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    • The timing and growth of follicles relative

    2022-03-18

    The timing and growth of follicles relative to follicle deviation was assessed in 49 cows (Fig. 4) that had NFWE following treatment onset and that had not already established dominance. Because day of deviation did not differ due to treatment, day of estrous cycle, or replicate, data are presented as pooled means. Deviation of the dominant follicle was observed 3.3 ± 0.13 days after emergence and averaged 7.61 ± 0.23 mm the day before and 9.5 ± 0.2 mm on the day of deviation. Size of the largest subordinate follicle on the day before deviation was 6.39 ± 0.23 mm and averaged 5.78 ± 0.22 mm on the day of deviation. Mean serum concentration of LH following treatment with GnRH-1 (Bee Synch It) or saline (Bee Synch IIt) are illustrated in Fig. 5. As expected, mean concentrations were greater (P <  0.0001) after GnRH in Bee Synch It than for Bee Synch IIt after saline. Peak serum concentrations of LH at 120 min after GnRH injection on day 3 of the estrous cycle were greater (4.28 ± 0.71 ng/mL; P <  0.05) than on days 7 (2.70 ±0.23 ng/mL) and 10 (1.86 ± 0.22 ng/mL) of the cycle, respectively.
    Discussion Results of the current experiment indicate that, although GnRH-1 resulted in synchronized NFWE in 68.6% of Bee Synch It protocol between 1and 4 days after injection, compared to 38.9% for the Bee Synch IIt protocol, its use failed to increase size of the dominant follicle or frequency of estrus at 66 h after CIDR removal, or to decrease mean interval to ovulation. Furthermore, greater than 90% of Bee Synch It-treated cows had a synchronized NFWE if this interval was expanded to include days 0–4 after treatment, compared to 72.2% for Bee Synch IIt. Results in earlier reports from our group indicate that use of protocols such as Ovsynch (Williams et al., 2002), 7-Day CO-Synch + CIDR (Saldarriaga et al., 2007; Zuluaga et al., 2010), and 5-Day CO-Synch + CIDR (Williams et al., 2011, 2013) fail to yield consistent and acceptable FTAI pregnancy rates in Bos indicus-influenced beef cows. This is in ryanodine to results in straightbred Bos taurus cows where FTAI pregnancy rates of 50% or greater have been reported consistently, as reviewed by Lamb and Mercadante (2016). Thus, modifications were made in the 5-Day CO-Synch + CIDR regimen to address this issue and initially included the addition of PGF on Day 0 of the protocol (Bee Synch I; Williams et al., 2011, 2013) to presumptively lower overall circulating P4 in a majority of cycling cows during the estrous synchronization period. We had hypothesized that Bos indicus-influenced cattle might be more sensitive to the negative feedback effects of P4 on secretion of LH, thus delaying maturation of the dominant follicle. Use of this strategy (Bee Synch I) resulted in FTAI pregnancy rates of 50% or greater relatively consistently in these types of cattle using a CIDR in combination with GnRH and PGF (Williams et al., 2011). Those studies were followed by preliminary field trials in which Bee Synch It was modified further by eliminating GnRH on Day 0. By default, the latter obviated the need for the double dose of PGF at Day 5 (Bee Synch II; Williams et al., 2015), since no new CL were induced by GnRH on Day 0. The basis of this modification was from earlier observations (Saldarriaga et al., 2007; Zuluaga et al., 2010) that ovulation rates to GnRH in mature, randomly estrous-cycling Bos indicus-influenced cows were often low and highly variable. Therefore, we tested the hypothesis that FTAI pregnancy rates would be similar between Bee Synch I (full protocol) and Bee Synch II (modified to eliminate GnRH on Day 0). This hypothesis was similar to that of Cruppe et al. (2014) in which elimination of GnRH on Day 0 in the standard 5-day CO-Synch + CIDR regimen using straight Bos taurus heifers resulted in pregnancy rates that did not differ from the original regimen that included GnRH. Although no statistically significant differences in FTAI pregnancy rates between Bee Synch I and II protocols have been observed in preliminary field trials, intensive studies to examine ovarian events associated with onset of treatments relative to stage of the follicle wave/estrous cycle have not been performed. Indeed, the inclusion of GnRH-1 in the current study resulted in measurable effects on synchrony of NFWE during the first 4 days after treatment onset. However, at 66 to 72 h after CIDR removal, the time at which mature cows estrous synchronized with both Bee Synch I and II are inseminated in a FTAI setting, there were no measurable differences due to treatment in follicle characteristics known to affect FTAI pregnancy rates.