Factors affecting estrogen-induced LH release in the cow

Magnitude and persistence of higher estrus-associated temperatures in beef heifers and suckled cows

Higher estrus-associated temperatures (HEAT) are a hallmark feature in sexually active females. The overarching aim of this study was to characterize the variability, magnitude, and persistence of HEAT in heifers and suckled beef cows as well as identify associated factors when occurring during thermoneutral conditions at the onset of the spring breeding season. In both heifers and cows, estrus was induced using a 7-d controlled internal drug release (CIDR)-PGF2α protocol. Vaginal temperature after prostaglandin F2α administration was recorded every 5 min using a Thermochron iButton affixed to a blank CIDR (containing no progesterone). Estrus was defined as when a heifer first stood to be mounted or when a cow had an Estrotect patch score of 3 or 4. Level of HEAT varied among individual animals. When comparing common HEAT variables using a mixed model with date nested within a year, maximum HEAT (39.9 ± 0.1 and 40.0 ± 0.1 °C) and duration (15.5 ± 0.8 and 15.4 ± 0.7) were similar in heifers and cows, respectively. However, the magnitude and persistence of HEAT differed. Total area under the HEAT curve was 117.1 ± 13.5 and 158.7 ± 12.3 for heifers vs cows, respectively (P = 0.0571). Further, 42.9% of heifers and 49% of cows had maximum HEAT ≥ 40 °C which persisted up to 6.5 and 10 h, respectively. When ambient conditions were predominantly thermoneutral, temperature humidity index had minimal impact on HEAT (mixed model, repeated measures over time). Toward identifying associated factors with different aspects of HEAT using best fit hierarchical linear regression models, baseline vaginal temperature and baseline duration were the most highly associated independent variables. Follicle size, estradiol and progesterone levels, and other available animal-related variables (e.g., age, weight, hair coat score) explained only a small amount of variation in HEAT. In summary, level of HEAT varies in estrus females even under thermoneutral conditions. Because HEAT can persist for an extended time, direct effects on fertility important components are unavoidable. Whether HEAT is a good or bad component of the periovulatory microenvironment is the basis of ongoing and future studies.

Parental Behavior in Bovines

After approximately 9 months, cows give birth to one calf. They usually try to separate from the herd if kept outdoors, but this depends on the possibilities to hide. When the calf is born, the cow immediately stands up and starts licking the calf, and this is most intense for the first hour. During this time, the cow is very protective of her calf and may attack anyone trying to come close to it, especially if the cow is not so used to human handling. The calf tries to stand up and falls in the beginning, but it usually stands about half an hour later. Once standing, the calf searches for the udder, and after approximately 2 h, the calf is suckling the colostrum from the cow's teats. The calf is born without an immune defense, and it, therefore, needs to suckle enough colostrum from the cow to get protection against infections. If the calf does not succeed in suckling within 4-6 h after birth, the farmer must milk the cow and give the calf at least 2-4 l of colostrum in a teat bottle. If the farmer has frozen colostrum of good quality in the freezer, and it is either difficult to milk the cow or her colostrum is of too low quality, the frozen colostrum can be warmed up and given to the calf. Beef calves are usually allowed to stay with their mother until they are 6-8 months old, when they are separated from the cow and then weaned. Dairy calves are usually separated from their mother immediately or within a few days after birth and raised by artificial milk feeding. In the beginning or during the whole milk period (6-12 weeks), they may be kept individually indoors in crates or outdoors in hutches. After weaning, they are usually group housed.

Day of prostaglandin F2α administration after natural ovulation affects the interval to ovulation, the type of ovulated follicle, and the failure to induce ovulation in cows

The factors that affect the interval to ovulation, the type of ovulated dominant follicle (DF), and the cause of anovulation after prostaglandin (PG) treatment were investigated. Nine cows were assigned to six groups (54 cows in total) but the group size was later fixed at eight cows (48 in total). They received 25 mg tromethamine dinoprost as dinoprost on Day 6 (Group D6), Day 7 (Group D7), Day 8 (Group D8), Day 9 (Group D9), Day 10 (Group D10), or Day 11 (Group D11) after natural ovulation (Day 0). If the DF did not ovulate, then the cow was assigned to Group NO. In Group D6, the 1st DF ovulated in all cows 4 days after PG treatment, whereas in Groups D9, D10, and D11, the 2nd DF ovulated in all cows 4 to 7 days after PG treatment. In 10 cows, the DF did not ovulate, and late anovulation was significantly higher in Group D6 cows than in Group D11 cows. The progesterone (P₄) levels decreased to less than 1 ng/ml in all groups on the day after PG treatment. The estradiol-17β (E₂) levels began to increase after PG treatment and peaked at 2 days before ovulation in the cows that ovulated. In anovulated cows, E₂ tended to be higher and there was no clear E₂ peak in some cows. These results indicated that the number of days to ovulation, the type of ovulated DF, and anovulation were affected by factors that were associated with the DF when it was producing E₂.

Presynchronization by induction of a largest follicle using a progesterone device in GnRH-based-ovulation synchronization protocol in crossbred dairy cows

The objective was to compare the fertility of dairy cows using a presynchronization protocol by induction of a largest follicle using a progesterone intravaginal device prior to an Ovsynch protocol (P4synch) with the Double-Ovsynch in lactating dairy cows. Lactating Bos indicus x Bos taurus crossbred cows (n = 440) were randomly allocated to one of two treatments: (I) Double-Ov (n = 228), GnRH (D-17), PGF2_α 7 days later (D-10) and GnRH 3 days later (D-7) followed by an Ovsynch protocol 7 days later (GnRH on D0, PGF on D7, GnRH on D9); (II) P4synch (n = 212), insertion of a sustained release progesterone intravaginal device (D-10), 10 days later (D0) an Ovsynch protocol was initiated (GnRH on D0, PGF on D7, GnRH on D9) with progesterone device withdrawal on Day 7. All cows were artificially inseminated (TAI) 16 h after the second GnRH of the Ovsynch protocol and pregnancy diagnosis was performed by transrectal ultrasonography 30 and 60 days after TAI. A subset of cows (n = 52 for Double-Ov and n = 50 for P4synch) ultrasonography was performed on days 0, 7, 9 and 24 of the experimental period. There were no differences among treatments on presynchronization rate [presence of a follicle>12 mm on D0, Double-Ov 94.2% (49/52) and P4synch 92.0% (46/50); P = 0.66], follicular diameter on the 1st GnRH (Double-Ov 17.2 ± 0.7 mm e P4synch 18.6 ± 0.9 mm; P = 0.28), ovulation rate to the 1st GnRH [Double-Ov 86.3% (44/51) and P4synch 81.2% (39/48); P = 0.50], synchronization rate [presence of a follicle>12 mm on D9; Double-Ov 84.6% (44/52) and P4synch 86.0% (43/50); P = 0.84], follicular diameter on the 2nd GnRH (Double-Ov 17.5 ± 0.6 mm and P4synch 18.0 ± 0.5 mm; P = 0.48), ovulation rate to the 2nd GnRH [Double-Ov 90.9% (40/44) and P4synch 86.0% (37/43); P = 0.48] and CL diameter on D24 (Double-Ov 27.9 ± 0.7 mm and P4synch 29.4 ± 0.9 mm; P = 0.19). Corpus luteum presence on D0 was different (P = 0.03) among treatments [Double-Ov 57.7% (30/52) and P4synch 36.0% (18/50)]. There was no difference (P = 0.85) among the pregnancy per AI on day 30 [Double-Ov 39.0% (89/228) and P4synch 40.1% (85/212)], on day 60 [Double-Ov 34.8% (79/227) and P4synch 38.7% (82/212); P = 0.41] and pregnancy loss [Double-Ov 10.2% (9/88) and P4synch 3.5% (3/85); P = 0.08]. The presynchronization by induction of a largest follicle using a sustained release progesterone device prior to Ovsynch yielded similar results compared with the Double Ovsynch protocol on follicular development patterns and on the fertility of lactating dairy cows.

