GnRH increased pregnancy risk in suckled beef cows that did not display estrus when subjected to a split-time artificial insemination program

Abstract Text: We hypothesized GnRH would induce ovulation in a split-time AI program by increasing pregnancy risk (PR) when estrus was not detected. A total of 1,236 suckled beef cows at 12 locations in 3 states (CO, KS, and ND) were enrolled. Before applying the fixed-time AI program, BCS was assessed. Cows were treated on d –7 with a CIDR insert concurrent with 100 µg GnRH and on d 0 with 25 mg PGF_2α plus removal of the insert. Estrotect patches were affixed to cows at CIDR insert removal. Estrus was defined to have occurred when an estrus-detection patch was >50% colored (activated). Cows in estrus by 65 h (n = 758; 61.3% of all cows) were allocated randomly to 2 treatments: 1) GnRH and early AI at 65 h (E+G; n = 373), or 2) AI only at 65 h (E–G; n = 385). Remaining cows were allocated randomly to 2 treatments: 1) GnRH injection at 65 h and late AI at 84 h (L+G; n = 252), or 2) AI only at 84 h (L–G; n = 226). Pregnancy was determined 35 d after AI via transrectal ultrasound. Pregnancy risk did not differ (P = 0.68) between E+G and E–G cows (61.9 vs 60.4%), respectively. Conversely, for cows inseminated at 84 h, PR was greater (P = 0.01) in cows that received GnRH at 65 h compared with their herd mates not receiving GnRH (41.7 vs 30.8%), respectively. Of those cows not in estrus by 65 h, 57.7% displayed estrus by 84 h for a total expression of estrus by all cows of 77.6%. Pregnancy risk was greater (P< 0.01) in cows not detected in estrus by 84 h when treated with GnRH at 65 h compared with no GnRH (+G = 33.4% [n = 146] vs. –G = 15.0% [n = 128]), whereas no difference in PR was detected for cows detected in estrus (+G = 65.3% [n = 103] vs. –G = 61.7% [n = 97]). Neither estrus expression by 65 or 84 h nor pregnancy risk was influenced by BCS, parity, or days postpartum at AI. Cows had greater PR when they displayed estrus before AI and cows that did not display estrus by 65 h benefited from an injection of GnRH at 65 h before insemination occurred at 84 h.

Keywords: beef cows, GnRH, split-time AI