An analysis addressing factors affecting risk of ovulation, pregnancy, and pregnancy loss in lactating dairy cows submitted to fixed-time artificial insemination programs with special reference to the diameter of the ovarian follicle.

Because reduced fertility in studies resulted from GnRH-induced ovulation of small or large follicles in lactating dairy cattle, the objective was to determine the association of follicle diameter and risks of ovulation and pregnancy per artificial insemination (P/AI). An analysis was conducted of data from 14 studies in 82 herd-year-seasons in which ovulation was synchronized before a fixed-time AI. Follicle diameter was included as a continuous variable in a random-effects mixed model that used herd-year-season as a random surrogate for study. Effects of presynchronization, parity, cycling status, body condition score, occurrence of estrus, service sire, and progesterone were included as appropriate in initial models plus all two-way interactions. Ovulation risk after first GnRH treatment of Ovsynch increased (P < 0.001) with increasing follicle diameter. Ovulation risk after the breeding GnRH treatment increased (P = 0.03) in multiparous but not in primiparous cows as follicle diameter increased. Pregnancy per AI at d 28-37 and at d 56-84 was not associated with follicle diameter but was affected by presynchronization treatment (P = 0.03), ovulation after the breeding GnRH treatment (P = 0.003), parity (P < 0.001), prebreeding cycling status (P < 0.001), and progesterone (P < 0.001) before AI. As follicle diameter before AI increased, pregnancy loss by d 56-84 after AI decreased (P = 0.006), especially in cows of greater body condition. These results do not support any effect of follicle size on P/AI in cows whose ovulation is synchronized before AI, but less pregnancy loss was associated with increasing follicle diameter.