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 ... -  《-》

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 《-》

Synchronization rate, size of the ovulatory follicle, and pregnancy rate after synchronization of ovulation beginning on different days of the estrous cycle in lactating dairy cows.

Vasconcelos JL ，Silcox RW ，Rosa GJ ，Pursley JR ，Wiltbank MC ... -  《THERIOGENOLOGY》

Comparison of protocols to synchronize estrus and ovulation in estrous-cycling and prepubertal beef heifers.

The objective of the experiment was to compare follicular dynamics, ovulatory response to GnRH, and synchrony of estrus and ovulation among estrous-cycling and prepubertal beef heifers synchronized with a controlled internal drug-release (CIDR)- based or GnRH-PGF(2alpha) (PG) protocol. Estrous-cycling beef heifers were randomly assigned to 1 of 4 treatments (C1, C2, C3, C4), and prepubertal beef heifers were randomly assigned to 1 of 2 treatments (P1, P2) by age and BW. Blood samples were taken 10 and 1 d before treatment to confirm estrous cyclicity status (progesterone > or =0.5 ng/mL estrous cycling). The CIDR Select (C1, n = 12; P1, n = 14)-treated heifers received a CIDR insert (1.38 g of progesterone) from d 0 to 14, GnRH (100 microg, i.m.) on d 23, and PG (25 mg, i.m.) on d 30. Select Synch + CIDR (C2, n = 12; P2, n = 11)-treated heifers received a CIDR insert and GnRH on d 23 and PG at CIDR removal on d 30. The CIDR-PG (C3, n = 12)-treated heifers received a CIDR insert on d 23 and PG at CIDR removal on d 30. Select Synch (C4, n = 12)-treated heifers received GnRH on d 23 and PG on d 30. HeatWatch transmitters were fitted at CIDR removal (C1, C2, C3, P1, and P2) or at GnRH administration (C4) for estrus detection. Ultrasound was used to determine the response to GnRH and the timing of ovulation after estrus. Among the estrous-cycling heifers, ovulatory response to GnRH and estrous response did not differ (P > 0.05). Among the prepubertal heifers, more (P = 0.02) P1 heifers responded to GnRH than P2 heifers, but estrous response did not differ (P > 0.05). Among the estrous-cycling heifers, variance for interval to estrus after PG was reduced (P < 0.05) for C1 compared with each of the other treatments, and C3 [corrected] was reduced (P < 0.05) compared with C2 [corrected] Variance for interval to ovulation after PG was reduced (P < 0.05) for C1 compared with each of the other treatments. Among the prepubertal heifers, there was no difference (P > 0.05) in variance for interval to estrus or ovulation. Results from C1 and P1 (T1) and C2 and P2 (T2) were combined to compare T1 and T2 among mixed groups of estrous-cycling and prepubertal heifers. Response to GnRH was greater (P < 0.01; 81% T1 and 39% T2), and variances for interval to estrus and ovulation for T1 were reduced (P < 0.01) compared with T2. In summary, CIDR Select improved (P < 0.01) the synchrony of estrus and ovulation compared with Select Synch + CIDR.

Leitman NR ，Busch DC ，Bader JF ，Mallory DA ，Wilson DJ ，Lucy MC ，Ellersieck MR ，Smith MF ，Patterson DJ ... -  《-》

Influence of premature induction of a luteinizing hormone surge with gonadotropin-releasing hormone on ovulation, luteal function, and fertility in cattle.

Mussard ML ，Burke CR ，Behlke EJ ，Gasser CL ，Day ML ... -  《-》