Progesterone (CIDR)-based timed AI protocols using GnRH, porcine LH or estradiol cypionate for dairy heifers: ovarian and endocrine responses and pregnancy rates

The overall objective was to compare the efficacy of GnRH, porcine LH (pLH) and estradiol cypionate (ECP), in a modified Ovsynch/fixed-time AI (FTAI) protocol that included a controlled internal drug [progesterone] release (CIDR) device. In Experiment 1, heifers received a CIDR on Day -10, and PGF (25mg) on Day -3. At CIDR insertion, heifers received 100 microg of GnRH (n=6), 0.5mg of ECP (n=6), 5.0mg of pLH (n=6) or 2 mL of saline (n=7); these treatments were repeated on Day -1, except for ECP, that was repeated on Day -2, concurrent with CIDR-removal. The 5.0 mg pLH was the least effective with a longer interval to ovulation than the other groups combined (102 versus 64 h; P<0.05). Overall mean LH concentrations (1.6 ng/mL) and area under the curve (AUC) did not differ among treatments, but mean peak LH concentration was lower in heifers given 5 mg of pLH compared to all other groups (4.5 versus 10.3 ng/mL; P<0.05). In Experiment 2, heifers on CIDR-based Ovsynch protocols were given 12.5mg pLH (n=6; pLH-low), 25.0 mg pLH (n=6, pLH-high), or 100 microg GnRH (n=5; control). Heifers in the pLH-high group had greater (P<0.01) plasma LH concentrations (between 12 and 20 h) than GnRH-treated heifers, but the pLH treatments did not differ (P>0.10). Area under the curve for LH (ng/32 h) was at least 50% greater (P<0.01) in pLH-treated heifers compared to GnRH-treated heifers (mean, 41.3, 56.3 and 20.3 for pLH-low, pLH-high and GnRH, respectively). Ovulation occurred in 15 of 17 heifers. Progesterone concentrations were higher on Days 9 and 14 in heifers given 25mg of pLH, suggesting enhanced CL function. In Experiment 3, 240 heifers were assigned to CIDR-based Ovsynch/FTAI protocols. The first and second hormonal treatments (with an intervening PGF treatment on Day -3) were GnRH/GnRH (100 microg), ECP/ECP (0.5 mg), pLH/pLH (12.5 mg) or GnRH/ECP, respectively; pregnancy rates were 58.7, 66.1, 45.9 and 48.3%, respectively (ECP/ECP>both pLH/pLH and GnRH/ECP; P相似文献   

Continuous administration of low-dose GnRH in mares II. Pituitary and ovarian responses to uninterrupted treatment beginning near the autumnal equinox and continuing throughout the anovulatory season

We tested the hypothesis that continuous subcutaneous treatment with low-dose GnRH, administered to mares from late September/early October through March, would prevent the development of seasonal anovulation. Quarter Horse mares (n=20) were stratified by age and body condition score and assigned randomly to either a saline control (n=9) or a GnRH (n=11) treatment group. Gonadotropin-releasing hormone was delivered continuously via osmotic minipumps, with sham pumps placed in control mares. Initial pumps were inserted on Day 3 following ovulation or during the follicular phase if the next anticipated ovulation did not occur by 9 October. Delivery rate of GnRH was 2.5 microg/h (60 microg/day) for the first 60 days, followed by 5.0 microg/h (120 microg/day) thereafter. Pumps were replaced every 30 days. Eighty and 100% of all mares had become anovulatory by 1 November and 1 December, respectively, and remained anovulatory through the end of February. Neither serum concentrations of LH throughout the study nor total releasable pools of LH in March differed between groups. Although control mares that exhibited ovulatory cycles after study onset had greater (P<0.05) mean concentrations of LH during the follicular phase and metestrus compared to GnRH-treated mares, neither size of ovulatory follicles nor interovulatory intervals differed between groups. Serum concentrations of FSH were not affected by treatment, but were lowest (P<0.05) from November through January. Continuous infusion of low-dose GnRH, beginning soon after autumnal equinox and continuing until just after vernal equinox, failed to prevent the occurrence of or to hasten transition from seasonal anovulation.     

