Effects of a gonadotropin-releasing hormone antagonist and altrenogest on luteinizing hormone concentration and ovulation in the mare

The objective of Experiment 1 was to determine a dose and frequency of gonadotropin-releasing hormone (GnRH) antagonist administration to effectively suppress serum luteinizing hormone (LH) concentration and to delay ovulation when administered to mares. The objectives of Experiment 2 were 1) to determine the effects of subcutaneous or intravenous administration of a GnRH antagonist or oral altrenogest on serum LH concentration in the estrual mare; and 2) to determine the effectiveness of human chorionic gonadotropin (hCG) in inducing ovulation in mares with suppressed LH concentrations. In Experiment 1, mares (N = 20) were randomly assigned and treated with either 5% mannitol (control, single subcutaneous injection, 1 mL, at time 0; n = 5); low-dose GnRH antagonist (single subcutaneous injection, 0.01 mg/kg, at time 0; n = 5); frequent low-dose GnRH antagonist (subcutaneous injections, 0.01 mg/kg, at 0, 6, 18, and 24 hours; n = 5); or high-dose GnRH antagonist (single subcutaneous injection, 0.04 mg/kg, at time 0; n = 5). Both the frequent low-dose and high-dose GnRH antagonist treatments resulted in significantly lower LH concentrations compared with controls at 90, 102, and 114 hours after treatment (P < .05). In Experiment 2, mares (N = 38) were randomly assigned and treated with subcutaneous sterile saline (control), altrenogest (oral), subcutaneous GnRH antagonist, or intravenous GnRH antagonist. LH concentration for the altrenogest group was lower than the control group at 3, 4, 18, and 30 hours after treatment (P < .05). LH concentration for both the subcutaneous and intravenous GnRH antagonist groups were lower compared with the control group at several time points (P < .05). Based on these data, dose but not frequency of administration of a GnRH antagonist lowered LH concentration in the estrous mare but did not delay ovulation. In addition, serum LH concentrations can be lowered and ovulation effectively postponed in mares treated with altrenogest followed by administration of hCG. This indicates that serum LH concentrations can be lowered and ovulation effectively postponed in mares treated with altrenogest followed by administration of hCG.     

Insights into the mechanism by which kisspeptin stimulates a preovulatory LH surge and ovulation in seasonally acyclic ewes: Potential role of estradiol

We have previously demonstrated that a constant intravenous infusion of kisspeptin (Kp) for 48 h in anestrous ewes induces a preovulatory luteinizing hormone (LH) surge followed by ovulation in approximately 75% of animals. The mechanisms underlying this effect are unknown. In this study, we investigated whether Kp-induced preovulatory LH surges in anestrous ewes were the result of the general activation of the whole gonadotropic axis or of the direct activation of central GnRH neurons required for the GnRH/LH surge. In the first experiment, a constant iv infusion of ovine kisspeptin 10 (Kp; 15.2 nmol/h) was given to 11 seasonally acyclic ewes over 43 h. Blood samples were taken every 10 min for 15 h, starting 5 h before the infusion, and then hourly until the end of the infusion. We found that the infusion of Kp induced a well-synchronized LH surge (around 22 h after the start of the Kp infusion) in 82% of the animals. In all ewes with an LH surge, there was an immediate but transient increase in the plasma concentrations of LH, follicle-stimulating hormone (FSH), and growth hormone (GH) at the start of the Kp infusion. Mean (± SEM) concentrations for the 5-h periods preceding and following the start of the Kp infusion were, respectively, 0.33 ± 0.09 vs 2.83 ± 0.49 ng/mL (P = 0.004) for LH, 0.43 ± 0.05 vs 0.55 ± 0.03 ng/mL (P = 0.015) for FSH, and 9.34 ± 1.01 vs 11.51 ± 0.92 ng/mL (P = 0.004) for GH. In the first experiment, surges of LH were observed only in ewes that also had a sustained rise in plasma concentrations of estradiol (E₂) in response to Kp. Therefore, a second experiment was undertaken to determine the minimum duration of Kp infusion necessary to induce such a pronounced and prolonged increase in plasma E₂ concentration. Kisspeptin (15.2 nmol/h) was infused for 6, 12, or 24 h in seasonally acyclic ewes (N = 8), and blood samples were collected hourly for 28 h (beginning 5 h before the start of infusion), then every 2 h for the following 22 h. Kisspeptin infused for 24 h induced LH surges in 75% of animals, and this percentage decreased with the duration of the infusion (12 h = 50%; 6 h = 12.5%). The plasma concentration of E₂ was greater in ewes with an LH surge compared to those without LH surges; mean (± SEM) concentrations for the 5-h period following the Kp infusion were, respectively, 2.23 ± 0.16 vs 1.27 ± 0.13 pg/mL (P < 0.001). Collectively, our results strongly suggest that the systemic delivery of Kp induced LH surges by activating E₂-positive feedback on gonadotropin secretion in acyclic ewes.     

