PS11-23. Assessing the Effects of Stocking Rate on Bermudagrass Herbage Mass and Stocker Average Daily Gain as Influenced by Climate Variability and Soil Moisture Using a Forage-Animal Interface Model.

Abstract Beef cattle production on pastures is a major economic activity in the southern United States. Because of complex relationships between grazing systems and the environment, the impacts of stocking rate and soil moisture condition on forage growth and animal performance as influenced by interannual climate variability in this region are still unknown. Using a forage-animal interface model developed for stockers grazing bermudagrass, this study assessed the effects of stocking rate, soil moisture condition, and ENSO (El Niño-Southern Oscillation), an oceanoatmospheric phenomenon having significant influence on interannual climate variability in this region, on bermudagrass herbage mass (BHM), BHM grazed per hectare (HMG_H) and per stocker (HMG_S), stocker seasonal weight gain (SWG), and stocker average daily gain (ADG). Three levels of stocking rate were considered: low (LO: 5 hd ha-1), medium (ME: 8 hd ha-1), and high (HI: 11 hd ha-1). For soil moisture condition, rainfed and fully-irrigated farming systems were considered. These effects were studied using the historical daily weather data spanning 83 years (1943-2025) collected at Overton, TX. Simulations were performed for 498 scenarios (83 years × 3 stocking rates × 2 moisture conditions). For statistical analyses, the R software was used. For significance tests, the pairwise Wilcoxon rank sum test was used. For ENSO analyses, the 83 years were assigned to a specific ENSO phase. The total number of years available under El Niño, La Niña, and Neutral phases were 20, 21, and 42, respectively. For drought analyses, the ARID drought index was used. Using the daily weather data, the seasonal values of ARID were computed for the 83 years. For each stocking rate, the relative values of BHM, HMG_S, SWG, and ADG were calculated by dividing the corresponding rainfed values by irrigated ones. The relative values were then subtracted from one to compute the fraction of losses of these variables due to drought. Results showed that, regardless of ENSO and moisture condition, the effects of stocking rate on HMG_S, SWG, and ADG were significant (P < 0.05). These variables were greatest at LO and least at HI, whereas BHM and HMG_H were greatest at HI and least at LO. The BHM was greatest at HI and least at LO under irrigated conditions. Under rainfed conditions, no stocking rate effect on BHM was found. The loss of BHM because of drought was found to increase at an increasing rate with an increase in drought intensity. This phenomenon was independent of stocking rate. The losses of HMG_S, SWG, and ADG due to drought also showed similar patterns. However, the losses became more pronounced with an increase in stocking rate. These results can help stakeholders assess the effects of management on forage and animal production and develop tailored management strategies for stockers. For image description, please refer to the figure legend and surrounding text.