263. Reproductive and Growth Performance of Beef Cow-calf Pairs Under Contrasting Grazing Strategies.

Abstract Adaptive grazing is a flexible decision-based approach that supports managed decisions based on forage responses and seasonal variability, improving forage utilization and grazing distribution compared to conventional strategies. However, its effects on animal performance in cow-calf systems remain uncertain. This 3-yr study evaluated cow reproductive performance and calf growth responses under contrasting grazing strategies on native rangeland in Southern Oklahoma. Two grazing strategies were evaluated: Adaptive (AD), characterized by high stocking density and flexible rest periods, and Prescriptive (PR), characterized by low stocking density and fixed 75-d calendar rotations. Similar numbers of Angus-crossbred cow-calf pairs were assigned to each treatment each year (totaling 102, 78, and 112 cows in 2023, 2024, and 2025, respectively) with balanced total body weight (BW) between treatments. Pregnancy diagnosis was conducted via palpation in 2023 and blood tests in 2024-2025. Reproductive variables (pregnancy, calving, and weaning rates) were calculated relative to cows exposed to bulls and analyzed using binomial generalized linear mixed models. Calf performance variables (205-d adjusted weaning BW, calf average daily gain [ADG], and calf efficiency, defined as kg weaned BW/cow exposed) were analyzed using linear mixed models. Fixed effects included treatment, year, and their interaction. Cow BW, age, and parity and calf sex were included as covariates. Least square means were estimated using the emmeans package in R with Tukey-adjusted comparisons. Pregnancy rate was not affected by grazing treatment (P = 0.8265), year (P = 0.5608), or their interaction (P = 0.1811), averaging 77 ± 3%. Similarly, calving and weaning rates did not differ by grazing treatment (P = 0.66 and P = 0.64, respectively) but differed among years (P < 0.001), with lower values in 2023 compared to 2024 and 2025. Calving rates were 46 ± 5%, 76 ± 5%, and 71 ± 4%, and weaning rates were 41 ± 5%, 76 ± 5%, and 69 ± 4% in 2023, 2024, and 2025, respectively. The 205-d adjusted weaning BW was greater for calves managed under PR compared to AD grazing (211 vs. 203 ± 3 kg; P = 0.018). Similarly, calf ADG was 5% greater under PR than AD grazing (0.88 vs. 0.84 ± 0.01 kg d-1; P = 0.018). Both weaning BW and ADG differed among years (P < 0.001), with lower calf performance in 2024 compared to 2023 and 2025. Calf efficiency was not affected by grazing strategy (P = 0.94) but differed among years (P < 0.001), with the lowest efficiency observed in 2023. Differences among years were likely associated with environmental variation, particularly drought conditions that limited forage availability in 2023 and 2024. These results indicate that although prescriptive grazing improved individual calf growth, there was no effect of grazing strategy on reproductive performance or calf efficiency.