Behavioral and hormonal pattern of repeat breeder cows around estrus

Repeat breeder (RB) cows were compared with normal (CTRL) ones with respect to behavioral estrus intensity, endocrine patterns and concentrations of plasma estradiol, progesterone and LH around estrus, and ovulation timing. A total of 27 and 31 cycles in 12 RB and 18 CTRL cows, respectively, were synchronized by means of the Ovsynch protocol followed by a single PG injection after 7 days. Behavioral estrus and ovulation were observed in 81.5 and 83.8% of the synchronized cycles in the RB and CTRL cows respectively. The RB and CTRL groups had similar estrus durations of 21.4 and 19.6 h respectively, but estrus was more intense in the RB, as indicated by numerically higher overall activity indexes and higher peak neck activity. The interval from PG injection to estrus onset (considered as proestrus) was 8.2 h shorter in RB than in CTRL cows, at 47.9 and 56.1 h respectively (P<0.007), but the average preovulatory follicle size was similar. The estradiol concentration at peak was numerically higher (21%) and the AUC tended to be higher in the RB cows than in the CTRL cows. LH secretion during the period from 18 to 3 h before the LH peak was also lower in RB than in CTRL cows: 2.5 and 4.6 ng/ml respectively (P<0.01). In conclusion, the behavioral estrus was more intense in the RB cows; nevertheless, short proestrus and subdued LH concentrations before the LH peak, which could impair oocyte competence and development, were first reported in RB cows.

Ovulatory responses and plasma luteinizing hormone concentrations in dairy heifers after treatment with exogenous progesterone and estradiol benzoate

The objectives of this experiment were to determine the effects of 0.5 mg estradiol benzoate, administered intramuscularly 24 h after removal of CIDR-B progesterone containing intravaginal devices, on the time to estrus, ovulation and peak LH concentration in dairy heifers. Ovulatory responses and plasma LH concentrations were examined using 14 Friesian dairy heifers in 2 separate treatment periods. All heifers received a CIDR-B progesterone-containing intravaginal device with an attached 10-mg estradiol benzoate capsule for 12 d. Within each period, 24 h after CIDR-B removal, 7 heifers received an intramuscular injection of 0.5 mg estradiol benzoate while the remaining 7 heifers received an intramuscular injection of a placebo. Blood samples for LH assay were collected at 0, 6 and 12 h, and then every 4 h for 60 h after estradiol injection. Detection of estrus was conducted at 4-h intervals, and ultrasonographical examination to detect ovulation was conducted every 8 h for 88 h after removal of the CIDR-B device. Treatment with estradiol benzoate tended to reduce the time from device removal to the LH peak in Period 1 (median time to LH peak 40.1 vs 63.9 h; P = 6.07). In Period 2, treatment with estradiol had no significant effect on the time to the LH peak, standing estrus or ovulation. We hypothesize that the period effect was due to the stage of cycle at the time of treatment. For heifers treated in Period 1, the stage of cycle was random. However, because of the prior synchronization of estrus, which was implicit in the experimental design, heifers in Period 2 tended to be in late diestrus. The administration of estradiol benzoate after treatment with exogenous progesterone appears to overcome the variability in timing of LH peaks typically occurring in a herd of synchronized heifers due to different stages of follicular development.

Milking Frequency, Estradiol Cypionate, and Somatotropin Influence Lactation and Reproduction in Dairy Cows

Our objectives were to determine lactational and reproductive outcomes in response to increased milking frequency (MF), injection of estradiol cypionate (ECP), and treatment with bovine somatotropin (bST). Lactating dairy cows (n = 144) were blocked by lactation number (1 vs. 2+) and assigned randomly to a 2 x 2 x 2 factorial experiment consisting of 8 treatment combinations: 1) MF consisting of 4x daily milking (4x) for the first 30 d in milk (DIM) vs. 2x daily milking (2x), with all cows milked 2x after 30 DIM; 2) 10 mg of ECP given postpartum at 8 +/- 3 DIM versus controls that received ECP diluent (oil); and 3) biweekly bovine somatotropin (bST), starting sometime after 60 DIM, versus no bST. Ovulation before the first artificial insemination was synchronized by using Heatsynch (GnRH injection 7 d before PGF2alpha followed in 24 h by ECP), and cows were artificially inseminated after detected estrus or at 48 h after ECP, whichever came first. Pregnancy was assessed by transrectal ultrasonography 28 to 30 d after artificial insemination. Daily yield and weekly components of milk were measured during the first 90 DIM. Intervals to first and second postpartum ovulation were unaffected by treatment, but cows were in estrus earlier after 2x (24 +/- 4 d) than 4x (41 +/- 4 d) daily MF, and sooner after ECP (25 +/- 3 d) than after oil (39 +/- 4 d) treatment. Pregnancy rates among 4x cows increased for ECP versus oil (52.8 vs. 27.8%) more than for cows with 2x MF treated with ECP versus oil (50.0 vs. 39.4%). Increased MF increased daily milk yields and energy-corrected milk yields during the first 30 DIM. Although milk yields were increased acutely by ECP during the 10 d after its injection, subsequent milk yields were decreased for ECP-treated cows previously milked 4x daily. Treatment with bST increased overall daily milk yields most in cows previously milked 2x daily and treated with oil and those milked 4x daily and treated with ECP. We concluded that early postpartum ECP injection increased pregnancy rates, but generally had detrimental effects on milk yields after 30 DIM for ECP-treated cows previously milked 4x daily, unless those cows also were treated with bST.

