New Insights into Photoperiod Effects on Double Ovulation Rates in Dairy Cattle. Influence of Estrous Cycle Length on such Effects

This study was designed to identify factors, including estrous cycle length, affecting the incidence of double ovulations in dairy cows. The study population comprised 748 primiparous cows undergoing spontaneous estrous cycles that had undergone their first postpartum artificial insemination following the second observed estrus. A subset of 341 cows with inter-estrus intervals (IEIs) of 18 to 30 days were selected to investigate the impacts of IEI, which was classified as normal (18–23 days) or lengthened (24–30 days). The odds ratio (OR) for double ovulations was 0.82 (p <0.0001) for each unit increase in genomic prediction values for twin pregnancies, 2.3 (p <0.0001) for cows inseminated during the negative photoperiod, compared to cows inseminated during the positive photoperiod, and 2.8 (p = 0.02) for cows with lengthened cycles, compared to cows with normal cycles. The OR for lengthened IEIs was 0.91 (p = 0.001) for each unit increase in genomic prediction value, and 4.4 (p <0.0001) for cows inseminated during the negative photoperiod. In conclusion, lengthened estrous cycles were associated with double ovulations, genomic prediction values were able to identify the risk of lengthened cycles, and the negative photoperiod was found to favor both lengthened cycles and double ovulations.