Effects of Human Chorionic Gonadotrophin Administration on Artificial Insemination Pregnancy Rates in Beef Heifers

The objectives of this study were to determine whether administering human chorionic gonadotropin (hCG) at 5 d post-AI improves fertility in beef heifers by increasing serum progesterone concentrations. Nulliparous crossbred beef heifers from separate breeding seasons [early (EBH; n = 48) and late (LBH; n = 48) breeding heifers] were stratified by BW, body condition score (BCS), and age within breeding season and randomly allotted to one of two treatments. All heifers were synchronized for standing estrus with the 7-11 MGA®-Select Synch protocol. Control heifers received a saline injection (3 mL; i.m.) and heifers in the treatment (hCG) group received hCG (3300 IU; i.m.) at 5 d post-AI. Blood samples were collected on d −35 and −28 to determine estrous cycling rates and for 5 consecutive d (beginning 4 d post-AI) for serum progesterone analysis. No treatment × breeding season interaction occurred; therefore, EBH and LBH data were pooled to analyze for main effects. First-service conception rates, estrous cycling status, and estrous cycling response were similar (P>0.10) for control vs hCG heifers. Serum progesterone concentrations were similar (P>0.10) on d 4, 5, and 6 post-AI for control vs hCG heifers; however, heifers receiving hCG had greater (P<0.05) serum progesterone concentrations on d 7 and 8 of the estrous cycle post-AI than did control heifers. Administration of hCG at 5 d post-AI increased serum progesterone concentrations by d 7 of the estrous cycle post-AI; however, AI pregnancy rates did not improve for beef heifers receiving hCG, suggesting that elevating progesterone synthesis does not improve fertility in beef heifers.