Effects of ovarian input on GnRH and LH secretion immediately postovulation in pony mares

The potential involvement of ovarian factors in regulating GnRH and LH postovulation was studied in ovarian intact (Group 1; n=3) and ovariectomized (OVX; Group 2; n=3) mares (OVX within 12 hr of ovulation). Blood samples were collected every 10 min for 6 hr from jugular vein (JV) and intercavernous sinus (ICS) during estrus and on Day 8 postovulation for LH and GnRH analysis. Additionally, JV samples were collected twice daily (12-hr intervals) for 30 days for LH and progesterone (P4) analysis. A significant treatment x day effect (P<0.0001) describes declining plasma LH concentrations in intact mares, and regression analysis indicated that response curves were not parallel (P<0.001). Plasma LH concentrations remained elevated in OVX mares. LH increased further in OVX mares by Day 8 post-OVX (P<0.06), reflecting the increased (P<0.07) LH episode amplitude. GnRH decreased from estrus to Day 8 in both groups reflecting an effect of sampling period (P<0.03). GnRH episode amplitude declined (P<0.08) from estrus (62.8+/-3.1 pg/mL) to Day 8 (46.3+/-3.1 pg/mL) in OVX mares, but not in control mares (intact estrus, 36.5+/-6.4; intact Day 8, 37.5+/-7.3; OVX estrus, 62.8+/-3.1; OVX Day 8, 46.3+/-3.1 pg/mL). In conclusion, we propose that postovulatory LH decline requires ovarian feedback in mares, and that OVX alters GnRH secretory dynamics such that LH concentrations does not decline postovulation and, in fact, is further elevated with time after OVX.     

Pharmacologic application of native GnRH in the winter anovulatory mare,II: Accelerating the timing of pregnancy

Onset of the winter anovulatory period in mares is associated with a marked diminution in adenohypophyseal synthesis and release of LH. Native GnRH, unlike its synthetic agonists, stimulates the synthesis and secretion of LH in mares without pituitary refractoriness. Herein we tested the hypotheses that (1) the average Julian day of pregnancy can be accelerated by up to 2 months in winter anovulatory mares treated continuously with native GnRH beginning on February 1 and (2) mares will sustain luteal function and pregnancy after treatment withdrawal. Forty-two winter anovulatory mares were stratified by age, body condition score, and size of the largest follicle across two locations in a randomized design and assigned to one of three groups (n = 14 per group): (1) Control: untreated, (2) GnRH-14: GnRH delivered subcutaneously in saline at a rate of 100 μg/h for 8 weeks (February 1–March 29) using four consecutive 14-day pumps (Alzet 2ML2), or (3) GnRH-28: GnRH delivered as in (2), but using two 28-day pumps (Alzet 2ML4). On development of a 35-mm follicle and expression of estrus, mares were bred the following day and treated with hCG. Pregnancies were confirmed using transrectal ultrasonography on Days 14, 24, 33, and 45, with blood samples collected to assess luteal function. Mares treated with GnRH (GnRH-14 and GnRH-28) did not differ reproductively in their responses and data were pooled for statistical comparisons. Mares treated with GnRH exhibited marked increases (P ≤ 0.04) in the frequency of development of a 35-mm follicle, submission rate for live cover and/or artificial insemination, ovulation, and pregnancy compared with control mares on treatment Day 56 (March 29). Interval to the first 35-mm follicle was 51.8 ± 4.9 and 19.3 ± 3.5 days (least square mean ± standard error of the mean) for control and GnRH-treated mares, respectively. Interval to pregnancy was 65.3 ± 6.7 and 28.6 ± 4.8 days (least square mean ± standard error of the mean) for control and GnRH-treated mares, respectively, excluding one GnRH-14 mare that failed to become pregnant over four cycles. By the end of the treatment period (March 29), only 21% of control mares were pregnant compared with 79% of GnRH-treated mares. Furthermore, mean serum concentrations of progesterone were similar to (GnRH-28; P = 0.26) or greater than (GnRH-14; P = 0.01) that of control mares from Day 0 to 46 postbreeding. Data illustrate that continuous administration of native GnRH is a highly efficient option for managing seasonal anovulation in mares and could be effectively used in the breeding industry if a user-friendly delivery option were available.     

Comparison of two Ovsynch protocols (GnRH versus LH) for fixed timed insemination in buffalo (Bubalus bubalis)