Luteolysis and associated interrelationships among circulating PGF2α, progesterone, LH, and estradiol in mares

The changing concentrations and temporal relationships among a PGF2α metabolite (PGFM), progesterone (P₄), LH, and estradiol-17β (E₂) before, during, and after luteolysis were studied in 10 mares. Blood samples were collected every hour for ≥4 d beginning on day 12 after ovulation. The luteolytic period extended from a decrease in P₄ at a common transitional hour (Hour 0) at the end of preluteolysis and beginning of luteolysis to a defined ending when P₄ reached 1 ng/mL. The length of luteolysis was 22.9 ± 0.9 h, contrasting with 2 d in published P₄ profiles from sampling every 6 to 24 h. In mares with complete data for Hours −40 to −2 (n = 6), PGFM concentrations remained below assay sensitivity (n = 2) or two or three small pulses (peak, 29 ± 4 pg/mL) occurred. During luteolysis, the pulses became more prominent (peak, 193 ± 36 pg/mL). Rhythmicity of PGFM pulses was not detected by a pulsatility program during preluteolysis but was detected in seven of nine mares during luteolysis and postluteolysis combined. The nadir-to-nadir interval for LH pulses and the peak-to-peak interval between adjacent pulses were longer (P < 0.05) during preluteolysis than during luteolysis (nadir to nadir, 5.2 ± 0.3 h vs 3.6 ± 0.4 h; peak to peak, 9.4 ± 1.0 h vs 4.7 ± 0.5 h). Unlike reported findings in cattle, concentrations of P₄ decreased linearly within the hours of each PGFM pulse during luteolysis, and a positive effect of an LH pulse on P₄ and E₂ concentration was not detected. The reported balancing of P₄ concentrations between a negative effect of PGF2α and a positive effect of LH in heifers was not detected in mares.     

Subclinical, chronic intramammary infection lowers steroid concentrations and gene expression in bovine preovulatory follicles

Various doses of estradiol-17β (E₂) were used in heifers to induce a pulse of 13,14-dihydro-15-keto-prostaglandin F_2α (PGFM). The effect of E₂ concentration on the prominence of PGFM pulses and the relationship between prominence and intrapulse concentration of progesterone (P₄), LH, and luteal blood flow were studied. A single dose of 0 (vehicle), 0.01, 0.05, or 0.1 mg of E₂ was given (n = six/group) 14 d after ovulation. Blood samples were collected, and luteal blood flow was evaluated hourly for 10 h after the treatment. The 0.05-mg dose increased and the 0.1-mg dose further increased the prominence of the induced PGFM pulse, compared with the 0.0-mg dose and the 0.01-mg dose. The PGFM pulses were subdivided into three different prominence categories (<50, 50 to 150, and >150 pg/mL at the peak). In the 50 to 150 category, P₄ concentration increased (P < 0.05) between −2 h and 0 h (0 h = peak of PGFM pulse). In the >150 category, P₄ decreased (P < 0.05) between −1 h and 0 h, LH increased (P < 0.05) at 1 h, and luteal blood flow apparently decreased (P < 0.05) at 2 h of the PGFM pulse. The novel results supported the following hypotheses: (1) an increase in E₂ concentration increases the prominence of a PGFM pulse, and (2) greater prominence of a PGFM pulse is associated with a greater transient intrapulse depression of P₄ at the peak of the PGFM pulse. In addition, the extent of the effect of prostaglandin F_2α on the increase in LH and changes in blood flow within the hours of a PGFM pulse was related positively to the prominence of the PGFM pulse.     

Effects of plasma progesterone concentrations on LH release and ovulation in beef cattle given GnRH

The effects of plasma progesterone concentrations on LH release and ovulation in beef cattle given 100 microg of GnRH im were determined in three experiments. In Experiment 1, heifers were given GnRH 3, 6 or 9 days after ovulation; 8/9, 5/9 and 2/9 ovulated (P<0.02). Mean plasma concentrations of progesterone were lowest (P<0.01) and of LH were highest (P<0.03) in heifers treated 3 days after ovulation. In Experiment 2, heifers received no treatment (Control) or one or two previously used CIDR inserts (Low-P4 and High-P4 groups, respectively) on Day 4 (estrus=Day 0). On Day 5, the Low-P4 group received prostaglandin F(2alpha) (PGF) twice, 12 h apart and on Day 6, all heifers received GnRH. Compared to heifers in the Control and Low-P4 groups, heifers in the High-P4 group had higher (P<0.01) plasma progesterone concentrations on Day 6 (3.0+/-0.3, 3.0+/-0.3 and 5.7+/-0.4 ng/ml, respectively; mean+/-S.E.M.) and a lower (P<0.01) incidence of GnRH-induced ovulation (10/10, 9/10 and 3/10). In Experiment 3, 4-6 days after ovulation, 20 beef heifers and 20 suckled beef cows were given a once-used CIDR, the two largest follicles were ablated, and the cattle were allocated to receive either PGF (repeated 12h later) or no additional treatment (Low-P4 and High-P4, respectively). All cattle received GnRH 6-8 days after follicular ablation. There was no difference between heifers and cows for ovulatory response (77.7 and 78.9%, P<0.9) or the GnRH-induced LH surge (P<0.3). However, the Low-P4 group had a higher (P<0.01) ovulatory response (94.7% versus 61.1%) and a greater LH surge of longer duration (P<0.001). In conclusion, although high plasma progesterone concentrations reduced both GnRH-induced increases in plasma LH concentrations and ovulatory responses in beef cattle, the hypothesis that heifers were more sensitive than cows to the suppressive effects of progesterone was not supported.     

