Effect of gonadotropin-releasing hormone in long-term controlled internal drug-release protocols on pregnancy rates in beef heifers artificially inseminated after observed estrus or fixed time

Beef heifers at 4 ranches were assigned to 1 of 2 estrus synchronization protocols: CIDR Select heifers received a controlled internal drug-release device (CIDR) on d 0, which was removed on d 14, gonadotropin-releasing hormone (GnRH) on d 23, and prostaglandin (PG) F2α on d 30; and CIDR-PG heifers received CIDR on d 0, which was removed on d 14, and PG on d 30. At 3 ranches (n = 264, Exp. 1), heifers were observed for estrus and AI 12 h after exhibition of estrus (EAI) or were fixed-time AI (TAI) with GnRH at 72 h. Heifers at the fourth ranch (n = 320, Exp. 2) were fitted with estrus-detection patches at PG administration, which were scored 72 h later at TAI. In Exp. 1, the percentage of heifers observed in estrus, the interval to observed estrus, variance for the interval to estrus, and the distribution of observed estrus did not differ (P > 0.52) between treatments. Pregnancy rate to EAI did not differ (P = 0.18) between treatments (71% for CIDR Select and 59% for CIDR-PG). There was a treatment × ranch interaction (P = 0.01) for TAI pregnancy rate. Pregnancy rate to EAI was greater (P = 0.003) than pregnancy rate to TAI (64 vs. 45%). In Exp. 2, estrus-detection patches on heifers synchronized with CIDR Select tended (P = 0.08) to have less wear than patches on heifers synchronized with CIDR-PG. Fixed-time AI pregnancy rate in Exp. 2 did not differ (P > 0.67) between treatments, averaging 50%. Timed-AI pregnancy rate was greater (P = 0.05) for heifers with estrus-detection patches that were 50 or 100% color change than for heifers with patches that had no color change. Inclusion of GnRH in long-term CIDR protocols did not improve synchrony of estrus or AI pregnancy rates.