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.

Colazo MG ，Kastelic JP ，Davis H ，Rutledge MD ，Martinez MF ，Small JA ，Mapletoft RJ ... -  《DOMESTIC ANIMAL ENDOCRINOLOGY》

Progesterone concentration, estradiol pretreatment, and dose of gonadotropin-releasing hormone affect gonadotropin-releasing hormone-mediated luteinizing hormone release in beef heifers.

We examined whether progesterone (P4)-induced suppression of LH release in cattle can be overcome by an increased dose of exogenous gonadotropin-releasing hormone (GnRH) or pretreatment with estradiol (E2). In Experiment 1, postpubertal Angus-cross heifers (N = 32) had their 2 largest ovarian follicles ablated 5 d after ovulation. Concurrently, these heifers were all given a once-used, intravaginal P4-releasing insert (CIDR), and they were randomly assigned to be given either prostaglandin F(2alpha) (Low-P4) or no treatment (High-P4) at follicle ablation, and 12 h later. Six days after emergence of a new follicular wave, half of the heifers in each group (n = 8) were given either 100 or 200 microg of GnRH i.m. Plasma luteinizing hormone (LH) concentrations were higher in the Low- vs High-P4 groups, and in heifers given 200 vs 100 microg of GnRH (mean +/- SEM 15.4 +/- 2.2 vs 9.1 +/- 1.2, and 14.8 +/- 2.1 vs 9.8 +/- 1.4 ng/mL, respectively; P < or = 0.01). Ovulation rate was higher (P = 0.002) in the Low-P4 group (15/16) than in the High-P4 group (6/16), but it was not affected by GnRH dose (P = 0.4). In Experiment 2, heifers (n = 22) were treated similarly, except that 5.5 d after wave emergence, half of the heifers in each group were further allocated to be given either 0.25 mg estradiol benzoate i.m. or no treatment, and 8 h later, all heifers were given 100 microg GnRH i.m. Both groups treated with E2 (Low- and High-P4) and the Low-P4 group without E2 had higher peak plasma LH concentrations compared to the group with high P4 without E2 (12.6 +/- 1.8, 10.4 +/- 1.8, 8.7 +/- 1.3, and 3.9 +/- 1.2 ng/mL, respectively; (P < 0.04)). However, E2 pretreatment did not increase ovulation rates in response to GnRH (P = 0.6). In summary, the hypotheses that higher doses of GnRH will be more efficacious in inducing LH release and that exogenous E2 will increase LH release following treatment with GnRH were supported, but neither significantly increased ovulation rate.

Dias FC ，Colazo MG ，Kastelic JP ，Mapletoft RJ ，Adams GP ，Singh J ... -  《-》

Effect of the timing of controlled internal drug-releasing device insertion on the gonadotropin-releasing hormone-induced luteinizing hormone surge and ovulatory response.

Concentrations of progesterone have been reported to influence GnRH-induced LH surges. At the beginning of many synchronization protocols, GnRH is used to synchronize follicular growth. Therefore, the objective of this study was to determine the effect of elevated concentrations of progesterone from a controlled internal drug-releasing device (CIDR) on the GnRH-induced LH surge and ovulatory response. Angus-cross beef heifers (n = 113; 41 pubertal and 72 prepubertal) were assigned to 1 of 3 treatments: 1) GnRH at CIDR insertion (CIDR-0), 2) GnRH 6 h before CIDR insertion (CIDR-6), or 3) GnRH 48 h after CIDR insertion (CIDR+48). Follicle size was determined before GnRH administration, and ovulatory response was determined 2 d later. Blood samples were collected from a subset of 60 heifers at -30, 0 (GnRH administration), 30, 60, 90, 120, 150, 180, 210, 240, 300, and 360 min after GnRH. Heifers receiving CIDR+48 had greater (P < 0.01) concentrations of progesterone compared with those receiving CIDR-0 and CIDR-6. There was no difference (P > 0.76) between treatments in concentrations of estradiol. There tended to be a cycling status x ovulation interaction on concentrations of progesterone (P = 0.11), and there was a cycling status x ovulation interaction on concentrations of estradiol (P = 0.02). The estradiol-to-progesterone ratio was significant because of treatment (P = 0.002), cycling status (P = 0.001), and a treatment x cycling status interaction (P = 0.02). Cycling status tended (P = 0.11) to have an influence on ovulation (29/41 and 42/72 for pubertal and prepubertal heifers). Ovulation was induced in more (P < 0.05) CIDR-0 (26/38) and CIDR-6 (28/37) heifers than CIDR+48 (17/38) heifers. There was no influence of treatment (P = 0.19), concentrations of estradiol (P = 0.90), or the estradiol-to-progesterone ratio (P = 0.21) on concentrations of LH, but there was an effect (P < 0.01) of progesterone on LH concentrations. Heifers with elevated progesterone at GnRH administration had a reduced LH surge compared with heifers with decreased concentrations of progesterone. Heifers that ovulated tended to have a greater (P = 0.11) magnitude of LH surge than heifers that did not ovulate. In summary, elevated concentrations of progesterone at GnRH administration decreased the GnRH-induced LH surge, and heifers in the CIDR+48 treatment had a decreased ovulatory response. However, there tended to be a difference in the magnitude of the LH surge only between heifers that did and did not ovulate.

Perry GA ，Perry BL 《-》

Influence of inducing luteal regression before a modified controlled internal drug-releasing device treatment on control of follicular development.