Hypothalamic deafferentation in prepuberal beef heifers: Effects of gonadotropin-releasing hormone and estradiol benzoate on luteinizing hormone secretion

Hypothalamic control of luteinizing hormone (LH) secretion was investigated in crossbred beef heifer calves by comparing anterior (AHD), posterior (PHD), and complete (CHD) hypothalamic deafferentation with sham operated controls (SOC). Heifers (n = 16) were fitted with an indwelling jugular catheter for 6 days before cranial surgery, and assigned randomly to treatments. Blood for radioimmunoassay of LH was collected sequentially at 15-min intervals during an 8-h period on days ? 1 before and day 6 after hypothalamic deafferentation or sham operation. On the day of surgery, blood samples were collected sequentially at 15-min intervals 2 h before induction of anesthesia and throughout surgery and early recovery. Seven months after hypothalamic deafferentation, all experimental and sham operated heifers were ovariectomized and treated with vegetable oil (i.m.) plus saline (i.v.), vegetable oil plus gonadotropin releasing hormone (GnRH), estradiol benzoate (EB, 1 mg) in vegetable oil. After ovariectomy basal plasma concentrations of LH increased (P < 0.01) compared with the low circulating hormone levels before ovariectomy. The amplitude of LH response to GnRH was greater (P < 0.01) in CHD and PHD when compared with SOC and AHD heifers. Injection of EB failed to induce a LH surge in CHD and PHD 900–1100 min later when compared with the robust response seen in SOC and AHD heifers. Injection of EB plus GnRH elicited LH release in all deafferentated and sham operated heifers. These results indicate a transient change in LH secretion after AHD or CHD in prepuberal heifers with intact ovaries. After OVX, the integrity of the neural connection of the posterior hypothalamus is required for EB-induced LH release in beef heifers.     

Use of gonadotropin-releasing hormone for hastening ovulation in transitional mares

Natural GnRH and its analog have potential for hastening ovulation in mares. A study was conducted to evaluate the efficacy of a GnRH agonist given either as an injectable or s.c. implant for induction of ovulation in mares. Forty-five seasonally anestrous mares (March) were assigned to one of three groups (n = 15/group): 1) untreated controls; 2) i.m. injection of the GnRH agonist buserelin at 12-h intervals (40 micrograms/injection for 28 d or until ovulation) and 3) GnRH agonist administered as a s.c. implant (approximately 100 micrograms/24 h for 28 d). Six mares per group were bled on d 0, 7, 14 and 21 after injection or insertion of implant. Samples were taken at -1, -.5 and 0 h and at .5, 1, 1.5, 2, 4, 6 and 8 h after GnRH. Additional daily samples were drawn for 28 d after injection or until ovulation. Samples were assayed for concentration of LH and FSH. Progesterone concentrations were determined in samples collected on d 4, 6 and 10 after ovulation. Number and size of follicles and detection of ovulation were determined by ultrasonography. Number of mares induced to ovulate within 30 d was 0 of 15, 7 of 15 and 9 of 15 for groups 1, 2 and 3, respectively. During treatment, follicle sizes were smaller for mares in group 3 (implant). The LH response to GnRH agonist (area under curve) was similar among groups at d 0 but was greater (P less than .05) for mares in group 3 on d 7 and 14 and groups 2 and 3 on d 21 than for controls. A similar pattern was detected for peak concentrations of LH after GnRH on d 0, 7, 14 and 21. Daily concentrations of LH remained low in untreated control mares compared with GnRH-treated mares throughout the sampling period. Concentrations of LH for mares in group 3 that ovulated were elevated greatly above those for group 2 mares, whereas concentrations of FSH were similar in both treatment groups prior to ovulation.     

Effects of gonadotropin-releasing hormone infused in a pulsatile or continuous fashion on serum gonadotropin concentrations and ovulation in the mare.

