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  • New follicular wave emergence is associated closely

    2021-10-25

    New follicular wave emergence is associated closely with the stage of the estrous kinetin (Sirois and Fortune, 1988). As indicated by results from the present experiment, cows that had regression of the largest follicle on Day 0 of treatment onset had follicle wave characteristics that were consistent with those described by Sirois and Fortune (1988) and Ginther et al. (1989). Based on these previous observations, NFWE occurs approximately on the day of ovulation and again on about the 10th day of the estrous cycle in cows with two follicular waves and on days 0, 9, and 15 of the estrous cycle for cows with three follicular waves. Cows with initiation of Bee Synch It or IIt treatments 3 days after ovulation had a delayed occurrence of NFWE at approximately 4.3 ± 0.3 days, which represents day 7.3 ± 0.3 of the estrous cycle. These responses are markedly consistent with the timing of the natural second follicular wave. The same pattern was observed for cows beginning both treatments on day 7, where NFWE occurred on average at 1.4 ± 0.2 days after treatment onset. This coincides with day 8.4 ± 0.2 of the estrous cycle. For day 10 of the estrous cycle, average interval to NFWE was 0.5 days. Ovulation after CIDR removal was detected in 21% of all cows (15/71) before 72 h (average of 68 ± 4 h), of which 80% (12/15) occurred in cows starting treatment on day 10 of the estrous cycle and 83.3% (10/12) of these were observed in Bee Synch IIt-treated cows. This suggests that treatment with GnRH at treatment onset in Bee Synch It reduced the incidence of earlier ovulation after CIDR removal, which could have a negative impact on fertility by loss of oocyte viability if FTAI is conducted at 72 h. This risk can potentially be decreased at the proposed FTAI at 66 h after CIDR removal. Overall mean concentrations of serum P4 during CIDR insertion for both treatments were similar, followed by a decline to below 1 ng/mL in all cows after CIDR removal and PGF. Because either naturally-occurring ovulations or ovulations induced by GnRH on Day 0 of 5- day protocols result in CL that do not respond to standard doses of PGF on Day 5, studies have been conducted to determine efficacy of different doses and split application. Nascimento et al. (2014) tested single (25 mg), double (50 mg), and double-split (two 25-mg injections 8 h apart) injection of PGF on Day 5 in Holstein cows. Results showed a dramatic superiority in efficacy of either the double or double-split doses of PGF compared to the single dose for regressing 5-day CL. In field trials, Rabaglino et al. (2010) observed no difference in pregnancy rate of dairy heifers estrous-synchronized using either single or double doses of PGF at Day 5 with the standard 5-day protocol. However, incomplete luteolysis has also been reported even when the double dose (50 mg of PGF) is applied (Nascimento et al., 2014). In the current study, because there was no induction of ovulation with GnRH after CIDR removal, most cows did not ovulate for 4–7 days after CIDR removal. Thus, serum concentrations of P4, on average, remained low during the blood sampling period following CIDR removal and leading up to spontaneous ovulation. However, serum P4 in the five cows that ovulated after GnRH-1 in the Bee Synch It group, while decreasing to < 1 ng/ml initially, had a resurgence beginning 2 days after CIDR removal and PGF. Thus, as observed in Fig. 3, it appears that a suboptimal response to PGF in these 5-day-old CL resulted in incomplete functional luteolysis (Lauderdale et al., 1974; Macmillan, 1983). Nonetheless, the temporary 3-day suppression of P4 in those 5 cows permitted final maturation of the dominant follicle and subsequent ovulation of an ovulatory follicle. Therefore, cows in this category would actually bear 2 CL following the synchronized ovulation, both of which would be maintained if cows were bred and became pregnant. In summary, results of the present experiment indicate that greater synchronization of NFWE and a reduced incidence of early ovulations with use of the Bee Synch It protocol provide a mechanistic basis for improved fertility at FTAI using Bee Synch I vs. II. However, in spite of these observations, GnRH-1 did not enhance the synchronized development of a dominant follicle at 66 h after CIDR removal, the time at which FTAI is employed for both Bee Synch I and II in Bos indicus-influenced beef cows. Consistent with these findings, no differences in FTAI pregnancy rates between Bee Synch I and II have been observed to date. A full report of field data comparing the two methodologies is in preparation (Williams and Stanko, unpublished).