Reproductive Success in Beef Cows: Bayesian Evaluation of Genotype and Herbage Allowance Under Grazing Conditions.

Abstract

Reproductive losses from breeding to weaning jeopardise productivity and sustainability in extensive beef cattle systems. This study evaluated the effects of cow genotype (purebred vs. crossbred) and herbage allowance (high vs. low) on reproductive success across three biologically relevant periods: (I) breeding to pregnancy diagnosis, (II) pregnancy diagnosis to calving and (III) calving to weaning. A total of 1069 reproductive events from Hereford, Angus and reciprocal crossbred cows were analysed between 2007 and 2022 under native pasture conditions in Uruguay. Bayesian generalised linear mixed models were applied to estimate posterior probabilities of reproductive success, accounting for repeated measures and individual cow effects. Crossbred cows consistently exhibited higher probabilities of success than purebreds across all reproductive periods. Under low herbage allowance, the crossbred advantage was most pronounced in Period III (Δ = 0.22; P (Δ > 0) = 1.00), followed by Periods I and II (Δ ≈ 0.13; P (Δ > 0) ≥ 0.99), indicating a moderate genotype × nutrition interaction. High herbage allowance improved outcomes for all cows, particularly during the calving-to-weaning stage. Cumulative probabilities (weaned/served) were approximately 0.43 for purebred-low, 0.63 for purebred-high and 0.78 for crossbreds under both herbage levels, corresponding to 30-35 more calves weaned per 100 cows served when using crossbred females. These results demonstrate that both crossbreeding and adequate herbage allowance enhance reproductive performance under grazing conditions and offer robust, cost-effective strategies to improve efficiency and resilience in extensive beef production systems.