Studies were conducted to compare continuous vs pulsatile i.v. infusion of GnRH on serum gonadotropin concentrations and ovulation in seasonally anestrous mares and in cycling mares. Anestrous mares (Exp. 1) received no treatment (control; n = 3), 2, or 20 micrograms of GnRH/h continuous infusion (CI) (n = 4 and n = 6, respectively), or 20 micrograms of GnRH/h pulsatile infusion (PI) (n = 5). After initiation of GnRH infusion, serum LH levels increased earlier, and to a greater extent, in the PI group than in other groups (P less than .05). In contrast, serum FSH concentrations did not differ among groups. The number of days to development of the first 35-mm follicle was not different among GnRH treatment groups; however, mares receiving PI ovulated on d 9.4 of treatment, 2.8 d earlier than those receiving 20 micrograms of GnRH/h CI (P less than .05). Mares given 2 micrograms of GnRH/h CI failed to ovulate spontaneously after 16 d of treatment, but each one ovulated within 2 to 4 d after injection of 2,000 IU of hCG on d 16. Control mares did not ovulate or show any significant follicular development throughout the experiment. Cycling mares (Exp. 2) received no treatment (control; n = 6), 20 micrograms of GnRH/h CI, or 20 micrograms of GnRH/h PI (n = 4) beginning on d 16 of an estrous cycle (d 0 = day of ovulation). Serum LH concentrations in all groups increased after initiation of treatment; however, on the day of ovulation LH concentrations were lower in the CI group than in the PI or control groups (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Luteinizing hormone release after administration of the gonadotropin-releasing hormone agonist Fertilan (goserelin) for synchronization of ovulation in pigs

The generic GnRH agonist, Fertilan (goserelin), was tested for the ability to induce an LH surge and ovulation in estrus-synchronized gilts. Three experiments were performed to 1) examine the effect of various doses of Fertilan on secretion of LH in barrows, to select doses to investigate in gilts (Exp. 1); 2) determine doses of Fertilan that would induce a preovulatory-like rise of LH in gilts (Exp. 2); and 3) determine the time of ovulation after Fertilan treatment (Exp. 3). In Exp. 1, 10 barrows were injected on d 1, 4, 7, 10, and 13 with 10, 20, or 40 microg of Fertilan; 50 microg of Gonavet (depherelin; GnRH control) or saline (negative control); and sequential blood samples were collected for 480 min. There was a dose-dependent stimulation (P < 0.05) of LH release. Maximal plasma concentrations of LH (LH(MAX)) were 2.1 +/- 0.2, 4.1 +/- 0.3, 2.6 +/- 0.4, and 3.4 +/- 0.3 ng/mL after 10, 20, and 40 microg of Fertilan and 50 microg of Gonavet, respectively, and duration of release was 78 +/- 9, 177 +/- 12, 138 +/- 7, and 180 +/- 11 min, respectively. Fertilan doses of 10 and 20 microg were deemed to be the most suitable for testing in gilts. In Exp. 2, 12 gilts received (after estrus synchronization with Regumate and eCG) injections of 10 or 20 microg of Fertilan or 50 microg of Gonavet 80 h after eCG to stimulate a preovulatory-like LH surge and ovulation. An LH surge was induced in 3 of the 4 gilts in both of the Fertilan groups and in all of the Gonavet-treated gilts. Characteristics of induced release of LH did not differ among groups: LH(MAX), 5.0 +/- 0.9 vs. 4.6 +/- 1.8 vs. 6.6 +/- 1.1 ng/mL; duration, 11.7 +/- 2.0 vs. 12.3 +/- 2.2 vs. 14.3 +/- 0.5 h; interval from GnRH injection to LH(MAX), 4.0 +/- 2.0 vs. 6.7 +/- 1.3 vs. 5.8 +/- 1.6 h. In Exp. 3, estrus-synchronized gilts were injected with 20 microg of Fertilan (n = 8) or 50 microg of Gonavet (n = 4), and the time of ovulation was determined by repeated endoscopic examination. Time of ovulation ranged from 34 to 42 h postGnRH; however, ovulation occurred earlier in the Gonavet compared with the other groups (P < 0.05). Results of these experiments indicate that 1) barrows are an appropriate model for determining GnRH doses that can be effective in inducing a preovulatory-like LH surge in females; 2) the generic GnRH agonist Fertilan, at doses of 10 to 20 microg, can stimulate an LH surge in gilts, with subsequent ovulation; and 3) Fertilan at doses of 10 and 20 microg should be examined further for use in fixed-time insemination protocols.     

Use of recombinant gonadotropin-releasing hormone antigens for immunosterilization of beef heifers

The objectives of this study were to evaluate the effects of immunization against recombinant GnRH fusion proteins and growth promotants on onset of puberty, feedlot performance, and carcass characteristics of beef heifers. Heifers were immunized against an ovalbumin fusion protein containing 7 GnRH peptides (oGnRH, n = 12), a thioredoxin fusion protein containing 7 GnRH peptides (tGnRH, n = 12), a combination of oGnRH plus tGnRH (otGnRH, n = 12), or a combination of ovalbumin and thioredoxin (control, n = 11). Each heifer received a primary immunization containing 1 mg of protein in 1 mL of adjuvant injected into the mammary gland at wk 0 (mean age = 38 wk) and booster immunizations at wk 6 and 12. Six heifers within each treatment received Synovex H implants at wk -2. Weekly blood samples were collected from wk -2 to 26 for determination of serum progesterone concentrations and GnRH antibody titers. In GnRH-immunized heifers, GnRH antibody titers increased after the first booster injection, peaked after the second booster injection, and remained elevated through the end of the study (P < 0.01). Heifers immunized against oGnRH achieved greater (P < 0.05) GnRH antibody titers than tGnRH heifers but did not differ (P = 0.20) from otGnRH heifers. During the 26-wk study, ovulation was prevented (P < 0.05) in 10 out of 12, 12 out of 12, 11 out of 12, and 0 out of 11 tGnRH, oGnRH, otGnRH, and control heifers, respectively. At slaughter, uterine weights were lighter (P < 0.01) for GnRH-immunized heifers than control heifers. Synovex H-implanted heifers had greater (P < 0.05) ADG from wk -2 to 26, greater LM area, and lesser percentages of KPH, yield grade, and quality grade than nonimplanted heifers, regardless of the immunization treatment. Immunization against GnRH fusion proteins resulted in production of antibodies against GnRH that prevented ovulation in 92% of the heifers without affecting feedlot or carcass performance. Implanting heifers with Synovex H improved ADG, LM area, and yield grade. Improvements in delivery of the oGnRH vaccine may provide a feasible alternative to surgical spaying of heifers.     

