Optimizing timed AI protocols for Angus beef heifers: Comparison of induction of synchronized ovulation with estradiol cypionate or GnRH

This study compared estradiol cypionate (ECP) or GnRH as ovulation inducers at the end of a timed AI (TAI) protocol in Angus heifers. On day 0, heifers (n = 415), between 22 and 24 months of age, were treated with an intravaginal 1 g progesterone (P4) insert and 2 mg of estradiol benzoate. On day 8, heifers had P4 removed, received 500 μg cloprostenol, and were randomized into two groups: ECP [n = 214; 0.5 mg of ECP on day 8] or GnRH [n = 201; 25 μg of GnRH analog licerelin acetate on day 10]. All heifers received TAI on day 10; 48–50 h after P4 insert withdrawal. Estrus was determined by removal of tail paint. Ovaries of heifers were evaluated by ultrasound on day 0 to determine CL presence (with CL = 213, without CL = 202) and on day 10 to measure preovulatory follicle size. Heifers were divided into three categories based on preovulatory follicle diameter: