Effects of gonadotropin-releasing hormone and human chorionic gonadotropin on pregnancy rate in dairy cattle.

This study was to determine if gonadotropin-releasing hormone, human chorionic gonadotropin, or their interaction would affect pregnancy rate or milk progesterone profiles in dairy cattle. Cows and heifers eligible for first, second, and third services were assigned to four treatments: 1) saline i.m. within 5 min after AI (d 0) and saline 15 d after AI (n = 222 AI); 2) gonadotropin-releasing hormone (100 micrograms) i.m. within 5 min after AI and saline on d 15 (n = 223); 3) saline after AI and human chorionic gonadotropin (3500 IU) i.m. on d 15 (n = 196); 4) gonadotropin-releasing hormone at AI and human chorionic gonadotropin on d 15 (n = 195). Progesterone was quantified in milk samples collected twice weekly for 5 wk from all cows. Neither gonadotropin-releasing hormone, human chorionic gonadotropin, gonadotropin-releasing hormone x human chorionic gonadotropin interaction, age, nor service number affected pregnancy rate, which averaged 46.2%. Pregnancy and human chorionic gonadotropin affected milk progesterone profiles. Pregnancy maintained progesterone concentrations and human chorionic gonadotropin on d 15 increased them. This study does not support the general use of gonadotropin-releasing hormone at AI or human chorionic gonadotropin 15 d after AI as methods for enhancing pregnancy rates of dairy cattle.