Effect of GnRH Dose on Occurrence of Short Oestrous Cycles and LH Response in Cyclic Dairy Heifers

Prostaglandin F_2α (PGF_2α) and GnRH treatments given 24 h apart have been shown to result in short oestrous cycles (8–12 days) in some cows and heifers. The differences in responses may depend on the dose of GnRH. Therefore, the effect of the dose of GnRH on occurrence of short cycles and LH response was studied here. Oestrus was induced with dexcloprostenol (0.15 mg) in two groups of Ayrshire heifers. A second luteolysis was induced similarly on day 7 after ovulation; 24 h after PGF_2α treatment, the heifers were administered either a high (0.5 mg, n = 15, group T500) or low (0.1 mg, n = 10, group T100) dose of gonadorelin. Blood samples for progesterone analyses were collected daily from the second PGF_2α administration to the second ovulation after the PGF_2α injection. Beginning 24 h after the GnRH treatment, ovaries were examined by transrectal ultrasonography every 6 h until ovulation, and daily between day 4 and the next ovulation. Five heifers from both groups were sampled for LH analyses via a jugular catheter every 30 min from 1 h before to 6 h after the GnRH administration. Short oestrous cycles were detected in 7 of 10 cases in group T100 and in 12 of 15 cases in group T500. No significant differences in LH responses were detected between the groups. In group T500, the rise in LH concentration tended to be somewhat slower than in group T100. The dose of GnRH (0.1 vs 0.5 mg) did not affect the occurrence of short oestrous cycles and LH response.     

Effect of luteinizing hormone oscillations on progesterone concentrations based on treatment with a gonadotropin-releasing hormone antagonist in heifers

Close temporality has been reported between the episodic secretion of luteinizing hormone (LH) and progesterone (P4) during the midluteal phase and preceding the beginning of luteolysis in cattle. In the present studies, the relationship between LH and P4 was examined by blocking LH oscillations with the gonadotropin-releasing hormone (GnRH) antagonist, acyline. In a titration study, the minimal single acyline dose for blocking LH oscillations in heifers was 3 μg/kg. The main experiment compared LH and P4 concentrations and oscillations between a group treated with acyline on day 15 after ovulation (n = 8) and a control group (n = 4). Concentrations of P4 in blood samples collected every 8 h on days 13 to 18 indicated that acyline treatment did not alter the time that luteolysis began or the length of the luteolytic process. In blood samples collected every hour for 24 h beginning at the hour of treatment, acyline reduced the LH concentrations and blocked LH oscillations. The hourly LH means were 0.06 to 0.08 ng/mL, comparable to the mean concentration at the nadirs of LH oscillations in controls (0.07 ng/mL). During the hourly sampling, the GnRH antagonist produced the following P4 responses: (1) lower P4 concentrations, (2) fewer and reduced prominence of P4 oscillations, and (3) increased length and variability in the interval between the peaks of P4 oscillations. Results indicated that LH oscillations affect both the prominence and the rhythmicity of P4 oscillations during preluteolysis but not the onset and length of luteolysis.     

The effects of 3 gonadorelin products on luteinizing hormone release,ovulation, and follicular wave emergence in cattle

