Research of Kojima et al DeJarnette et al and
Research of Kojima et al. (2000); DeJarnette et al. (2001), and Saldarriaga et al. (2007) inferred that synchronization of NFWE is related to ovulation response after GnRH-1. In the research of Saldarriaga et al. (2007) with Bos indicus-influenced beef cows, ovulation rate was 40% and estrous synchronization rate was greater in cows having ovulations after GnRH-1 administration (88%) compared to those not having ovulations (42%). Vasconcelos et al. (1999) suggested that ovulation rates following GnRH in growing, dominant, and regressing phases of dominant follicles (days 3, 7, and 10 of the Swainsonine australia in the present experiment) vary because of differences in ovulatory capacity related to the number of LH receptors. In this previous experiment, administration of GnRH resulted in ovulation in 23%, 96%, 54% and 77% of cows when treatment was administered on days 1–4, 5–9, 10–16, and 17–21 of the estrous cycle, respectively. Sartori et al. (2001) made similar observations, reporting a low rate of ovulation for day 3 (0%) and a high rate of ovulation for day 6 (100%) of the estrous cycle, respectively. Conversely, in Bos taurus beef heifers, Martinez et al. (1999) administered GnRH on days 3, 6 and 9 of the estrous cycle and obtained ovulation in 89%, 56% and 22%, respectively. Martinez et al. (1999) indicated that all heifers with a dominant follicle larger than 9 mm ovulated in response to GnRH, which supports more recent work by Ginther et al. (2016) in which follicle deviation was shown to occur at a diameter of 8.5 mm, and by Sartori et al. (2001), where ovulatory capacity was achieved immediately after deviation. In our study, mean largest follicle size on Day 0 of treatment onset for Bee Synch It (GnRH-1) was 11.1 mm ± 0.2 and deviation of the synchronized NFW in both treatments was achieved at 9.5 ± 0.2 mm. Therefore, ovulation after GnRH-1 treatment was expected to occur in cows with follicles larger than 9.5 mm in diameter already present at treatment onset. Sartori et al. (2001) has also indicated that ovulatory capacity is dependent upon both the amount of LH released and diameter of the dominant follicle. The mean peak concentration of LH observed at 120 min after GnRH-1 on day 10 of the estrous cycle in Bee Synch It-treated cows in the current study was 1.86 ng/mL compared to 4.3 ng/mL on day 3 of the estrous cycle. Such differences are consistent with observations by Stevenson and Pulley (2016) where follicle size, presence of a CL, and concentrations of estradiol and P4 had major effects on the magnitude of GnRH-induced release of LH in lactating dairy cows. In addition, maximum concentrations of LH observed following GnRH in the current study were less than overall mean values observed previously (Saldarriaga et al., 2007), which may explain the relatively lesser ovulation response after GnRH-1 in this study. Interestingly, the lack of any ovarian response to GnRH-1 in the current study was 10-fold less (2.9%) than that reported by Zuluaga et al. (2010; ˜29%), as well as Saldarriaga et al. (2007) and Barros et al. (2000) where about 21% of Bos indicus-influenced cows failed to have any follicular response to GnRH-1 on Day 0 of 7-day CO-Synch + CIDR protocols. Nonetheless, estrous-synchronized NFWE after GnRH-1 or saline in the present experiment was quite high (72% for days 1–4 and 94.3% for days 0–4 after treatment), which is consistent with the findings of Twagiramungu et al. (1995) and Martínez et al. (2000), both of which indicated that both follicle regression and ovulation are equally effective for inducing NFWE. Mean size of the dominant follicle at 66 h after CIDR removal for cows in the present study was always greater than 11.5 mm except for individuals starting treatment on day 3 of the cycle, regardless of treatment. Sartori et al. (2001) reported that 11.5 mm is the apparent threshold size for optimal fertility after ovulation in Holstein cows. This observation is consistent with the findings of Perry et al. (2005), who reported that induction of ovulation in follicles smaller than 11 mm resulted in a significant lowering of pregnancy rates in Bos taurus females. Lowered fertility as a consequence of smaller follicles occurs because the oocyte is physiologically-immature, resulting in increased embryonic loss. Moreover, because smaller follicles generate smaller CL, concentrations of P4 are reduced during the luteal phase. Thus, based on dominant follicle size, it is possible to infer that cows in the present experiment could have been induced to ovulate with GnRH at 66 h after CIDR removal without negative effects on ovulation rate or fertility, except for cows starting Bee Synch II treatment on day 3 of the estrous cycle.