Short communication: Presynchronization using a single injection of PGF2α before synchronized ovulation and first timed artificial insemination in dairy cows

Abstract

Conception to synchronized ovulation (Ovsynch) using injections of GnRH and PGF2α and timed artificial insemination has been shown to be maximized when the program is initiated 5 to 12 d after estrus. The objective of this double-blinded field trial was to assess the effect of one injection of PGF2α, 10 d before the Ovsynch program, on the probability of pregnancy at first insemination in lactating dairy cows. The hypothesis was that cows that underwent luteolysis in response to PGF2α would be between 5 and 8 d postestrus at the start of Ovsynch. In five commercial dairy herds in Ontario, Canada, at 52 ± 12 d in milk, 506 cows were assigned at random to receive either one i.m. injection of 500 μg of cloprostenol or saline. Ten days later, all cows received 100 μg of GnRH i.m., followed in 7 d by 500 μg of cloprostenol i.m. and 100 μg of GnRH i.m. 48 h later. All cows were artificially inseminated 0 to 20 h after the second injection of GnRH, without regard to detection of estrus. Pregnancy was diagnosed by transrectal palpation at least 35 d after insemination. The probability of pregnancy after first insemination was modeled with logistic regression, accounting for the correlation of cows with herd and the effect of season of calving. There was no difference in pregnancy risk between cows that received PGF2α presynchronization and controls (37.3 and 36.6%, respectively; odds ratio = 1.03, 95% confidence interval, 0.88 to 1.20). Parity and days in milk at insemination were not significant covariates.