At the initiation of most controlled internal drug-releasing (CIDR) device protocols, GnRH has been used to induce ovulation and reset follicular waves; however, its ability to initiate a new follicular wave is variable and dependent on stage of the estrous cycle. The objectives of the current studies were to determine 1) if inducing luteal regression before the injection of GnRH at time of insertion of a CIDR resulted in increased control of follicular development, and 2) if removing endogenous progesterone by inducing luteal regression before insertion of the CIDR decreased variation in LH pulse frequency. In Exp. 1 and 2, Angus-cross cycling beef heifers (n = 22 and 38, respectively) were allotted to 1 of 2 treatments: 1) heifers received an injection of PGF(2α) on d -3, an injection of GnRH and insertion of a CIDR on d 0, and a PGF(2α) injection and CIDR removal on d 6 (PG-CIDR) or 2) an injection of GnRH and insertion of a CIDR on d 0 and on d 7 an injection of PGF(2α) and removal of CIDR (Select Synch + CIDR). In Exp. 3, Angus-cross beef heifers (n = 15) were assigned to 1 of 3 treatments: 1) PG-CIDR; 2) PGF(2α) on d -3, GnRH on d 0, and PGF(2α) on d 6 (PG-No CIDR); or 3) Select Synch + CIDR. Follicular development and ovulatory response were determined by transrectal ultrasonography. Across all experiments, more (P = 0.02) heifers treated with PG before GnRH initiated a new follicular wave after the injection of GnRH compared with Select Synch + CIDR-treated heifers. In Exp. 1, after CIDR removal, interval to estrus did not differ (P = 0.18) between treatments; however, the variance for the interval to estrus was reduced (P < 0.01) in PG-CIDR heifers compared with Select Synch + CIDR heifers. In Exp. 3, there was a tendency (P = 0.09) for LH pulse frequency to be greater among PG-CIDR and PG-No CIDR compared with the Select Synch + CIDR, but area under the curve, mean LH concentrations, and mean amplitude did not differ (P > 0.76). In summary, induction of luteal regression before an injection of GnRH increased the percentage of heifers initiating a new follicular wave. Removal of endogenous progesterone tended to increase LH pulse frequency, and the modified treatment increased the synchrony of estrus after CIDR removal.

Grant JK ，Abreu FM ，Hojer NL ，Fields SD ，Perry BL ，Perry GA ... -  《-》

Gonadotropin-releasing hormone-induced ovulation and luteinizing hormone release in beef heifers: effect of day of the cycle.

The COSynch protocol has been used to synchronize ovulation and facilitate fixed-time AI in beef cattle. Establishment and maintenance of pregnancy was negatively affected, in previous studies, by GnRH-induced ovulation of small dominant follicles (</=11 mm). The reason for the presence of small follicles at the second GnRH (GnRH 2) is not clear. The objectives of this study were 1) to determine the effect of ovulatory response at the first GnRH (GnRH 1) on diameter and variation in diameter of the largest follicle at GnRH 2, and 2) to determine the effect of day of the cycle (stage of a follicular wave) on GnRH-induced luteinizing hormone (LH) release, and the resulting ovulatory response after GnRH 1 and 2. Two experiments used pubertal beef heifers synchronized to be on different days of the estrous cycle (d 2, 5, 10, 15, and 18 after estrus) in which a dominant follicle would or would not respond to GnRH 1. Ovulatory response to GnRH 1 did not affect size or variation in diameter of the largest follicle at GnRH 2 in Exp. 1 or 2. In Exp. 1, ovulatory response after GnRH 1 (0/14(a), 12/13(b), 4/13(ac), 9/13(bc), and 2/10(a) in the d 2, 5, 10, 15, and 18 groups; (a-c)P < 0.05) and GnRH 2 (13/14(a), 12/13(a), 12/13(a), 2/13(b), and 2/10(b) in the d 2, 5, 10, 15, and 18 groups, respectively; (a,b)P < 0.05) was affected by day of the cycle. In Exp. 2, day of the cycle also affected the proportion of heifers ovulating after GnRH 1 (0/7(a), 8/8(b), 0/6(a) 5/8(ab), and 5/8(ab) of the d 2, 5, 10, 15, and 18 heifers, respectively; (a-c)P < 0.05) and GnRH 2 (3/7(ab), 8/8(b), 5/6(b), 1/8(a), and 2/8(a) of the d 2, 5, 10, 15, and 18 heifers, respectively; (a,b)P < 0.05). In both experiments, heifers receiving GnRH 1 on d 15 and 18 had a greater (P < 0.05) occurrence of luteolysis before PGF(2alpha) injection and expression of estrus than heifers treated on d 2, 5, and 10. The GnRH-induced LH surge was of greatest magnitude in heifers receiving GnRH 1 on d 18 of the cycle followed by d 5, 15, 10, and 2 (9,054(b), 5,774(bc), 4,672(c), 2,548(c), and 915(d) arbitrary units; respectively; (a-d)P < 0.05). In summary, ovulatory response to GnRH 1 did not affect size of the dominant follicle at GnRH 2. Day of the cycle when GnRH 1 was delivered affected dominant follicle size at GnRH 2. Treatment with GnRH 1 in the earlier part of the estrous cycle (on or before d 10) increased the proportion of dominant follicles that were large enough to respond to GnRH 2 (>/=10 mm) and increased ovulatory response after GnRH 2.

Atkins JA ，Busch DC ，Bader JF ，Keisler DH ，Patterson DJ ，Lucy MC ，Smith MF ... -  《-》