We evaluated the efficiency of replacing GnRH with LH in the ovulation synchronization protocol in buffaloes. Buffaloes received GnRH on Day 0, (Buserelin; Conceptal, 20 microg), PGF2alpha (Luprostiol; Prosolvin, 15 mg) on Day 7 and GnRH (Buserelin; Conceptal, 10 microg; Group 1) or porcine LH (LH; Lutropin-V, 12.5 mg; Group 2) on Day 9. In Experiment 1, we studied the follicular dynamics of 30 buffaloes (Group 1, n = 15 and Group 2, n = 15). We performed ultrasonography every 12 h from Days 0 to 2, then on Day 7 and then every 6 h from the time of GnRH or LH treatment (Day 9) until the time of ovulation. All females not ovulating by 48 h after the second GnRH or LH injection were considered as nonresponders. In Experiment 2, we evaluated 305 buffaloes (Group 1, n = 154; Group 2, n = 151), using the same two treatments studied in Experiment 1. We also recorded and evaluated aspects like parity, lactational status, the presence of mucus, and uterine tone at the time of artificial insemination (Al). In Experiment 1, ovulation rate after the first GnRH was 86.6% (26/30). Ovulation rates were 93.3% (14/15; Group 1) after the second dose of GnRH and 93.3% (14/15) after LH (Group 2). Ovulation occurred 36.4+/-10.4 h after the first GnRH. The interval for treatment to ovulation was 26.5+/-9.6 h for buffaloes treated with GnRH (Group 1) and 24.4+/-7.9 h for buffaloes treated with LH (Group 2); the time of ovulation did not differ statistically between the two groups (GnRH versus LH; P > 0.05). In Experiment 2, conception rates of the animals AI in the field were 56.5% (Group 1) and 64.2% (Group 2), respectively (P = 0.08). The response to the treatment with LH was not different to the treatment with GnRH; however, multiparous buffaloes had higher conception rates than the primiparous buffaloes in both groups (P > 0.05). Buffaloes with mucus at the time of AI in Group 2 had higher conception rates than the buffaloes that had mucus in Group 1 (P < 0.05). Uterine tone and lactational status did not influence conception rates (P > 0.05). In summary, the results showed that both treatments resulted in synchronization of ovulation and acceptable conception rates. Therefore, the exogenous injection of LH can substitute the GnRH injections in the Ovsynch program in buffaloes.     

Pharmacologic application of native GnRH in the winter anovulatory mare,I: Frequency of reversion to the anovulatory state following ovulation induction and cessation of treatment

The continuous, subcutaneous infusion of native GnRH into seasonally anovulatory mares stimulates the synthesis and secretion of LH without pituitary refractoriness, offering opportunities to markedly accelerate the timing of ovulation within the operational breeding season. Herein, we tested the hypothesis that ovarian cycles induced in winter anovulatory mares using continuous administration of native GnRH for 28 days, beginning in either early February or early March (North America) would not revert to an anovulatory state after treatment withdrawal. Anovulatory mares received sham pumps (control) or native GnRH (100 μg/h) for 28 days beginning from February 2 or 3 (GnRH-Feb) or March 2 or 3 (GnRH-Mar). Mean concentrations of LH were five- to seven-fold greater during February in the GnRH-Feb group compared with control and GnRH-Mar groups through February and ending on March 2 or 3. However, concentrations of LH returned to the winter baseline within 3 to 11 days after pump removal and all GnRH-Feb mares failed to remain cyclic after treatment withdrawal. Correspondingly, during March, concentrations of LH in the GnRH-Mar group were greater (P < 0.001) than in the control and GnRH-Feb groups during the 28-day treatment period. Follicular growth and frequency of ovulation (6/10 GnRH-Feb; 9/10 GnRH-Mar, 1/11 controls, respectively) were greater (P < 0.01) in GnRH-treated mares. Ovulatory cycles continued in five of nine GnRH-Mar mares that ovulated, with interovulatory intervals of 15 to 24 days; whereas, three of nine mares had extended (33–42 days) interovulatory intervals and one of nine mares had a persistent CL after cessation of treatment. In summary, continuous administration of native GnRH for 28 days, beginning in early February or March, elevated circulating LH adequately to stimulate follicular growth and ovulation up to 60 days earlier than in untreated controls. However, if continuous, subcutaneous infusion of GnRH is selected as the only pharmacologic or managerial intervention, and mares are not pregnant, treatment must be continued at least until the end of March. This will improve the likelihood of a normal interovulatory interval after treatment withdrawal.     

Pulsatile administration of gonadotropin-releasing hormone advances ovulation in cycling mares

Cycling standardbred mares were infused with saline or 20 micrograms gonadotropin-releasing hormone (GnRH) in a pulsatile pattern (one 5-sec pulse/h, 2 h or 4 h) beginning on Day 16 of the estrous cycle. Although serum concentrations of luteinizing hormone (LH) increased significantly earlier in all three GnRH-treated groups (within one day of the initiation of infusion) compared to saline-infused controls, there were no differences in peak periovulatory LH concentrations among treatments (overall mean +/- SEM, 8.98 +/- 0.55 ng/ml). The number of days from the start of treatment to ovulation was significantly less in mares infused with 20 micrograms GnRH/h (mean +/- SEM, 2.9 +/- 0.6 days after the initiation of treatment, or 18.9 days from the previous ovulation; N = 7) compared to mares treated with saline (5.9 +/- 0.3 days, or 21.9 days from previous ovulation; N = 7) or 20 micrograms GnRH per 4 h (5.4 +/- 0.9 days or 21.4 days from previous ovulation; N = 5). Although mares infused with 20 micrograms GnRH/2 h ovulated after 4.3 +/- 0.7 days of treatment (Day 20.3; N = 7), this was not significantly different from either the control or 20 micrograms GnRH/h treatment groups. Neither the duration of the resulting luteal phase nor the length of the estrous cycle was different between any of the treatment groups (combined means, 14.7 +/- 0.2 days and 21.3 +/- 0.4 days, respectively). We conclude that pulsatile infusion of GnRH is effective in advancing the time of ovulation in cycling mares, but that the frequency of pulse infusion is a critical variable.     

