Estradiol Cypionate did not Affect Pregnancy Rates of Beef Cattle Synchronized with a Modified Timed Artificial Insemination Protocol

A study was conducted to determine the effect of inclusion of estradiol cypionate (ECP) in a conventional gonadotropin-releasing hormone (GnRH)-prostaglandin F2α (PGF2α)-GnRH-timed AI protocol (Cosynch) on conception and pregnancy rates in beef cattle. The objective was to determine if administration of 0.25 mg of ECP at the time of the second GnRH injection would effectively increase conception to AI and improve pregnancy rates. Cows were presynchronized with an i.m. injection of 25 mg of PGF2αon d 14. On d 0, Cosynch (COS) was initiated with administration of 100 μg GnRH (i.m.) followed 7 d later with 25 mg of PGF2α (i.m.). On d 9 cows were paired by age, BCS, and BW and assigned randomly to receive either GnRH (100 g) + 0.25 mg ECP (COSECP; n = 90) or GnRH (100 μg) + vehicle (COS-C; n = 93). Following treatment, all cows were immediately inseminated by a single inseminator. Pregnancy status was determined by ultrasonography 35 and 70 d postinsemination. Data were analyzed by logistic regression. Pregnancy per AI (53.3% COS-ECP vs. 57.0% COS-C) and 70-d pregnancy rates to AI and natural service (92.2% COS-ECP vs. 90.3% COS-C) did not differ between treatments. Days postpartum had no effect on the odds ratio of pregnancy. The results indicate that the incorporation of 0.25 mg of ECP into the Cosynch protocol did not affect pregnancy per AI and 70-d pregnancy rates in beef cattle.