The objective was to determine luteinizing hormone (LH) secretion and follicular dynamics in cattle following administration of 3 gonadorelin formulations that are commercially available in Canada. In experiment 1, nonlactating Holstein cows (n = 4 per group) were randomly assigned to receive 100 micrograms gonadorelin diacetate tetrahydrate, intramuscularly (C; Cystorelin, or FE; Fertagyl). Blood samples (for LH analysis) were collected 0, 1, 2, and 4 hours after treatment. In experiment 2, nonlactating Holstein cows (n = 10 per group) were randomly allocated to receive 100 micrograms gonadorelin, intramuscularly as follows: 2 mL of C; 1 mL of FE; or 2 mL of Factrel (FA, gonadorelin hydrochloride). Gonadorelin treatment was done on days 6 or 7 after ovulation and blood samples for LH analysis were collected at 0, 1, 2, 4, and 6 hours after treatment. Ovaries were examined by ultrasonography, twice daily, to detect ovulation. A replicate was conducted using only C (n = 10) or FE (n = 10); blood samples were collected at 0, 1, 2, 3, and 4 hours. In experiment 3, beef heifers (n = 10 per group) were randomly assigned to receive 1 of 3 GnRH gonadorelin treatments (as in the first phase of experiment 2) on days 6 or 7 after ovulation and blood samples were collected at 0, 0.5, 1, 1.5, 2, and 4 hours. In experiments 2 and 3, both mean and mean peak plasma LH concentrations were higher (P < 0.05) in cattle treated with C. The proportion of dominant follicles that ovulated was higher (P < 0.02) in Holstein cows treated with C than in those treated with FE or FA (18/19, 11/19, and 4/7, respectively), but there was no significant difference among the products in beef heifers (6/10, 6/10, and 4/10, respectively). No significant differences were found in the interval from treatment to the emergence of the next follicular wave. In summary, C induced a greater LH release and this resulted in a higher ovulatory rate in Holstein cows but not in beef heifers.     

Heat-tolerant versus heat-sensitive Bos taurus cattle: influence of air temperature and breed on the acute phase response to a provocative immune challenge

The difference in the acute phase response of a heat-tolerant and a heat-sensitive Bos taurus breed to a lipopolysaccharide (LPS) challenge when housed at different air temperatures (T_a) was studied. Angus (ANG; heat-sensitive; n = 11; 306 ± 26 kg BW) and Romosinuano (RO; heat-tolerant; n = 10; 313 ± 32 kg BW) heifers were transported from the USDA Agricultural Research Service SubTropical Agricultural Research Station in Florida to the Brody Environmental Chambers at the University of Missouri, Columbia. Heifers were housed in stanchions in 4 temperature-controlled environmental chambers. Initially, T_a in the 4 chambers was cycling at thermoneutrality (TN; 18.5°C–23.5°C) for a 1-wk adjustment period, followed by an increase in 2 of the 4 chambers to cycling heat stress (HS; 24°C–38°C) for 2 wk. On day 19, heifers were fitted with jugular catheters and rectal temperature (RT) recording devices. On day 20, heifers were challenged with LPS (0.5 μg/kg BW; 0 h), sickness behavior scores (SBSs) were recorded, and blood samples were collected at 0.5-h intervals from −2 to 8 h and again at 24 h relative to LPS challenge at 0 h. Serum was isolated and stored at −80°C until analyzed for cortisol and cytokine concentrations. A breed by T_a interaction (P < 0.001) was observed for RT such that the post-LPS average RT in RO heifers housed at TN was lower than the RT of all other treatment groups (P < 0.001), whereas ANG heifers housed at HS had greater post-LPS average RT than all other treatment groups (P < 0.001). In response to LPS, HS increased SBS after LPS in RO heifers compared to RO heifers housed at TN (P < 0.001), whereas HS decreased SBS after LPS in ANG heifers compared to ANG heifers housed at TN (P = 0.014). The cortisol response to LPS was greater in TN than in HS heifers (P < 0.01) and was also greater in RO than in ANG heifers (P = 0.03). A breed by T_a interaction (P < 0.01) was observed for tumor necrosis factor-α (TNF-α) concentration such that HS increased post-LPS serum concentrations of TNF-α in ANG heifers compared to ANG heifers housed at TN (P = 0.041), whereas HS decreased post-LPS concentrations of TNF-α in RO heifers compared to RO heifers housed at TN (P = 0.008). A tendency (P < 0.06) was observed for a breed by T_a interaction for IL-6 concentrations such that RO heifers had greater post-LPS concentrations of IL-6 than ANG heifers when housed at HS (P = 0.020). A breed by T_a interaction was observed for interferon-γ (IFN-γ; P < 0.01) concentrations such that HS decreased post-LPS concentrations of IFN-γ in ANG heifers compared to ANG heifers housed at TN (P < 0.001), and HS increased post-LPS concentrations of IFN-γ in RO heifers compared to RO heifers housed at TN (P = 0.017). These data indicate differences in the acute phase response between the heat-tolerant RO and heat-sensitive ANG heifers under different T_a which may aid in elucidating differences in productivity, disease resistance, and longevity among cattle breeds.     