Effect of the GnRH antagonist, acyline, on canine testicular characteristics

The primary objective was to assess the effects and clinical safety of a single high-dose of the third generation GnRH antagonist, acyline, on testicular characteristics in male dogs. Seven dogs were followed up weekly for six, 2-week periods (-2, -1, 1, 2, 3 and 4). At the end of the second period, they were given acyline (330 microg/kg sc). Responses to treatment varied among individual animals. Testicular consistency and scrotal diameter were slightly diminished (P>0.05) in Periods 1, 2, and 3. Libido and erection decreased during Periods 1 and 2 (P<0.05). Second and third fractions of the ejaculate volume, sperm count and motility varied throughout periods (P<0.01); there was a clear impairment of these parameters (< or =0.2 mL, < or =0.6 mL, < or =0.5 million/mL and < or =30%, respectively) around the second week of Period 1, followed by slow improvement (to the end of the study). Semen volumes and motility diminished during Period 1 (P<0.05). Sperm count decreased during Periods 1, 2, and 3, relative to Periods -2 and -1 (22.7+/-11.7, 62.8+/-19.9, and 51.0+/-25.4 versus 235.7+/-63.3 and 315.5+/-27.3, respectively; P<0.05; (L.S.M.+/-S.E.M.). Morphologically abnormal sperm increased during Periods 2 and 3 (up to 64.9+/-2.6%; P<0.05). Throughout the study, no dog had hematological, biochemical, local, or systemic side effects. In conclusion, a single high-dose acyline treatment severely decreased semen quality with no adverse effects.     

Treatment of seasonally anestrous Romney ewes with continuous infusion of low doses of GnRH: effects on estrus, ovulation and plasma progesterone concentration

The aim of this study was to assess the suitability of a GnRH infusion regimen (125 ng/h or 250 ng/h) to induce estrous behaviour, ovulation and normal corpus luteum function in progesterone-primed Romney ewes each month of seasonal anestrus (i.e. September to February inclusive) over two years. None of the progesterone-primed control ewes (i.e. no GnRH treatment; N = 120 observations) ovulated, showed normal corpus luteum function or displayed estrous behaviour at any time during anestrus. Approximately 27 and 50% of the respective 125 ng/h and 250 ng/h GnRH-treated ewes (N = 120 observations per GnRH treatment) ovulated and showed normal luteal function. Of those which ovulated 59.2% and 52.4% in the respective 125 ng/h and 250 ng/h GnRH treatment groups showed estrous behaviour. There was a significant effect of GnRH dose on the median number of ovulations (250 ng/h > 125 ng/h; P<0.01) but no overall difference (when both treatment years and GnRH doses were pooled) in the median number of ovulations per month of anestrus. The frequency of ewes with an ovulation rate >2 was low with only 4/95 treated ewes with more than 2 corpora lutea (CL). Treatment of progesterone-primed ewes with 250 ng/h GnRH increased plasma LH (P<0.01) but not FSH concentrations; a significant increase in LH pulse amplitude (P<0.05) but not LH pulse frequency was observed. The plasma gonadotropin levels in the 125 ng/h GnRH treatment groups were not studied. We suggest that in breeds such as the Romney which have a strict (i.e. 5-6 month) anovulatory interval, the GnRH-infusion technique may be of limited practical use for inducing pregnancies during the non-breeding season.     

Comparison of the effects of two GnRH antagonists on LH and FSH secretion,follicular growth and ovulation in the mare

The effects of two GnRH antagonists were tested in order to delay and/or synchronise ovulation in mares. Five mares received Antarelix (0.01 mg.kg(-1)), 5 mares received Cetrorelix (the same dose), 5 mares (control mares) received the vehicle intravenously, twice daily, for 8 days from the day the largest follicle reached 22 mm following prostaglandin administration. Ovulation was postponed in all mares injected with Antarelix (19.4 +/- 1.2 days after the beginning of the treatment) and in 2/5 mares injected with Cetrorelix (20 +/- 1 days) vs. 6.2 +/- 0.4 days in control mares. During the treatment, LH concentrations were strongly depressed in Antarelix and in Cetrorelix mares (1.6 +/- 0.1 and 3.8 +/- 0.5 ng.mL(-1) respectively vs. 21 +/- 2.5 ng.mL(-1) in control mares). In the 3 Cetrorelix mares which ovulated during the treatment. 2 initiated their LH surge at this moment. FSH concentrations were not affected in Antarelix or in Cetrorelix mares during the treatment (11.4 +/- 1.3 and 7.9 +/- 0.8 ng.mL(-1) respectively vs. 10.5 +/- 0.8 ng.mL(-1) in control mares). In conclusion, Antarelix seems more efficient than Cetrorelix for postponing ovulation in mares. The role of LH in antral follicular development before the preovulatory stage is confirmed.     