The variability in the interval between estrus and ovulation in cattle and its determinants

Fertility of Holstein cows has been decreasing for years and, to a lesser extent, the fertility of heifers too but more recently. A hypothesis to explain this phenomenon may be that the chronology of events leading to ovulation is different for those animals bred nowadays when compared to what was reported previously; this would result in an inappropriate time of insemination. Therefore, two experiments were designed to investigate the relationships among estrus behavior, follicular growth, hormonal events and time of ovulation in Holstein cows and heifers. In the first experiment, the onset of estrus, follicular growth, patterns of estradiol-17beta, progesterone and LH, and the time of ovulation were studied in 12 cyclic Holstein heifers that had their estrus synchronized using the Crestar method; this was done twice, 3 weeks apart. The intervals between estrus and ovulation, estrus and the LH peak, and between the LH peak and ovulation were, respectively, 38.5 h +/-3.0, 9.1 +/- 2.0 and 29.4 h +/-1.5 (mean+/- S.E.M). The variation in the interval between estrus and the LH peak explained 80.6% of the variation in the interval between estrus and ovulation. The intervals between estrus and the LH peak, and estrus and ovulation were correlated with estradiol-17beta peak value (r=-0.423, P <0.04 and r=-0.467, P<0.02, respectively). Positive correlation coefficients for the number of follicle larger than 5 mm, and negative correlation coefficients for the size of the preovulatory follicle with the intervals between estrus and LH peak, LH peak and ovulation, and estrus and ovulation suggest an ovarian control of these intervals. In respect to its role to explain the variation in the interval between estrus and ovulation, the variation in the interval between estrus and the LH peak was evaluated further in a second set of experiments utilizing 12 pubertal Holstein heifers and 35 Holstein cows. The duration of the interval between the beginning of estrus and the LH peak was longer in heifers than in cows (4.15 h versus -1.0 h; P <0.002); the variation for this interval was higher in cows than in heifers (S.E.M.= 1.2 h versus 0.8 h; P=0.01). According to the results of these studies it can be proposed that estradiol and other product(s) of ovarian origin regulate not only the duration of intervals between the onset of estrus and the LH surge but also between the LH surge and ovulation. From the results obtained in the first experiment, it may be postulated that differences observed between cows and heifers for the duration of the interval between onset of estrus and the LH surge as well as for the variation of this interval would be observed also for the interval between the onset of estrus and ovulation. Therefore, on a practical point of view, the long interval between the onset of estrus and ovulation and the high variation of this interval, especially in cows, may be a source of low fertility and should be considered when analysing reproductive disorders.

Use of Estradiol Cypionate as a Substitute for GnRH in Protocols for Synchronizing Ovulation in Dairy Cattle

Our purpose was to determine whether estradiol cypionate (ECP) could be substituted for the second GnRH injection of the standard Ovsynch protocol (injection of GnRH given 7 d before and 48 h after PGF(2alpha), with timed AI [TAI] 12 to 20 h after the second GnRH injection). Lactating dairy cows ranging from 61 to 82 d in milk at TAI were studied in 14 replicates. Main effects were hormone (ECP vs. GnRH) to induce ovulation and exposure to progesterone (P4) or not during the week preceding PGF(2alpha)-induced luteolysis. Four treatments were: 1) 100 microg of GnRH at 48 h after PGF(2alpha) (Ovsynch; n = 27); 2) same as Ovsynch, plus a P4-releasing intravaginal insert (CIDR) placed for 7 d beginning at the first GnRH injection (Ovsynch + CIDR, n = 20); 3) same as Ovsynch, but substituting 1 mg of ECP for GnRH, and injecting ECP at 24 h after PGF(2alpha) (Heatsynch; n = 33); or 4) Heatsynch + CIDR (n = 26). The largest follicle was identified by ultrasonography 24 h after PGF(2alpha) and was monitored every 6 h until ovulation. Incidence of estrus was less after GnRH (54%) than after ECP (87%), but more GnRH-treated cows had LH surges detected (95 vs. 65%) and ovulated (100 vs. 86%). Duration of LH surges, but not peak concentrations, was less after GnRH than after ECP (6.1 +/- 0.7 vs. 12.2 +/- 0.9 h). Pre-treatment with P4 reduced the incidence of LH surges but had no effects on incidence of estrus or ovulation. Intervals to the LH surge and ovulation were less after GnRH than after ECP, but intervals between onset of the LH surge and ovulation did not differ (26 +/- 2 vs. 30 +/- 3 h). We concluded that substituting ECP for GnRH resulted in more cows in estrus and slightly fewer ovulating.

Effects of suckling frequency and breed on productive performance, follicular dynamics and postpartum interval in beef cows

One experiment was conducted to determine the influence of suckling frequency (restricted suckling (RS) versus ad libitum suckling (AS)) and breed (Brown Swiss versus Pirenaica) on productive performance, follicular dynamics and postpartum cyclicity resumption in beef cows. Twenty-four spring-calving cows (12 Brown Swiss and 12 Pirenaica) were used. After calving cows were assigned, within breed, to the two suckling frequencies (RS versus AS). During the postpartum period, cows were fed 100% of the daily energy estimated requirements. The study was conducted from calving to either day 90 postpartum or to detection of the second ovulation whenever it came later. The pattern of follicular development was monitored daily by transrectal ultrasonography, and blood samples were collected three times a week to analyse progesterone concentrations by RIA. Body condition score at calving was 2.5 (from 1 to 5). Productive parameters were not significantly affected by suckling frequency. Pirenaica cows gained more weight (P<0.001), while Brown Swiss cows had higher milk yield (P<0.001) and thus their calves grew faster (P<0.05). In the present moderate conditions of nutrition, either suckling frequency or breed did not affect follicular dynamics (on average there were 6.5 follicles in emergence, growth rate of dominant follicle (DF) was 1.3mm per day and maximum diameter of dominant follicle was 13.9mm). There were gradual increases in maximum diameter and growth rate of dominant follicle related to the proximity of the ovulatory waves, and they were defined by third order polynomial equations. Postpartum anoestrous interval (PPI) was markedly affected by the interaction between suckling frequency and breed (P<0.001): ad libitum suckling prolonged significantly the interval from calving to first ovulation in Brown Swiss cows (31.8 day versus 82.7 day, in RS and AS groups, respectively, P<0.05), but not in Pirenaica cows (43.5 day versus 32.6 day, in RS and AS groups, respectively, not statistically significant). It is concluded that in moderate pre- and postpartum nutritional conditions Brown Swiss cows were more susceptible to the suppressor effect of ad libitum suckling on postpartum anoestrous interval than Pirenaica cows, although this breed difference did not seem to be mediated by differences in energy balance or in ovarian follicular dynamics.

