Effects of a single supplemental dose of gonadotropin-releasing hormone in 2 different fixed-time artificial insemination synchronization protocols among beef cows and heifers

Objective: Studies evaluating single and multiple gonadotropin-releasing hormone (GnRH) treatments at time of prostaglandin (PG) administration using the 7-d CO-Synch + controlled internal drug-release device (CIDR) protocol reported decreased interval to estrus and in-creased estrus expression, or increased conception rates, respectively. Therefore, the objectives of these studies were to determine effects of supplementing GnRH at PG on estrus expression and conception rates in beef females synchronized with different protocols.Materials and Methods: In Exp 1 and 2, heifers (n = 1,626) and cows (n = 1,409), respectively, were synchronized using the 7-d CO-Synch + CIDR fixed-time AI (FTAI) protocol. In Exp 3, heifers (n = 3,270) were synchronized using the melengestrol acetate-PG FTAI heifer protocol; all animals were randomly assigned to receive a single dose of GnRH (5 mu g) at PG or no GnRH (control). Estrus and pregnancy rates were analyzed using the GLIMMIX procedure of SAS.Results and Discussion: In all experiments, there was no effect of treatment on estrus (P >= 0.21), but there was an effect of estrus (P < 0.0001) on FTAI pregnancy rates. Estrual females had greater FTAI pregnancy rates compared with nonestrual females. In Exp 1 there was a treatment x estrus interaction (P = 0.01) on FTAI pregnancy rates. Heifers in the control group that exhibited estrus had greater (P <= 0.04) FTAI pregnancy rates (72.8 +/- 3.2%) compared with estrual heifers that received 5 mu g of GnRH (67.2 +/- 3.4%) and compared with heifers that did not exhibit estrus in the control (54.4 +/- 4.7%) or 5 mu g of GnRH (60.8 +/- 4.6%) groups. There was no effect (P >= 0.17) of treatment on breeding-season pregnancy rates in any of the experiments.Implications and Applications: Utilization of a supplementary dose of 5 mu g of GnRH may be a useful tool to address differences in fertility of FTAI in short-term progestin synchronization protocols among animals that do not exhibit estrus.