Influence of small doses of gonadotropin-releasing hormone following controlled internal drug-release insert removal on reproductive parameters in beef cattle subjected to a fixed-time artificial insemination protocol

Heifers and cows that exhibit estrus before fixed-time AI have increased concentrations of estradiol and greater conception rates compared with those that do not. Our objective was to determine whether small doses of gonadotropin-releasing hormone following controlled internal drug-release insert removal would affect estradiol concentrations, estrus expression, or pregnancy success. Beef cattle (n = 247) were synchronized using the 7-d CO-Synch+CIDR protocol and assigned to 1 of 3 treatment groups (0 μg,10 μg, or 5 + 5 μg of gonadotropin-releasing hormone). Animals were observed for estrus, blood samples were collected, and animals were bred by appointment. Concentrations of estradiol were influenced by time (P < 0.0001) but not by treatment (P = 0.52) or a treatment-by-time interaction (P = 0.68). Estrus was not influenced by age (P = 0.36), treatment (P = 0.92), or a treatment-by-age interaction (P = 0.58). Conception rates were influenced by estrus (P < 0.01), treatment (P = 0.04), and a treatment-by-estrus (P < 0.01) interaction but were not influenced by age (P = 0.27), treatment by age (P = 0.84), or treatment by estrus by age (P = 0.94). Animals administered 5 + 5 μg had increased conception rates (P = 0.01; 69 ± 6%) compared with animals administered 10 μg and tended to be increased (P = 0.11) compared with 0 μg, which did not differ from 10 μg (53 ± 6% vs. 42 ± 8%, respectively). In conclusion, 5 + 5 μg did not affect concentrations of estradiol or estrus expression but did improve conception to AI.