Recent advances in bovine reproductive endocrinology and physiology and their impact on drug delivery system design for the control of the estrous cycle in cattle

When methods of drug intervention are being developed to control estrous cycles, a thorough understanding of the endocrine and functional changes together with the reproductive behavior of the animals are essential. This review presents our current knowledge on reproductive endocrinology, physiology and behavior, and the methods of drug intervention to control estrous cycles. It also describes current efforts to develop advanced drug delivery systems that meet the animal scientist's demands to control the estrous cycle in cattle.

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.

Local versus systemic effects of exogenous estradiol-17 beta on ovarian follicular dynamics in heifers with progestogen implants

Two experiments were designed to determine if the suppressive effect of estradiol treatment on ovarian follicles in progestogen-implanted heifers is mediated directly at the ovary or systemically, at a higher level. The purpose of Experiment 1 was to determine a minimal effective dose of estradiol-17beta (E-17beta) that would induce follicle regression in progestogen-implanted heifers. Beef heifers were implanted with progestogen on Day 2 (Day 0=ovulation) and were assigned randomly to five groups: control (sesame seed oil, n=9); 0. 1 mg of E-17beta (n=8); 0.5 mg of E-17beta (n=8); 1 mg of E-17beta (n=8); or 5 mg of E-17beta (n=8) by intramuscular (im) injection on Day 3. Treatment with 5 and 1 mg of E-17beta resulted in smaller (P<0.05) day-to-day diameter profiles of the dominant follicle compared with controls, whereas 0.1 mg of E-17beta did not have an apparent effect on follicle growth. The effect of a dose of 0.5 mg was intermediate and tended (P<0.06) to result in a smaller diameter profile of the dominant follicle compared with control heifers. Experiment 2 was designed to utilize a subminimal dose of E-17beta (0.1 mg), locally, to determine whether estradiol treatment induces follicle regression through a direct action on the ovary. Beef heifers received a progestogen ear implant on Day 2 and were assigned randomly to five groups on Day 3: control (sesame seed oil, n=8); 5 mg of E-17beta im (n=8); 0.1 mg of E-17beta im (n=8); 0.1 mg of E-17beta given into the wall of the uterus, near the tip of the horn ipsilateral to the dominant follicle (intrauterine (iu), n=8); or 0.1 mg of E-17beta given into the stroma of the ovary, immediately adjacent to the dominant follicle (intraovarian (io), n=6). Local (iu and io) treatments were given via a transvaginal ultrasound-guided needle injection. Treatment with 5 mg of E-17beta im resulted in suppression of the dominant follicle of the first follicular wave and early emergence of the second follicular wave (P<0.05). Diameter profiles of the dominant follicle in heifers treated with 0.1 mg im or 0.1 mg iu differed from those of control heifers on Day 5, whereas diameter profiles of the dominant follicle in heifers treated with 0.1 mg io did not differ from the controls. Daily changes in diameter of the dominant follicle did not differ among the three groups treated with 0.1 mg of E-17beta (im, iu and io). Hourly changes in circulating concentrations of FSH and LH were not detected following estradiol treatment either before or after the results were combined for all estradiol-treated groups. Results are supportive of the hypothesis that the suppressive effect of estradiol in cattle is exerted indirectly through a systemic route rather than directly at the ovary. Although low plasma concentrations of FSH and LH were not detected, systemic treatments with high E-17beta dosages resulted in follicular suppression whereas local treatments with subminimal dosages, within the ovary bearing the dominant follicle, were without effect.

Synchronization of ovulation in beef cows (Bos indicus) using GnRH, PGF2alpha and estradiol benzoate

The objective of this study was to evaluate protocols for synchronizing ovulation in beef cattle. In Experiment 1, Nelore cows (Bos indicus) at random stages of the estrous cycle were assigned to 1 of the following treatments: Group GP controls (nonlactating, n=7) received GnRH agonist (Day 0) and PGF2alpha (Day 7); while Groups GPG (nonlactating, n=8) and GPG-L (lactating, n=9) cows were given GnRH (Day 0), PGF2alpha (Day 7) and GnRH again (Day 8, 30 h after PGF2alpha). A new follicular wave was observed 1.79+/-0.34 d after GnRH in 19/24 cows. After PGF2alpha, ovulation occurred in 19/24 cows (6/7 GP, 6/8 GPG, 7/9 GPG-L). Most cows (83.3%) exhibited a dominant follicle just before PGF2alpha, and 17/19 ovulatory follicles were from a new follicular wave. There was a more precise synchrony of ovulation (within 12 h) in cows that received a second dose of GnRH (GPG and GPG-L) than controls (GP, ovulation within 48 h; P<0.01). In Experiment 2, lactating Nelore cows with a visible corpus luteum (CL) by ultrasonography were allocated to 2 treatments: Group GPE (n=10) received GnRH agonist (Day 0), PGF2alpha (Day 7) and estradiol benzoate (EB; Day 8, 24 h after PGF2alpha); while Group EPE (n=11), received EB (Day 0), PGF2alpha (Day 9) and EB (Day 10, 24 h after PGF2alpha). Emergence of a new follicular wave was observed 1.6+/-0.31 d after GnRH (Group GPE). After EB injection (Day 8) ovulation was observed at 45.38+/-2.03 h in 7/10 cows within 12 h. In Group EPE the emergence of a new follicular wave was observed later (4.36+/-0.31 d) than in Group GEP (1.6+/-0.31 d; P<0.001). After the second EB injection (Day 10) ovulation was observed at 44.16+/-2.21 h within 12 (7/11 cows) or 18 h (8/11 cows). All 3 treatments were effective in synchronizing ovulation in beef cows. However, GPE and, particularly, EPE treatments offer a promising alternative to the GPG protocol in timed artificial insemination of beef cattle, due to the low cost of EB compared with GnRH agonists.

Lack of LH response to exogenous estradiol in heifers with ACTH-induced ovarian follicular cysts

The aim of the present study was to examine the LH response to exogenous estradiol in 4 heifers with ACTH-induced ovarian follicular cysts. During the control experiment, administration of estradiol 24 hr after PGF2alpha in luteal phase heifers resulted in a LH response in all 4 heifers. The LH response was obtained between 16-20 hr after estradiol administration. The peak LH concentration (Mean +/- SEM; 5.1 +/- 0.8 ng/ml) during the control study was significantly different (P<0.05) from the concentration after cyst formation. None of the 4 heifers responded to estradiol after ovarian cyst formation. This result suggests that heifers with ACTH-induced ovarian follicular cysts may have a defective hypothalamio-pituitary response to exogenous estradiol similar to cows with spontaneous ovarian cysts.