Effect of dose of GnRH analog on ovulation in mares

Proper timing of insemination for optimal conception is accomplished by frequent palpations per rectum, by ultrasonography of the preovulatory follicle and/or by treatment with hCG or GnRH. Sustained release of GnRH from implants has been shown to hasten ovulation. Therefore, 2 studies were conducted to evaluate the efficacy of a GnRH analog, deslorelin, for hastening ovulation in nonlactating cyclic mares. The GnRH implant was 2.3 x 3.7 mm and released deslorelin for 2 to 3 days. In Experiment 1, 60 nonlactating, cycling mares were assigned to 1 of 5 doses: 0, 1.2, 1.7, 2.2 and 2.7 mg per implant. Mares were assigned sequentially on the first day of estrus (Day 1). Ovaries were examined per rectum and with ultrasonography every 12 h until ovulation. Once the mares obtained a follicle >30 mm, they were injected subcutaneously with a GnRH implant. The mares were inseminated every other day during estrus with semen from 1 of 3 stallions. Pregnancy was determined with ultrasonography. Experiment 2, 40 nonlactating, cyclic mares were assigned to 1 of 5 treatments (same treatments as in Experiment 1). Data were obtained on interval to ovulation, duration of estrus and pregnancy rates at 12, 18 and 35 d after ovulation. Time to ovulation was shorter (P<0.05) in GnRH-treated mares than in control mares in the Experiment 1. Mean time to ovulation was 68, 49, 48, 47, 44 h in Experiment 1, and 91, 66, 58, 46, 58 h in Experiment 2 for mares given 0, 1.2, 1.7, 2.2 and 2.7 mg/mare in the 2 trials. Averaged for both experiments, the proportion of mares ovulating within 48 h of treatment was 40, 75, 85, 90 and 90% for 0, 1.2, 1.7, 2.2 and 2.7 mg/mare. For both experiments, there was no effect of GnRH on pregnancy rate. In summary, a subcutaneous implant containing GnRH analog induced ovulation in most mares by 48 h of injection, and there was no advantage of doses higher than 2.2 mg/mare.     

Influence of reproductive stage at PRID insertion on synchronization of estrus and ovulation in mares

In the present study, we investigated the effects of reproductive status, size of follicles and plasma progesterone concentrations of mares at PRID insertion on the efficacy of the treatment, estrous cycle patterns, plasma concentrations of progesterone and LH. The progesterone-releasing device (PRID) was administered intravaginally to 28 Haflinger mares for 11 days at different reproductive stages: anestrus (n=6), estrus (n=11) and diestrus (n=11). Plasma concentrations of progesterone at insertion (Day 1) of PRID differed among treatment groups (anestrus: 0.2-0.6 ng mL(-1), estrus: 0.2-0.5 and diestrus: 1.6-10.8 ng mL(-1); P<0.001). Total secretion of progesterone (area under curve (AUC)) during treatment period revealed highest values in diestrus (38.2+/-3.1 ng mL(-1)h(-1)) followed by estrus (25.1+/-2.7) and anestrus (21.0+/-0.4 ng mL(-1)h(-1); P<0.05). Progesterone area under curve (AUC) was positively correlated with initial progesterone concentrations (R=0.5; P<0.05), but it did not correlate with the interval from PRID removal to ovulation. Plasma concentrations of LH during treatment period, were significantly lower in anestrous mares (184.6+/-28.6 ng mL(-1)h(-1)) when compared to estrous and diestrous mares (349.7+/-53.3 and 370.5+/-40.3 ng mL(-1)h(-1); P<0.05). Follicular size at PRID insertion had no effects on the intervals from PRID removal to subsequent estrus and ovulation. Follicle diameters at removal of PRID were significantly correlated with the interval from coil removal to estrus (R=-0.55, P<0.05) and ovulation (R=-0.72, P<0.0004) in cyclic mares. In anestrus 0 of 6 (0%) mares, in estrus 5 of 11 (45.5%) and in diestrus 6 of 11 (54.5%) mares ovulated within a defined interval of 1 day before to 1 day after mean interval from PRID removal to ovulation. In cyclic mares, response to treatment was significantly higher when compared to anestrous mares: almost all mares responded with estrus and ovulation independent from the stage of the estrous cycle at the start of treatment. However, accuracy of synchronization was still unsatisfactory. In cyclic mares, the plasma progesterone concentrations at insertion of PRID seem to be more important for the efficacy of the treatment than the assignment to estrous cycle stages.     

