Diameter of the ovulatory follicle at timed artificial insemination as a predictor of pregnancy status in lactating dairy cows subjected to GnRH-based protocols

Abstract

Ovulation of an early-stage dominant follicle induced by exogenous GnRH reportedly results in a reduced ovulatory follicle size and fertility in beef and dairy cattle. Objectives were to examine factors associated with ovulatory follicle diameter and the relationship between ovulatory follicle diameter and pregnancy per artificial insemination (P/AI) and pregnancy loss in lactating dairy cows subjected to GnRH-based programs for timed artificial insemination (TAI). A total of 1048 ovulations from 1576 breedings of cows that had or had not been presynchronized and subjected to a 5- or a 7-day GnRH-based protocol with or without progesterone supplementation between initial GnRH and PGF2α were analyzed. Ultrasonography was used to determine cyclicity, ovulation, and pregnancy status and to measure diameter of the ovulatory follicle at the time of TAI. Mean (±standard error) diameter of the ovulatory follicle was 16.4 ± 0.1 mm (range, 11–25 mm; n = 1048); the most common diameters were 15 (n = 170), 16 (n = 169), and 17 mm (n = 170). Multiparous cows and those subjected to a 7-day protocol had larger ovulatory follicles (P < 0.05). On average, multiparous cows that did not ovulate after the initial GnRH had the largest ovulatory follicles (17.0 ± 0.1 mm; n = 290), whereas all cows that ovulated after the initial GnRH and were subjected to a 5-day GnRH-based protocol had the smallest ovulatory follicles (15.6 ± 0.2 mm; n = 207). The associations between ovulatory follicle diameter and P/AI at 32 and 60 days after TAI were not significant. However, a significant linear relationship between ovulatory follicle diameter and pregnancy loss between 32 and 60 days after TAI was observed, with an increased (P < 0.05) probability of pregnancy loss in cows with an ovulatory follicle greater than 20 mm in diameter. In summary, the diameter of the ovulatory follicle preceding TAI was affected by interactions between ovulatory response to initial GnRH and parity and between ovulatory response to initial GnRH and the length of ovulation synchronization protocol. Ovulatory follicle diameter was not associated with P/AI at 32 or 60 days, but large ovulatory follicle size was identified as a predictor of pregnancy loss in lactating dairy cows subjected to GnRH-based protocols for TAI.