424 The influence of hCG on cyclicity status of beef cows bred via natural service.

Abstract A primary goal of the cow-calf industry is to get females pregnant within the first 21 days of the breeding season and ensure they maintain this pregnancy to term. This study aimed to evaluate the effects of human chorionic gonadotropin (hCG) in a natural service breeding system and assess its ability to stimulate cyclicity in anestrous females. A total of 920 multiparous Bos Taurus cows were enrolled across five locations L1 = 154), L2 (n = 172), L3 (n = 194), L4 (n = 90), L5 (n = 308). Treatment 1 (Trt1; n = 304) received no estrus synchronization, treatment 2 (Trt2; n = 306) received a single injection of prostaglandin F2α (PGF2α; 500μg im; Estrumate; cloprostenol; Merck Animal Health) on D14, and treatment 3 (Trt3; n = 308) received 2000 IU of human chorionic gonadotropin (hCG; Chorulon; Merck Animal Health) on D0, followed by a 500μg injection of PGF2α on D14. Bulls having passed an industry standardized breeding soundness exam were evenly distributed among treatments at a ratio of 1:20 on D10. Pregnancy was determined by transrectal ultrasonography at 35, 90 and/or 125 days after initial bull exposure, and confirmed positive by the visualization of an embryonic or fetal heartbeat. Gestational age was estimated by measuring crown-rump length to determine the cycle in which conception was successful. Prior to the onset of the study, no differences were observed between treatments in body condition score (P = 0.99) nor cyclicity status (P= 0.50). Treatment had no effect on the proportion of cows bred in the first 21 days of the breeding season (P = 0.61) or the overall timing of conception during the 90-day bull exposure period (P = 0.44). However, cows that had a corpus luteum (CL) present on D0 were more likely to become pregnant than those without a CL (P < 0.01). Despite this, no interaction was observed between treatment and CL presence for pregnancy outcomes (P = 0.55). Cyclicity was assessed in a subset of cows prior to the onset of the study (n = 297), revealing that 32% (n = 97) were anestrous, which was determined by the absence of a CL on two consecutive scans 14 days apart. Of these, 45% (n = 43) conceived within the first 21 days of the breeding season, but treatment did not affect this outcome (P = 0.30). In a subset of Trt3 cows (n = 101), ovulation response to hCG was recorded seven days post-treatment, with 64% exhibiting a response; however, pregnancy rates were not influenced (P = 0.41). As no significant effects between treatments were observed, future research should evaluate increasing the interval between hCG administration and bull turnout, as well as optimizing hCG dosage.