Response to GnRH on day 6 of the estrous cycle is diminished as the percentage of Bos indicus breeding increases in Angus, Brangus, and Brahman x Angus heifers

Angus (n=6), Brangus (5/8 Angus x 3/8 Brahman, n=6), and Brahman x Angus (3/8 Angus x 5/8 Brahman, n=6) heifers exhibiting estrous cycles at regular intervals were used to determine if the percentage of Bos indicus breeding influenced the secretory patterns of LH in response to a GnRH treatment on Day 6 of the estrous cycle. Heifers were pre-synchronized with a two-injection PGF(2 alpha) protocol (25 mg i.m. Day -14 and 12.5 mg i.m. Day -3 and -2 of experiment). Heifers received 100 microg GnRH i.m. on Day 6 of the subsequent estrous cycle. Blood samples were collected at -60, -30, and -1 min before GnRH and 15, 30, 60, 90, 120, 150, 180, 240, 300, 360, 420, and 480 min after GnRH to determine concentrations of serum LH. Estradiol concentrations were determined at -60, -30, and -1 min before GnRH. On Day 6 and 8, ovaries were examined by ultrasonography to determine if ovulation occurred. On Day 13, heifers received 25 mg PGF(2 alpha) i.m. and blood samples were collected daily until either the expression of estrus or Day 20 for heifers not exhibiting estrus to determine progesterone concentrations. There was no effect (P>0.10) of breed on ovulation rate to GnRH as well as size of the largest follicle, mean estradiol, and mean corpus luteum volume at GnRH. Mean LH was greater (P<0.05) for Angus (7.0+/-0.8 ng/mL) compared to Brangus (4.6+/-0.8 ng/mL) and Brahman x Angus (2.9+/-0.8 ng/mL), which were similar (P>0.10). Mean LH peak-height was similar (P>0.10) for Brangus (13.9+/-3.4 ng/mL) compared to Angus (21.9+/-3.4 ng/mL) and Brahman x Angus (8.0+/-3.4 ng/mL), but was greater (P<0.05) for Angus compared to Brahman x Angus. Interval from GnRH to LH peak was similar (P>0.10) between breeds. As the percentage of Bos indicus breeding increased the amount of LH released in response to GnRH on Day 6 of the estrous cycle decreased.     

Gonadotropin-releasing hormone treatment induces follicular growth and ovulation in seasonally anestrous mares

A study was conducted to evaluate the effectiveness of gonadotropin-releasing hormone (GnRH) pulse infusion to stimulate follicular development and induce ovulation in seasonally anestrous standardbred mares. Seventeen mares were selected for use in this experiment, on the basis of a previous normal reproductive history, and were housed under a photoperiod of 8L:16D beginning one week prior to the start of the experiment (second week in January). Mares were infused with 20 micrograms (n = 7) or 2 micrograms (n = 6) GnRH/h, or were subjected to photoperiod treatment only (controls, n = 4). Serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and progesterone did not vary, and neither significant follicular development nor ovulation was observed in any control mare throughout the experimental period (greater than 60 days). By contrast, both groups of GnRH-treated mares showed elevated serum concentrations of LH and FSH within one day after the start of infusion. Mares infused with 20 micrograms GnRH/h had at least one follicle greater than or equal to 25 mm in 7.4 +/- 1.3 (mean +/- SEM) days following the start of infusion, and ovulated in 12.0 +/- 0.7 days. In the 2-microgram-GnRH/h treatment group, a 25-mm follicle was detected in 5.7 +/- 0.7 days, and ovulation occurred after 10.0 +/- 0.3 days of infusion. Ovulation in every instance was followed by a functional luteal phase, as indicated by the profiles of progesterone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Treating heifers with GnRH from 4 to 8 weeks of age advanced growth and the age at puberty

In heifer calves, an early transient increase in circulating concentrations of LH is associated with early follicular development and is thought to regulate the timing of puberty. In an attempt to hasten the onset of sexual maturity, the early rise in LH concentration was advanced by injecting heifer calves with 120 ng/kg of GnRH (n=6) twice daily from 4 to 8 weeks of age; control calves received saline (n=6). Blood samples were collected every 15 min for 10h at 4, 8, 14, 20, 26, 32, 38, 44 and 50 weeks of age. Treatment with GnRH increased mean circulating concentrations of LH at 8 weeks of age (P<0.05), LH pulse frequency at 4 and 8 weeks of age (P<0.05), and reduced the mean age at puberty by 6 weeks (56.8+/-1.7 versus 62.8+/-2.4 weeks of age, for GnRH treated and control calves, respectively; P=0.04). Body weight gain was greater in GnRH-treated calves than control calves (P<0.05), and the rate of weight gain was shown to be a significant covariate within age at puberty. In conclusion, we suggest that the timing of the early rise in LH concentrations is a critical signal involved in the timing of puberty in heifers.     