Timing of Ovulation and Fertility of Heifers After Synchronization of Oestrus with GnRH and Prostaglandin F2α

Synchronization of oestrus and/or ovulation can reduce workload in heifer reproductive management. The objective of this study was to compare two protocols to synchronize oestrus and/or ovulation using GnRH and prostaglandin F_2α (PGF_2α) in dairy heifers concerning their effect on follicular dynamics and reproductive performance. Four trials were carried out. In trial 1, 282 heifers were treated with GnRH and PGF_2α 7 days apart (GP protocol). One group was inseminated on detection of oestrus (IDO 1), and the other group received two timed artificial inseminations (AI) 48 and 72 h after PGF_2α administration (TAI 1). In trial 2, 98 heifers were synchronized with the same GP protocol. Heifers in IDO 2 were treated as in IDO 1, heifers in TAI 2 received two TAI 48 and 78 h after PGF_2α administration. In trial 3, heifers in IDO 3 (n = 71) were again treated as in IDO 1. Heifers in TAI 3 (n = 166) received a second dose of GnRH 48 h after PGF_2α (GPG protocol) and TAI together with this treatment and 24 h later. Trial 4 compared the timing of ovulation after the GP and the GPG protocol, using a subgroup of the heifers from trials 1 to 3. The ovaries of the heifers were scanned via ultrasound at 48, 56, 72, 80, 96 and 104 h after PGF_2α administration. Timing of ovulation and size of the ovulatory follicles were compared between the two groups. In trials 1 to 3, conception rates to first service were between 49 and 66%. They did not differ significantly between IDO and TAI groups within or between trials. Pregnancy rates per synchronization were numerically higher in the TAI groups, but the difference was not significant. Conception rates to breeding on spontaneous oestrus in heifers returning to oestrus were higher than that after synchronized oestrus. In trial 4, more heifers ovulated before the end of the observation period in GPG than in GP (96.5% vs 74.7%; p < 0.001). Overall, ovulatory follicles were smaller in GPG (13.1 ± 1.9 mm vs 14.3 ± 1.9 mm; p < 0.001).     

Ultrasonographic and Endocrine Evaluation of Three Regimes for Oestrus and Ovulation Synchronization for Sheep in the Subtropics

This study aimed to evaluate three regimes for oestrus and ovulation synchronization in Farafra ewes in the subtropics. During autumn, 43 ewes were assigned to (i) controlled internal drug releasing (CIDR)‐eCG group, treated with CIDR for 12 days and eCG at insert withdrawal, n = 13; (ii) PGF₂α‐PGF₂α group, treated with two PGF₂α injections at 11 days interval, n = 14; and (iii) GnRH‐PGF₂α‐GnRH group, treated with GnRH, followed 5 days later with PGF₂α and 24 h later with a second GnRH, n = 16. Oestrus‐mating detection was carried out at 4 h intervals starting on day 0 [the day of CIDR withdrawal (CIDR‐eCG group), the day of second PGF₂α treatment (PGF₂α‐PGF₂α group) and the day of PGF₂α treatment (GnRH‐PGF₂α‐GnRH group)]. Ovarian dynamics was monitored by ultrasound every 12 h beginning on day 0 and continued for 4 days. Blood samples were obtained daily for progesterone (P₄) and oestradiol 17β (E₂) estimation starting on day 0 and continued for 4 days. The obtained results showed that, oestrus expression, ovulation and conception were greater (p < 0.05) in CIDR‐eCG and PGF₂α‐PGF₂α groups than in GnRH‐PGF₂α‐GnRH group. All ewes of PGF₂α‐PGF₂α group presented, on day of second PGF₂α injection with mature CL (P₄ > 2.0 ng/ml), compared to 42.9% in GnRH‐PGF₂α‐GnRH group (p = 0.01). The peak of oestrus occurred 32–52, 48–60 and 28–96 h after the end of treatment in CIDR‐eCG, PGF₂α‐PGF₂α and GnRH‐PGF₂α‐GnRH groups, respectively. Ovulation started 48 h after treatment in all groups and extended for 24, 36 and 48 h for CIDR‐eCG, PGF₂α‐PGF₂α and GnRH‐PGF₂α‐GnRH groups, respectively. Results demonstrated that oestrus and ovulation synchronization could be efficiently achieved in Farafra ewes using either CIDR‐eCG or PGF₂α‐PGF₂α regimes; however, the GnRH‐PGF₂α‐GnRH treatment induced a more spread oestrus and ovulation that may make the protocol inadequate for timed artificial insemination.     

Gonadotropin response by postpartum mares to gonadotropin-releasing hormone

We hypothesized that the LH response to GnRH would be greater as the interval from foaling increases, whereas the FSH response would decrease, and that corpus luteum function after the first ovulation would be similar to that after the second ovulation. At parturition, mares were assigned to receive GnRH (2 micrograms/kg) intravenously on 1) d 3 postpartum (n = 6); 2) d 6 postpartum (n = 6); 3) d 1 of first postpartum estrus (foal estrus) and again on d 1 of second postpartum estrus (n = 8). Blood was collected through an indwelling cannula at -2, -1 and 0 h relative to GnRH stimulation (basal concentrations) and at .25, .5, .75, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 h post-GnRH. Samples were assayed for concentrations of LH and FSH. Basal concentrations of LH were lower (P less than .05) for mares given GnRH on d 3 postpartum than for mares on d 1 of foal estrus. A rise in concentrations of LH was noted within 30 min in all groups, but the response to GnRH on d 1 of the first estrus was less (P less than .05) than on d 1 of second postpartum estrus. As the interval from parturition increased, the amount of LH secreted in response to GnRH increased. The maximum response to GnRH was greater (P less than .05) during d 1 of the first estrus than on d 3 or 6 postpartum and was greater on d 1 of cycle 2 than on d 1 of cycle 1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of Three Estrous Synchronization Protocols in Beef Heifers

