More male calves born after Presynch-Ovsynch protocol with 24-hour timed AI in dairy cows

Various factors including synchronization treatments have been reported to influence sex ratio of offspring in cattle. The present study was conducted to investigate the effect of treatment with Presynch-Ovsynch protocol on sex ratio of offspring in Holstein dairy cows. Healthy Holstein cows (N = 1102) were randomly assigned to Presynch-Ovsynch (N = 564) or control (N = 538) group by parity. Cows in Presynch-Ovsynch group received two administrations of PGF2α 14 days apart started at Days 23 to 27 postpartum. Twelve days after the second PGF2α treatment, Ovsynch protocol, consisting of an administration of GnRH followed by administration of PGF2α 7 days later and a final administration of GnRH given 48 hours after the third PGF2α treatment, began. Cows were subjected to fixed-time AI 24 hours after the last administration of GnRH. Cows in control group received no treatment and were inseminated 12 hours after standing estrus. Sex of calves conceived by the first service postpartum was determined after parturition and used for calculation of sex ratio. Parity, season, sire, and calving to conception interval were considered as covariates in statistical analysis. Sex ratio of calves in Presynch-Ovsynch group (1.64) was higher than that in control group (1.09; odds ratio = 1.51; P < 0.05). Moreover, male to female ratio was higher in cows conceived in summer, fall, and winter than in cows that conceived in spring (P < 0.05). In conclusion, cows treated with Presynch-Ovsynch protocol and inseminated 24 hours after the last GnRH administration of Ovsynch had a higher sex ratio than cows inseminated 12 hours after standing estrus.