Ovarian follicles, ovulations and progesterone concentrations in aged versus young mares

The objectives of this study were: 1) to document age-related ovulation failure in mares and 2) to contrast the number of ovarian follicles, occurrence of ovulations, and postovulatory concentrations of progesterone in aged versus young mares. In Experiment 1, 4 of 10 aged (25- to 33-years-old) mares were anovulatory between July 1 and September 1, 1989. In Experiment 2, two of 25 aged (20- to 30-years-old) and none of 21 young (3- to 12-years-old) mares were anovulatory between February 1 and June 30, 1990. The average (+/- SEM) day of the first ovulation was later (P<0.05) for aged versus young mares (May 9 +/- 7.1 vs April 25 +/- 7.4 days, respectively). There tended (P<0.10) to be fewer 11- to 20-mm ovarian follicles in aged versus young mares (2.8 +/- 0.2 vs 5.3 +/- 0.1, respectively), but there was no difference (P>0.10) in the total number of ovarian follicles in aged versus young mares (21.0 +/- 0.3 vs 26.1 +/- 0.2, respectively) during the pooled periovulatory period of the first and second (single) ovulations. The number of ovulatory cycles during the study period was less (P=0.01) for aged versus young mares (2.2 +/- 0.3 vs 3.2 +/- 0.3). Plasma progesterone concentrations on Days 10 and 15 of the first ovulatory cycle were higher (P<0.05) in aged versus young mares.     

Control of follicular development and luteal function in the mare: effects of a GnRH antagonist

Control of the equine estrous cycle was studied by suppressing gonadotropin secretion by administration of a GnRH antagonist to cyclic pony mares. Four mares received vehicle (control cycle) or a GnRH antagonist, Antarelix (100 microg/kg) on Day 8 of diestrus, and blood samples were collected at 15-min intervals from 0 to 16 h, 24 to 36 h, and daily until the next ovulation. Ovarian activity was monitored by transrectal ultrasonography, and measurement of plasma concentrations of progesterone and estradiol. Antagonist treatment eliminated large diestrous pulses of LH. Progesterone concentrations had fallen significantly in all mares by the day after treatment and, in three of the four mares, remained low until luteolysis. However timing of luteolysis (ie., progesterone concentrations <1 ng/mL) was not affected by antagonist treatment. The preovulatory surges of estradiol and LH were significantly delayed in the treatment cycle, as was the appearance of a preovulatory follicle >30 mm. Cycle length was significantly longer during the treatment than the control cycle. These results show that treatment of diestrous mares with a GnRH antagonist attenuated progesterone secretion, indicating a role for LH in control of CL function in the mare, and delayed ovulation presumably because of lack of gonadotropic support.     

Effects of medroxyprogesterone acetate (MAP) on ovarian antral follicle development, gonadotrophin secretion and response to ovulation induction with gonadotrophin-releasing hormone (GnRH) in seasonally anoestrous ewes

When ovulation is induced with gonadotrophin-releasing hormone (GnRH) in anoestrous ewes, a proportion of animals fail to form normal (full-lifespan) corpora lutea (CL). Progesterone treatment before GnRH prevents luteal inadequacy. It remains uncertain whether a similar effect, achieved with medroxyprogesterone acetate (MAP) from intravaginal sponges, is mediated by influences on growing ovarian follicles and/or secretion of gonadotrophic hormones, before and after GnRH treatment. Two experiments were performed, on 13 and 11 anoestrous Western white-faced ewes, respectively. Seven and six ewes, respectively, received MAP-containing sponges (60 mg) for 14 days; the remaining ewes served as untreated controls. To test the effect of timing of GnRH administration after pre-treatment with MAP-releasing sponges, GnRH injections (250 ng every 2h for 24h followed by a bolus injection of 125 microg of GnRH i.v.) were given either immediately (Experiment 1) or 24h after sponge removal in the treated ewes (Experiment 2). Ovarian follicular dynamics (follicles reaching >or=5mm in size) and development of luteal structures were monitored using transrectal ultrasonography. In Experiment 1, the mean ovulation rate (0.7+/-0.3 and 1.0+/-0.4) and proportion of ovulating ewes (57 and 67%, respectively) did not vary (P>0.05) between MAP-treated and control ewes. Normal (full-lifespan) CL were detected in 29% of treated and 67% of control ewes (P>0.05). In Experiment 2, the mean ovulation rate (2.3+/-0.2 and 1.2+/-0.6; P<0.05) and percentage of ewes with normal (full-lifespan) CL (100 and 40%, respectively; P<0.10) were greater in the treated compared to control ewes. In Experiment 1, the mean peak concentration of the GnRH-induced LH surge was lower (P<0.05) in MAP-treated than in control ewes. There were no significant differences between MAP-treated and control ewes in the characteristics of follicular waves, mean daily serum FSH concentrations, and secretory parameters of LH/FSH, based on intensive blood sampling conducted 1 day before sponging and 1 day before sponge removal. It is concluded that treatment with MAP has no effect on the tonic secretion of LH/FSH or follicular wave development in anoestrous ewes. However, the GnRH-stimulated LH discharge was attenuated in the ewes that received MAP-impregnated sponges for 14 days and were treated with GnRH immediately after sponge withdrawal. Ovulatory response and CL formation were increased when GnRH was administered 24 h after sponge removal.     