The effect of bovine pestivirus infection on the superovulatory response of Friesian heifers

The pathogenesis of reproductive loss associated with bovine pestivirus infection during the preovulatory period was investigated using superovulated heifers. Twenty-five Friesian heifers were selected and randomly assigned to either a control group (n = 12) which did not become infected or to a treatment group (n = 13) which became infected following intranasal instillation of 2 ml of serum inoculum containing 5.5 log(10) TCID(50)/ml non-cytopathic virus, 9 d prior to artificial insemination (AI). Transrectal ultrasonography was used to monitor follicular development and ovulation during the superovulatory period. Animals were superovulated using a standard protocol of twice-daily injections of FSH-P and then were inseminated twice commencing 12 h after the onset of estrus. The intensity of expression of estrus was higher in the control heifers than in the pestivirus-infected heifers. Of 13 pestivirus-infected heifers, only 3 heifers displayed standing estrus compared with that in the control group, in which 10 of 12 heifers exhibited standing estrus. The mean number of ova/embryos recovered from the control group heifers was 5.75 +/-2.31, of which 4.00 +/- 0.72 were evaluated as transferable quality embryos. In comparison, heifers in the pestivirus-infected group yielded only a mean of 0.60 +/-0.34 ova/embryos, of which 0.23 +/- 0.22 were transferable quality embryos. Based on ultrasonographic examination, 24 h after the first AI 82% of the presumptive ovulatory follicles had ovulated in the control group compared with an ovulation rate of only 17% in the treated group. The results of this experiment demonstrated that bovine pestivirus infection during the preovulatory period could adversely affect ovulation, thus leading to a significant reduction in the number of palpable corpora lutea and in the number and quality of embryos recovered.

The effect of estradiol benzoate administration on estrous response and synchronized pregnancy rate in dairy heifers after treatment with exogenous progesterone

The objectives of this trial were to determine the effects of 0.5 mg estradiol benzoate administered intramuscularly 24 h after the removal of progesterone-containing intravaginal devices on the occurrence and timing of estrus, synchronized pregnancy rate and synchronized conception rate in dairy heifers. A clinical trial was conducted involving 750 dairy heifers in 13 herds. Within each herd heifers were randomly allocated to 1 of 2 estrus synchronization treatments. All heifers received a CIDR-B progesterone-containing intravaginal device containing a 10 mg estradiol benzoate capsule for 12 d. Twenty-four hours after CIDR-B removal one group received 0.5 mg, im estradiol benzoate while the other group received an intramuscular injection of a placebo. Estrus detection was performed at 48 and 72 h after intravaginal device removal, and heifers detected in estrus at those times were inseminated. Administration of estradiol benzoate 24 h after removal of CIDR-B devices significantly increased the number of heifers exhibiting estrus within the observation period (96.1 vs 90.5%; P < 0.01). It also altered the onset of estrus so that significantly more heifers were in estrus (86.6 vs 72.3%; P < 0.01) and conceived (47.1 vs 37.5%; P < 0.05) by 48 h after CIDR-B device removal. The synchronized conception rate was unaffected by treatment. The distribution of estrus was such that fixed-time insemination after estrus synchronization with this treatment program may be recommended.

Effect of induced suprabasal progesterone levels around estrus on plasma concentrations of progesterone, estradiol-17β and LH in heifers

A controlled study was carried out to investigate the effects of suprabasal plasma progesterone concentrations on blood plasma patterns of progesterone, LH and estradiol-17beta around estrus. Heifers were assigned to receive subcutaneous silicone implants containing 2.5 g (n=4), 5 g (n=4), 6 g (n=3), 7.5 g (n=3) or 10 g (n=4) of progesterone, or implants without hormone (controls, n=5). The implants were inserted on Day 8 of the cycle (Day 0=ovulation) and left in place for 17 d. The time of ovulation was determined by ultrasound scanning. Blood was collected daily from Days 0 to 14 and at 2 to 4-h intervals from Days 15 to 27. Control heifers had the lowest progesterone concentrations on Days 20.5 to 21 (0.5 +/- 0.1 nmol L(-1)); a similar pattern was observed in heifers treated with 2.5 and 5 g of progesterone. In the same period, mean progesterone concentrations in the heifers treated with 6, 7.5 and 10 g were larger (P < 0.05) than in the controls, remaining between 1 and 2.4 nmol L(-1) until implant removal. A preovulatory estradiol increase started on Days 16.4 to 18.4 in all the animals. In the controls and in heifers treated with 2.5 and 5 g of progesterone, estradiol peaked and was followed by the onset of an LH surge. In the remaining treatments, estradiol release was prolonged and increased (P < 0.05), while the LH peak was delayed (P < 0.05) until the end of the increase in estradiol concentration. The estrous cycle was consequently extended (P < 0.05). In all heifers, onset of the LH surge occurred when progesterone reached 0.4 to 1.2 nmol L(-1). The induction of suprabasal levels of progesterone after spontaneous luteolysis caused endocrine asynchronies similar to those observed in cases of repeat breeding. It is suggested that suprabasal concentrations of progesterone around estrus may be a cause of disturbances oestrus/ovulation.

Effect of estradiol valerate on ovarian follicles, emergence of follicular waves and circulating gonadotropins in heifers

An experiment was designed to examine the effect of estradiol valerate (EV) on the growth and regression of follicles of a wave and on the emergence of the next follicular wave. Twenty-six beef heifers were xamined daily by ultrasonography and randomly allocated to 1 of 4 treatment groups at the time of ovulation (Day 0): unterated control heifers and those that received 5 mg EV intramuscularly on Day 1, Day 3 or Day 6. Maximum diameter of the dominant follicle was greater (P<0.05) in control heifers than in heifers treated on Day 1 or Day 3. Mean day of onset of regression of the dominant follicle was later (P<0.05) in control heifers than in heifers treated on Day 1 but was not different from heifers treated on Day 3. In heifers treated on Day 6, cessation of growth, maximum diameter and onset of regression were not different from that of control heifers. The emergence of the next follicular wave was earlier (P<0.05) in heifers treated on Day 1 than in control heifers, whereas wave emergence was delayed (P<0.05) in heifers treated on Day 3 or Day 6. The mean day of maximum concentration of FSH prior to the emergence of the next wave was earlier in heifers treated with EV on Day 1 and later in heifers treated on Day 3 or Day 6 compared with that of the controls (P<0.05). Treatment on Day 1 or Day 3 resulted in a significant LH surge in 8 13 heifers, whereas no LH surges were detected in control heifers or in heifers treated on Day 6. The hypothesis that EV suppresses the growth of the dominant follicle, was supported. Estradiol valerate treatment resulted in early emergence of the next follicular wave in heifers treated on Day 1, but treatment on Day 3 or Day 6 resulted in delayed emergence of the next follicular wave.