Objectives of this study were to evaluate synchronization, conception, and pregnancy rates of heifers synchronized with melengestrol acetate (MGA)-prostaglandin F_2α (PGF_2α,), Select Synch, or Select Synch preceded by MGA (MGA-Select Synch). Heifers in the MGA-PGF_2α group (n = 209; BW = 378 kg) received MGA (0.5 mg/ d per heifer) for 14 d and PGF_2α (25 mg) 19 d later. Select Synch heifers (n = 213; BW = 374 kg) received gonadotropin-releasing hormone (GnRH; 100 μg) followed by PGF_2α (25 mg) 7 d later. The MGA-Select Synch heifers (n = 210; BW = 373 kg) were fed MGA (0.5 mg/d per heifer) for 7 d, GnRH (100 μg) the day following the last MGA feeding, and PGF_2α (25 mg) 7 d after GnRH. More (P<0.01) heifers were in estrus 1 to 4 d before PGF2a administration in both the Select Synch (20%) and MGA-Select Synch (24%) groups than in the MGA-PGF_2α (4%) group. Pregnancy rates for heifers in estrus early (d 1 to 4 before PGF_2α) were greater (P<0.05) for both Select Synch (55%) and MGA-Select Synch (63%) compared with MGA-PGF_2α heifers (18%). Synchronization rate (detected after PGF_2α) was greater (P<0.01) for MGA-PGF_2α heifers (86%) compared with Select Synch (66%) and MGA-Select Synch (68%) heifers; however, conception rate did not differ (P=0.13) and averaged 72, 63, and 62% for MGA-PGF_2α, Select Synch, and MGA-Select Synch heifers, respectively. Select Synch (52%), MGA-Select Synch (58%), and MGA-PGF_2α protocols (61%) provided similar (P=0.18) overall AI pregnancy rates; however, more heifers were in estrus before PGF_2α administration in protocols using GnRH.     

Seasonal differences in the parameters of luteinizing hormone release to exogenous gonadotropin releasing hormone in prepubertal Holstein heifers in Sapporo

Stress due to summer heat has adverse effects on reproduction in Holstein dairy cattle. Summer suppression of reproduction of Holsteins can pose an important economic problem, even in Hokkaido prefecture located in the northern region of Japan. Hokkaido is one of the most important dairy farming areas of Japan. This study is an attempt to clarify the seasonal differences in the parameters of luteinizing hormone (LH) response to exogenous gonadotropin releasing hormone (GnRH) in Sapporo, Hokkaido, Japan. A total of 12 prepubertal heifers received an injection with GnRH analogue intramuscularly in either May (n=4, May group), July (n=4, July group), or November (n=4, November group), and serial blood samples were collected to analyze the parameters of the LH response curve after GnRH injection. The parameters were as follows: the basal LH concentration, peak LH concentration, duration from the time of GnRH injection to the time of the peak LH concentration, and area under the LH response curve (AUC). There were no significant differences in the basal and peak LH concentrations or the AUC among the three groups. The July group reached the LH peak significantly (P<0.05) faster than the May group, but there was no significant difference with the November group. Therefore, the results of the present study do not demonstrate an effect of summer heat on the LH response to the exogenous GnRH in Holstein heifers.     

Secretion of luteinizing hormone into pituitary venous effluent of the follicular and luteal phase mare: novel acceleration of episodic release during constant infusion of gonadotropin-releasing hormone

We tested the hypothesis that continuous infusion of native GnRH into mares during the estrous cycle, at a dose of 100 μg/h, would elevate circulating concentrations of LH without disrupting the endogenous, episodic pattern of LH release. Ten cyclic mares were assigned to one of two groups (n = 5/group): (1) Control (saline) and (2) GnRH in saline (100 μg/h). On experimental day 0 (3 to 6 d after ovulation), osmotic pumps containing saline or GnRH were placed subcutaneously and connected to a jugular infusion catheter. Blood samples were collected from jugular catheters daily and at 5-min intervals from catheters placed in the intercavernous sinus (ICS) for 8 h on experimental day 4 (luteal phase; 7 to 10 d after ovulation), followed by an additional 6-h intensive sampling period 36 h after PGF(2α)-induced luteal regression (experimental day 6; follicular phase). Treatment with GnRH increased (P < 0.001) concentrations of LH by 3- to 4-fold in the peripheral circulation and 4- to 5-fold in the ICS. Continuous GnRH treatment accelerated (P < 0.01) the frequency of LH release and decreased the interepisodic interval during both luteal and follicular phases. Treatment with GnRH during the luteal phase eliminated the low-frequency, long-duration pattern of episodic LH release and converted it to a high-frequency, short-duration pattern reminiscent of the follicular phase. These observations appear to be unique to the horse. Further studies that exploit this experimental model are likely to reveal novel mechanisms regulating the control of gonadotrope function in this species.