Seasonal effects on attempts to synchronize estrus and ovulation by intravaginal application of progesterone-releasing device (PRID) in mares

To investigate seasonal effects on the efficacy of estrus synchronization in mares, we administered a progesterone-releasing device (PRID) intravaginally to eight Haflinger mares for 11 days. In January 3 of 8 mares responded to the treatment with estrus and ovulation, in March 7 with estrus and 6 of 7 mares with ovulation, in June 6 of 7 and in October 7 of 8 mares with estrus and ovulation. Follicle distribution patterns at PRID insertion were different between January/October, March/June and June/October (P<0.05). Number of follicles decreased during PRID treatment in January, March and June (difference of number of follicles at Day 12 minus number of follicles at Day 1: -4.2+/-2.7, -0.9+/-0.9 and -4.9+/-1.5 follicles), while it increased in October (3.9+/-1.2 follicles; P<0.05). Mean progesterone concentrations were lowest in January (0.3+/-0.1 ng mL(-1)) when compared with March (3.5+/-1.8 ng mL(-1); P=0.063), June (4.4+/-1.4 ng mL(-1); P<0.05) and October (2.2+/-0.9 ng mL(-1); P<0.05). At Day 2 of PRID treatment, mean progesterone concentrations significantly increased in all mares. Except from January, mean LH concentrations decreased within one day after PRID insertion and remained at low levels during treatments in January and March. Total secretion of LH during PRID-treatment was significantly lower in January and March when compared with June and October. In the 5 of 7 mares that ovulated during PRID treatment a distinct increase of plasma LH concentrations after ovulation was detected. Administration of the progesterone releasing intravaginal device PRID combined with the PGF2alpha analogue cloprostenol was able to induce estrus and ovulation in mares at different times of the year. However, efficacy of the treatment was not satisfactory concerning effectiveness in relation to season and synchrony of intervals from removal of PRID to ovulation in mares.     

Follicular growth and ovulation in postpartum beef cows following calf removal and GnRH treatment

This study investigated the effects of calf removal (CR) and gonadotrophin releasing hormone (GnRH) administration on the duration of the postpartum anoestrous period in suckled beef cows. Experiment 1 involved 20 multiparous suckled cows that were assigned to each of two treatments on Day 61 postpartum: (i) unlimited access to their calves (C; n=10) and (ii) calf removal for a period of 96 h (CR96, n=10). Experiment 2 involved 24 multiparous cows that were assigned to each of two treatments on Day 63 postpartum: (i) CR96 (n=12); and (ii) CR96 plus 250 microg of GnRH administered on the day before calf return (CR96+GnRH, n=12). Experiment 3 was a 3x2 factorial experiment, involving 48 multiparous cows assigned to the experiment on Day 58 postpartum. The factors were C, CR96 and calf removal for 144 h (CR144), and 0 or 250 microg GnRH administered on the day prior calf return. In Experiment 1, the number of cows that ovulated within 12 days of calf removal was higher (P<0.05) in CR96 group (3/9) compared to the C group (0/10). In Experiment 2, all 12 cows in the CR96+GnRH group ovulated. In contrast only 4/12 cows in the CR96 group ovulated in response to calf removal. The diameter of the ovulatory follicle tended (P=0.06) to be smaller in CR96+GnRH cows (9.8 +/- 0.3 mm) than in CR96 cows (11.3 +/- 0.9 mm). The maximum diameter attained by the corpus luteum (CL) also tended (P=0.08) to be smaller for cows in the CR96+GnRH than for cows in the CR96 group (12.1 +/- 2.4 mm versus 16.7 +/- 7.5 mm, respectively). Plasma progesterone concentrations 12 days after calf removal tended (P=0.06) to be lower in CR96+GnRH cows than in CR96 cows (0.66 +/- 0.1 ng/ml versus 2.00 +/- 1.1 ng/ml, respectively). Few cows in the CR96+GnRH group regained normal cyclical activity and the interval from onset of calf removal to conception was longer (P<0.05) compared to cows in the CR group (52.2 +/- 5.7 days versus 20.0 +/- 6.6 days). In Experiment 3, 5/8 cows on the CR144 group and all 8 cows in the CR144+GnRH group ovulated. However, the interval from CR to conception was similar for all treatments. Temporary (96-144 h) calf removal, particularly in combination with GnRH treatment, can induce a high proportion of beef cows to ovulate, but the restoration of oestrous cycles may not be achieved.