Effects of transcutaneous thermal and electrical stimulation of the teat on pituitary luteinizing hormone, prolactin and oxytocin secretion in ovariectomized, estradiol-treated beef cows following acute weaning

The objective of the study was to determine if chronic electrical or thermal stimulation of sensory neurons on the surface of the teat is able to activate pathways that suppress the weaning-induced increase in luteinizing hormone (LH) secretion in beef cows. Treatment groups (n = 5 per group) consisted of: 1) control suckled (CS); 2) weaned plus electrical stimulation of the teat (ESTT); 3) weaned plus electrical stimulation of the tail (ESTL); 4) weaned plus thermal stimulation of the teat (TTT); 5) weaned plus thermal stimulation of the tail (TTL) and 6) weaned (WN). Cows were ovariectomized on Day 5 post partum (PP) and were treated with estradiol-17beta to maintain a constant tonic baseline. Beginning on Days 17 to 21 post partum, cows were suckled by their own calf (control), weaned or weaned and electrically or thermally stimulated for 10 minutes every 6 hours for 4 days. Chronic transcutaneous electrical and thermal stimulation of the teat or tail failed to impede the unambiguous rise (P < 0.001) in LH pulse frequency and amplitude following weaning. Positive and negative feedback of estradiol on LH secretion was not affected by treatments. Relatively consistent episodes of oxytocin and prolactin release were observed following control-suckling, but responses to electrical and thermal stimulation were inconsistent. Chronic electrical or thermal stimulation of teat-specific or nonspecific loci did not attenuate heightened secretion of LH after weaning. The results are further evidence against a role for mammary somatosensory neurons in the suckling-mediated inhibition of LH secretion.

Lack of LH response to oestradiol treatment in cows with cystic ovarian disease and effect of progesterone treatment or manual rupture

The luteinising hormone (LH) surge in response to 1 mg oestradiol benzoate intramuscular injection was studied on 67 occasions in 45 cows with cystic ovarian disease 20 to 150 days post partum. Cows diagnosed as having luteal cysts were given 500 micrograms cloprostenol intramuscularly 24 hours before oestradiol, to induce luteolysis. Oestradiol benzoate was also given to eight post partum acyclic and eight cyclic cows and in all these cases a control LH response was characterised for comparison. Eight of 17 cows with luteal cysts (47 per cent), and 10 of 21 cows with follicular cysts (48 per cent), released LH in response to oestradiol. Some cows with cysts were given one of two treatments. Seven cows with follicular cysts were treated with a progesterone-releasing device (PRID) for seven days: all responded to a second oestradiol treatment given 24 hours after removal of the PRID. Luteal cysts in three cows and follicular cysts in nine cows were ruptured manually: only one cow (a luteal case) responded to the second oestradiol treatment given 24 hours after manual rupture. In eight cows initially diagnosed with luteal cysts, cloprostenol was not given and plasma progesterone concentration at the time of oestradiol treatment was high (over 0.9 ng ml-1): none released LH in response to oestradiol. As manual rupture did not improve the LH response to oestradiol, it is concluded that the defective LH response to oestradiol in cows with cystic ovarian disease was not influenced in the short-term by cyst fluid contents.(ABSTRACT TRUNCATED AT 250 WORDS)

Site of action for endotoxin-induced cortisol release in the suppression of preovulatory luteinizing hormone surges

The study was conducted to identify the mechanisms of endotoxin/cortisol action in the suppression of preovulatory LH surges in heifers infused with Escherichia coli (E. coli ) endotoxin. The hypotheses tested were that 1) endotoxin stimulates the release of progesterone, possibly from the adrenal leading to the LH blockade; 2) cortisol released in response to endotoxin infusion blocks the synthesis of estradiol at the ovarian level, culminating in a failure of the LH surge. Eight Holstein heifers were given two injections of prostaglandin F(2alpha) (PG), 11 d apart, to synchronize estrus. Starting from 25 h after the second injection of PG (PG-2), the uterus of each heifer was infused either with 5 ml of pyrogen-free water (control, n = 3) or with E. coli endotoxin (5 mug/kg of body weight) in 5 ml of pyrogen-free water (treated, n = 5), once every 6 h for 10 treatments. Blood samples were obtained every 15 min for 1 h before infusion and again 2 h after each infusion, then hourly until 1 h before the next infusion. After the tenth infusion, blood was collected daily until estrus. Serum progesterone concentrations remained at baseline values (< 1 ng/ml) in control and treated heifers. The total amount of progesterone measured starting 24 to 84 h after PG-2 injection was not different between control and treated heifers (P 0.05). In the control heifers, serum estradiol concentrations remained basal (< 10 pg/ml) until 4 h before the LH surge. Serum estradiol concentrations increased to 20 +/- 5.6 pg/ml, 4 h before the LH surge in control heifers (LH surge occurred 60 to 66 h after the PG-2 injection). There were no changes in serum estradiol concentrations in treated heifers during the sampling period, and the concentrations remained < 10 pg/ml. The total amount of estradiol measured in control heifers was higher (P < 0.05) than in treated heifers. The results if this study suggest that increases in cortisol concentrations after the infusion of endotoxin might block the synthesis of estradiol at the ovarian level, resulting in the failure of a preovulatory LH surge to occur.

Photoperiodic alteration of postpartum reproductive function in suckled cows

Three experiments were conducted using a total of 41 cows to determine if photoperiod modulates the establishment of postpartum estrous cycles and conception. Cows calving in the autumn and winter were exposed to either 18 hr light/day (18L:6D) or natural photoperiods. In Exp. 1, cows receiving 18L:6D had shorter (P<0.025) intervals from calving to estrus (61 +/- 3.8 days) than cows not receiving supplemental light (154 +/- 23.9 days). The same was true for primiparous cows in Exp. 2 (76 +/- 5.5 days vs 153 +/- 38.0 days; P<0.06) but not for the multiparous cows in Exp. 2 (56 +/- 5.2 days vs 40 +/- 7.4 days) or for all cows in Exp. 3 (60 +/- 10.1 days vs 70 +/- 13.5 days). Because conception rate was higher for cows exposed to 18L:6D for the multiparous cows in Exp. 2 and all cows in Exp. 3, interval to conception was significantly shorter for animals exposed to 18L:6D in all experiments. Generally, interval from calving to uterine involution was also reduced by exposure to 18L:6D. No effects of photoperiod were observed on body weight changes, serum levels of luteinizing hormone (LH), follicle stimulating hormone (FSH) or estradiol or on estradiol-induced release of gonadotropins. It was concluded that in certain situations day length can hasten estrus and conception in postpartum cows. The effect of photoperiod interacts with other conditions, one of which is parity. The endocrine basis for these effects are not known.

