytocin release from the posterior pituitary. A model for the postpartum ovulatory acyclicity and for the resumption of cyclicity is presented.

Serum luteinizing hormone, 13,14-dihydro-15-keto-prostaglandin F2α and cortisol profiles during postpartum anestrus in Brahman and Angus cows

Pluriparous suckled Brahman and Angus cows were utilized to evaluate the effect of breed, day after calving and endogenous opioid peptides (EOP) on hormonal profiles during postpartum anestrus. On Days 17 and 34 after calving, blood samples with and without heparin were collected at 15- and 30-min intervals, respectively, for a 7-h period via jugular cannula. Two hours after the start of blood sampling, cows of each breed were administered either 1 mg/kg iv naloxone or saline. Three hours later, all animals received 10 ng/kg iv GnRH. On Day 34 after calving cows received 0.2 IU/kg iv ACTH. Mean LH, basal LH and area under the LH curve increased (P < 0.01) from Day 17 to Day 34 after calving. Height of LH pulses increased (P < 0.05) by Day 34 after calving. Brahman cows had higher (P < 0.05) mean LH, basal LH, LH pulse frequency and area under the LH curve than Angus cows. Naloxone increased postchallenge area under the LH curve in treated cows above that of control cows (P < 0.06). Naloxone also increased the postchallenge area under the LH curve above that of the prechallenge level (P < 0.01). No breed differences in the response to the naloxone challenge were observed. The LH response to naloxone challenge occurred earlier on Day 34 than on Day 17 after calving but the amount of LH released was similar between days. The GnRH-induced LH release was greater in Brahman than in Angus cows (P < 0.04). Mean cortisol concentrations and area under the cortisol curve decreased (P < 0.05) between Day 17 and Day 34 after calving. Mean cortisol concentrations and area under the cortisol curve were lower (P < 0.01) in Brahman than in Angus cows. Cortisol secretion after ACTH treatment was similar between Brahman and Angus cows. The cortisol response after ACTH challenge was positively correlated (r=0.68; P < 0.001) to the prechallenge area under the cortisol curve. Under optimal environmental conditions Brahman cows have a greater LH release and their anterior hypophysis is more sensitive to GnRH challenge than the Angus cows.

Effects of suckling and of a long interval after ovariectomy on hypothalamo-hypophyseal responsiveness to naloxone, morphine and GnRH in beef cows

The response of serum luteinizing hormone (LH) to morphine, naloxone and gonadotropin-releasing hormone (GnRH) in ovariectomized, suckled (n=4) and nonsuckled (n=3) cows was investigated. Six months after ovariectomy and calf removal, the cows were challenged with 1mg, i.v. naloxone/kg body weight and 1 mg i.v. morphine/kg body weight in a crossover design; blood was collected at 15-minute intervals for 7 hours over a 3-day period. To evaluate LH secretion and pituitary responsiveness, 5 microg of GnRH were administered at Hour 6 on Day 1. On Days 2 and 3, naloxone or morphine was administered at Hour 3, followed by GnRH (5 microg/animal) at Hour 6. Mean preinjection LH concentrations (3.6 +/- 0.2 and 4.7 +/- 0.2 ng/ml), LH pulse frequency (0.6 +/- 0.1 and 0.8 +/- 0.1 pulses/hour) and LH pulse amplitude (2.9 +/- 0.5 and 2.9 +/- 0.6 ng/ml) were similar for suckled and nonsuckled cows, respectively. Morphine decreased (P < 0.01) mean serum LH concentrations (pretreatment 4.2 +/- 0.2 vs post-treatment 2.2 +/- 0.2 ng/ml) in both suckled and nonsuckled cows; however, mean serum LH concentrations remained unchanged after naloxone. Nonsuckled cows had a greater (P < 0.001) LH response to GnRH than did suckled cows (area of response curve: 1004 +/- 92 vs 434 +/- 75 arbitrary units). We suggest that opioid receptors are functionally linked to the GnRH secretory system in suckled and nonsuckled cows that had been ovariectomized for a long period of time. However, gonadotropin secretion appears not to be regulated by opioid mechanisms, and suckling inhibits pituitary responsiveness to GnRH in this model.

Effect of dietary energy, body condition and calf removal on pituitary gonadotropins, gonadotropin-releasing hormone (GnRH) and hypothalamic opioids in beef cows

Beef cows (n = 64) were slaughtered to evaluate effects of dietary energy and calf removal (CR) on hypothalamic and adenohypophysial endocrine characteristics. From d 190 of gestation until parturition, cows received maintenance (ME; n = 32) or low (LE; n = 32) energy diets (ME = 100%, LE = 70% NRC recommendations). After parturition, half (n = 16) of each prepartum diet group received low (LE; n = 32) or high (HE = 130% NRC; n = 32) energy diets. At 30 d postpartum, cows were slaughtered 0 or 48 hr after CR. Hypothalami [preoptic area (POA), hypothalamus (HYP), stalk-median eminence (SME)] and pituitaries were collected. Basal and K(+)-induced release of GnRH from SME, and pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) did not differ among groups (P greater than .05). Hypophyseal LH was correlated (P less than .01) with body condition score (BCS) at parturition and slaughter (r = .36 and .47, respectively). Prepartum LE diet increased (P less than .05) met-enkephalin in POA compared to prepartum ME (.59 +/- .05 vs. .44 +/- .04 pmol/mg) regardless of postpartum diet or suckling status. Concentrations of beta-endorphin in combined HYP + POA were decreased (P less than .05) by 48 hr CR (15.1 +/- 1.1 vs. 18.1 +/- 0.7 fmol/mg).(ABSTRACT TRUNCATED AT 250 WORDS)

Extension of short cycles in postpartum beef cows by intrauterine treatment with catecholestradiol

Eight multiparous beef cows were used to examine the effects of intrauterine infusion of catecholestradiol (4-hydroxylated estradiol) on development and function of the first corpus luteum after parturition. Calves were weaned on day 1 (day 0 = parturition) to initiate formation of a corpus luteum (CL) by approximately day 10 or 11. Before CL formation, on days 5 to 9, cows received twice daily infusions of catecholestradiol (4 micrograms; n = 4) or vehicle (n = 4) into the uterine horn opposite the previous pregnancy. Plasma progesterone during the first estrous cycle was elevated longer (P less than .001) and reached a higher (P less than .001) concentration in cows treated with catecholestradiol. The decline in progesterone was associated with an increase in plasma 13,14-dihydro, 15-keto-prostaglandin F2 alpha (PGFM) in all cows infused with catecholestradiol. In contrast, a rise in PGFM at the end of the first short cycle was detected in only one of four cows treated with vehicle. Furthermore, PGFM concentrations were linearly related (R2 = .870; P less than .001) to concentrations of progesterone. Estradiol-17 beta concentrations were not different during the infusion period, but after formation of the first CL, estradiol remained elevated (P less than .01) in cows that received vehicle. Results of this experiment suggest that exposure of postpartum beef cows to catecholestradiol extended luteal function in association with enhanced PGFM release.