Photoperiod length and the estrus synchronization protocol used before AI affect the twin pregnancy rate in dairy cattle

This study addresses potential management risk factors affecting the incidence of twin pregnancies in high-producing dairy cows. Special attention was paid to the estrus synchronization protocol used before the AI resulting in pregnancy. Possible factors affecting the twin pregnancy rate were analyzed through binary logistic regression procedures on 2015 pregnant cows from July 2010 to July 2011. Twin pregnancy was recorded in 361 of the 2015 pregnancy diagnoses made (17.9%). Twin pregnancy rates differed among herds (P < 0.001) and ranged from 12.4% to 23.9%. Based on the odds ratios, the risk of twin pregnancy was reduced by factors of 0.65 or 0.71 when AI was performed during the warm season or an increasing photoperiod, respectively and increased by a factor of 1.11 for each unit increase in lactation number; by factors of 4.57 or 6.33 in cows that received a progesterone-releasing intravaginal device (PRID) plus 500 or 750 IU of equine chorionic gonadotropin (eCG) 28 days before the pregnancy AI, respectively; by a factor of 2.39 in cows with an ovarian cyst diagnosed in the 14 days prior to AI and treated with prostaglandins (PG); by factors of 1.94 or 3.91 in cows that received two PG doses during the 14 days prior to AI or cows that following failed PRID treatment had received PG started over the 28 days prior to AI, respectively; and by a factor of 2.58 in cows that had previously delivered twins compared to cows delivering singletons. Our results indicate that cow factors, such as lactation number and previous twining, as well as environmental factors, such as photoperiod and season and management related to synchronization protocols affect significantly the incidence of twin pregnancies.