The effect of method of GnRH administration and short-term calf removal on ovarian function and reproductive performance in postpartum suckled beef cows administered PGF2α for estrous synchronization

The first experiment was a 2 x 2 factorial experiment with calf removal (none or short-term) and method of GnRH administration (intramuscularly in saline or subcutaneously in gelatin capsules) as main effects. The durations of the GnRH-induced LH surges were similar among groups but the LH surges were delayed in the cows that received GnRH subcutaneously in gelatin capsules. Calf removal enhanced the GnRH-induced LH release for cows administered GnRH subcutaneously in a gelatin capsule but not for cows administered GnRH intramuscularly in saline. In the second experiment, 191 postpartum suckled beef cows were administered two injections of prostaglandin F(2)alpha(PGF(2)alpha) 11 days apart. After the second PGF(2)alpha injection, the cows were assigned to a 2 x 2 factorial experiment as in Experiment 1 plus one control group. Short-term calf removal (47 h) began 28 h after the second PGF(2)alpha injection. GnRH was administered 30 h after the time of calf removal. The number of cows that ovulated following the time of the GnRH treatment, the number that had abnormal luteal phases and the first-service pregnancy rates among treatment groups within the anestrous and cyclic cows classifications were not significantly different. However, several effects were detected and are reported.

Endocrine profiles and embryo quality in the PMSG-PGF2α treated cow

Plasma progesterone and LH concentrations around estrus were determined for both PMSG treated (experimental animals) and non-treated (control animals) dairy cows and heifers of the Holstein Friesian and Jersey breeds, and these hormone profiles were related to the embryo quality. Most experimental animals experienced an increase in progesterone concentrations following PMSG treatment and an abrupt decrease to values below 3 nmol/l after PG injection. The mean (+/-SE) intervals from prostaglandin treatment to estrus were 46.9+/-1.8 h and 64.5+/-4.8 h for experimental and control animals, respectively. At the onset of heat the progesterone concentration in experimental animals with optimal embryo quality (group I) was significantly lower (p<0.01) than in experimental animals which yielded unfertilized eggs (group II) (1.2+/-0.1 versus 3.9+/-0.8 nmol/l) and significantly higher than the level in the control group (0.6+/-0.1 nmol/l). Following estrus the progesterone profiles in all 3 groups were studied and the length of the superovulatory cycle was measured to 26.0+/-4.8 days. The preovulatory LH surge occurred sooner after prostaglandin injection in experimental (41 h) than in control animals (65 h). The LH surge in group I occurred within a narrow range and reached a higher average level than group II (24.2+/-2.2 ng/ml and 16.3+/-3.7 ng/ml, respectively). The control group attained an even higher LH surge (31.8+/-8.8 ng/ml) than did the experimental animals. The data presented in this experiment indicate that plasma levels of progesterone and LH in PMSG-PGF2alpha treated animals are related to embryo or egg quality.

Effects of energy level and monensin on reproductive performance and lactation of beef cows

This experiment consisted of a 2 year drylot study involving 80 multiparous, suckled beef cows (40 Angus and 40 Herefords). Experimental treatments (breed, monensin and year) were arranged in 2(3) factorial to evaluate the effect of breed and monensin on reproductive performance and lactation. Cows received 85% of the NRC total digestible nutrient (TDN) requirement for the first 56 days of the 140 day trial. Cows were synchronized with Synchro-Mate B and artificially inseminated 30 days into each trial with blood samples collected for luteinizing hormone (LH) analysis from one-half of each treatment group from 24 to 36 hours after implant removal. Progesterone determinations were made on plasma samples collected at day 9 and 16 after implant removal. On day 56, milk yield estimates were obtained by the weigh-suckle-weight technique. Following these collections, energy levels were increased by allowing ad lib consumption of forage. Calves, which were removed from the cows during feeding, were given access to a 75% TDN creep ration after day 56. Milk estimates were again evaluated at 140 days. Monensin supplementation did not result in a difference in cow weight change through the restricted energy period (first 56 days) or throughout the entire 140 day period. Milk yield estimates at 56 and 140 days and calf gains throught the trial, were unaffected by monensin supplementation. Monensin resulted in no effect on conception rate or services per conception. The time of the LH peak was shifted slightly forward by monensin although not significantly. Progesterone levels were not consistently affected by monensin supplementation.

GnRH induced LH release in suckled beef cows. II. The effects of exogenous corticoids and estradiol benzoate on luteinizing hormone release by GnRH

In Experiment 1, 24 suckled beef cows were assigned to 4 treatment groups (6 cows/group). Group I cows calved spontaneously. Parturition was induced in Groups 2, 3 and 4 with 20 mg dexamethasone (DEX) 8 to 12 days prior to expected calving date. Additionally, cows in Groups 3 and 4 received 8 mg triamcinalone acetonide (TA) 6 days prior to DEX treatment. Animals in Group 4 also received 10 mg estradiol benzoate (EB) with TA, and on alternate days until DEX, when 20 mg EB was given. Gonadotropin releasing hormone (GnRH, 100 mug) was given intramuscular (IM) to all cows on days 2 or 3 postpartum. Plasma LH increased (P< .05) following GnRH treatment in Groups 2, 3 and 4, but not in Group 1. LH release (area under the curve) following GnRH was greater (P< .05) for cows in Group 4 compared to cows in Groups 1, 2 or 3, and differences in LH release between Groups 1, 2 or 3 were not significant. In Experiment II, 36 mature Hereford cows were assigned to a 2 x 3 factorial experiment (6 cows/group). Groups 1 and 2, 3 and 5, and 4 and 6 received 0, 100, or 200 mug GnRH (IM) at 78 hr postpartum, respectively. In addition, cows in Groups 2, 5 and 6 received 5 mg EB at 36 hr postpartum. Plasma LH concentrations were not different (P <.05) among groups from 36 to 78 hr postpartum. A surge of LH in response to EB treatment was not detected at 54 to 62 hr (18 to 26 hr post EB), indicating a lack of response by the positive feedback mechanism at this early time postpartum. Mean plasma LH concentrations were elevated 78 to 82 hr postpartum for Groups 3 through 6. Treatment with EB at 36 hr caused a significantly greater (P< .05) response to GnRH with 200 mug of GnRH releasing more LH than 100 mug of GnRH.

