Gonadotropin-releasing hormone increased pregnancy risk in suckled beef cows not detected in estrus and subjected to a split-time artificial insemination program

We hypothesized that GnRH would increase pregnancy risk (PR) in a split-time AI program for cows in which estrus was not detected. A total of 1,236 suckled beef cows at 12 locations in 3 states (Colorado, Kansas, and North Dakota) were enrolled. Before applying the fixed-time AI program, BCS was assessed. Cows were treated on d -7 with a progesterone insert concurrent with 100 mu g GnRH and on d 0 with 25 mg PGF(2 alpha) plus removal of the insert. Estrus-detection patches were affixed to cows at 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 randomly allocated to 2 treatments: 1) 100 mu g GnRH and early + GnRH (E+G; n = 373) or 2) AI only at 65 h (early -no GnRH [E-G]; n = 385). The remaining cows were randomly allocated to 2 treatments: 1) 5(L+G; n = 252) or 2) AI only at 84 h (late no GnRH [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 (L+G) compared with their herd mates not receiving GnRH (L-G; 41.7 vs. 30.8%, respectively). Of those cows not detected in estrus by 65 h, 42.1% were detected by 84 h, for a total expression of estrus by all cows of 77.6%. Administration of GnRH increased (P < 0.01) PR in cows not detected in estrus by 84 h (+ GnRH = 33.4% [n = 146] vs. no GnRH = 15.0% [n = 128]) but had no effect in cows expressing estrus by 84 h (+ GnRH = 65.3% [n = 103] vs. no GnRH = 61.7% [n = 97]). Neither estrus expression by 65 or 84 h nor PR was influenced by BCS, parity, or days postpartum at AI. Cows had greater PR when they had been detected in estrus before AI, and PR was improved by administration of GnRH at 65 h after insert removal in cows that were not detected in estrus and inseminated at 84 h.