Reproductive Performance Following Long-Vs Short-Term Progesterone-Based Synchronization Protocols for Fixed-Time Artificial Insemination in Beef Heifers

The objective of this study was to determine the effect of long-term (CPG group; 14-d controlled internal drug release (CIDR)-prostaglandin (PG)-gonadotropin releasing hormone (GnRH) or short-term (FCC group; 5-d CO-Synch+CIDR) progestin-based synchronization protocols on artificial insemination (AI) pregnancy rate in beef heifers. The hypothesis was both long and short-term progesterone based protocols would result in similar AI pregnancy rates. Angus cross beef heifers (N = 1887) at nine locations were included in this study. All heifers received a body condition score (BCS) and reproductive tract score (RTS) and within the herd were randomly assigned to CPG or FCC protocol groups. Heifers in CPG group received a CIDR (1.38 g of progesterone; Eazi-Breed CIDR Cattle Insert®, Pfizer Animal Health, New York, NY) from day 0 to 14, followed by 25 mg of dinoprost (Lutalyse®, Pfizer Animal Health) 16 days later (day 30). Heifers in FCC group received a CIDR and 100 μg of gonadorelin hydrochloride (Factrel®, Pfizer Animal Health) on day 0 (day 25) followed by 25 mg of dinoprost at CIDR removal and a second dose of dinoprost, (25 mg), 6 h later on day 5 (day 30). Artificial insemination was performed at 72 h (day 33) for CPG group and at 56-h for FCC group (day 32) after CIDR removal, and all heifers were given GnRH (100 μg, IM) concurrently at the time of AI. Two weeks later, intact bulls were placed with the heifers for the remainder of the 60-70 d breeding season. Heifers were examined for pregnancy status 50 to 70 days after AI to determine if pregnancy was the result of AI or bull breeding.

The data were analyzed using mixed model procedure (PROC GLIMMIX SAS). The variables included in the model were treatment (CPG vs. FCC), BCS categories (4, 5 and 6, and 7), RTS (≤3, 4 and 5), treatment by RTS categories and treatment by BCS categories interactions. Year (location), AI sires and AI technicians were included as random variables in the model. The P value was set at > 0.1 for exclusion and α ≤ 0.05 for significance. No difference in AI pregnancy rates between CPG and FCC synchronization protocols [54.5% (489/897) 55.5% (549/990); (P=0.92)] was observed. The AI pregnancy for heifers with RTS ≤3, 4 and 5 were 52.6, 53.6 and 59.9% respectively (≤3 vs 5 P0.1). In conclusion, heifers synchronized for fixed time AI with 14-d CIDR-PGF-GnRH and 5-d CO-Synch+CIDR resulted in similar AI pregnancy rates and thus the short-term protocol offers an additional opportunity for open heifers to become pregnant.