Serum luteinizing hormone levels in Brangus cows following variable suckling intensity and administration of various levels of estrogen

Fifty Brangus cows were randomly allotted to suckled (S) or nonsuckled (NS) treatment groups on day 20 postpartum. Suckled cows were nursed at 6 hr intervals for 72 hours. Nonsuckled cows were separated from their calves for the entire 72 hours. At 24 hr after initial separation from calves, S and NS cows were given an I.M. challenge of 0, 0.5, 1.0, 2.0 or 4.0 mg estradiol-17beta (E2) to induce a luteinizing hormone (LH) surge (five cows per treatment group). Blood samples were taken at the time of E2 injection and at 2 hr intervals until hr 48 post-injection. Blood serum was analyzed for LH content via radioimmunoassay. Suckled and NS cows manifesting an LH surge after receiving less than 4 mg E2 were 2 of 15 vs 9 of 15 (P<.01), or 4 mg E2 dose were 5 of 5 vs 5 of 5, respectively. Greater serum LH concentrations in NS than S cows were found with dose levels of 0, 0.5 and 1.0 mg E2 (P<.005), but there was no difference by period. Differences by treatment (P<.05) and by period (P<.005) were found at the 2 mg E2 dose. Suckled and NS cows having an LH surge at less than a 4 mg E2 challenge had no differences in LH concentration or timing parameters. Four mg E2 hastened the time of onset of the LH surge (P<.025), time till peak height of the surge (P<.025) and completion of the surge (P<.10). No differences in postpartum interval or conception rate were found between S and nonsuckled. Suckling impairs hypothalamic/pituitary response to low E2 challenge dose and elicits changes in timing parameters in response to high E2 dosage.

Luteinizing hormone response to estradiol benzoate in cows postpartum and cows with ovarian cysts

Twenty-seven dairy cows were evenly assigned to one of three groups and given an intramuscular injection of 2 mg estradiol benzoate. Cows in group 1 were greater than 30 days postpartum at treatment and had been diagnosed via rectal palpation to have ovarian cysts. Cows in groups 2 and 3 were 12 to 14 and 30 to 40 days postpartum, respectively. Blood plasma was collected from all cows before treatment and then every three hours for 36 hours post-treatment. Concentrations of LH, estradiol-17 beta and progesterone in plasma were determined by radioimmunoassay. Four, zero and five cows in groups 1, 2 and 3, respectively, had concentrations of progesterone greater than 1.0 ng/ml before estradiol benzoate treatment. None of these cows had a peak LH release greater than 5 ng/ml following estradiol benzoate treatment. The numbers of cows with progesterone concentrations less than 1 ng/ml that released LH (>5 ng/ml) in response to estradiol benzoate were 3 of 5, 3 of 9, and 4 of 4 for groups 1, 2, and 3, respectively; the proportion for group 3 was higher (P<.05) than for group 2. Of the cows that released LH, mean peak LH concentrations were 33.3+/-5.4, 14.8+/-7.2 and 24.6+/-9.8 ng/ml for groups 1, 2 and 3, respectively, and the duration of the LH increase was 8.0+/-1.0, 8.0+/-2.0 and 13.0+/-4.0 hours. The time from estradiol benzoate treatment to peak LH release for cows with ovarian cysts (25+/-2 hours) was delayed (P<.05) compared with that for cows 30 to 40 days postpartum without ovarian cysts (16+/-1 hour). In summary, responsiveness to estradiol benzoate is regained between 2 to 4 weeks postpartum in most cows. In addition, some cows with ovarian cysts can release LH in response to estradiol benzoate, but peak LH release is delayed compared to cows at a comparable stage postpartum without ovarian cysts.

Effect of variable suckling intensity and estrogen administration upon serum luteinizing hormone in Brahman cows

Ten mature Brahman cows were randomly allotted within calving intervals to either a suckled (S) or nonsuckled (NS) treatment group. All cows received a 20 mg intramuscular injection of estradiol-17beta (E2), suspended in 2 ml of corn oil, to determine the effect of suckling on the estrogen induced LH surge. Starting on day 21 postpartum the S cows were suckled at six hour intervals for 24 hours, at which time they were challenged with a 20 mg E2 injection. The suckling regimen was continued for 48 hours postinjection. The NS cows were separated from their calves on day 21 postpartum and received no suckling stimulus for 72 hours. At 24 hours after calf separation, the NS cows were challenged with a 20 mg E2 injection. Blood samples were removed at two hour intervals beginning 10 hours post E2 injection until 36 hours postinjection, at which time blood samples were removed at four hour intervals until 48 hours postinjection. Blood samples were processed to yield serum and assayed for luteinizing hormone (LH) via radioimmunoassay. The injection of a 20 mg dose of E2 induced an LH surge in all cows. The NS cows were found to exhibit a longer (P<.05) duration of the estrogen induced LH surge than the S cows, 15.6 +/- .98 and 12.4 +/- .75 hours, respectively. The timing parameters (time to start of LH surge, time to peak LH value and time to end of surge) and LH concentration parameters (LH concentration at start of LH surge, peak value of LH surge and LH concentration at end of LH surge) were not different between suckling regimens. No blockage of the LH response to estrogen challenge was found on day 22 postpartum. Suckling did depress the duration of the LH surge indicating some blockage due to suckling stimuli.

Effect of days postpartum and exogenous GnRH on reproductive hormone and ovarian changes in postpartum suckled beef cows

Two experiments were conducted to determine the effect of days postpartum and exogenous gonadotropin releasing hormone (GnRH) on reproductive hormone and ovarian changes in postpartum suckled beef cows. In experiment 1, eight suckled cows were bled at .5 hour intervals for 4 hours on days 7, 14, 21 and 28 postpartum. Although mean concentrations of plasma luteinizing hormone (LH) were positively correlated with days postpartum, mean concentrations did not differ. The mean maximum change and the variance of plasma LH were low on days 7, 14, 21 and 28 postpartum. Although the number of cows with an ovarian follicle and follicular size increased with days postpartum, mean concentrations of estradiol-17beta did not change. The interval from parturition to the first detected ovarian follicle and the first postpartum estrus was 17.5 +/- 2.6 days and 36.0 +/- 2.2 days, respectively. An elevation in plasma progesterone was detected about one week prior to the first postpartum estrus in 6 of the eight cows in the absence of corpora lutea. In experiment 2, gonadotropin releasing hormone (GnRH) induced ovulation in 4 of the 8 cows treated on day 27, 28 or 29 postpartum whereas none of the 8 saline treated cows ovulated to treatment. The interval from parturition to first estrus and conception were similar for both groups (P >.10).