Modelling ruminant grazing systems based on native grasslands. 2. Evaluation of the animal dynamics module

• Quantitative tools are necessary to scale out ecologically intensive cow-calf systems. • The model enable the evaluation of management strategies in time and space. • Model performance was evaluated for experimental and on-farm data. • Daily weight gain, intake, body condition score and pregnancy were simulated well. • The model synthesizes scientific and practical knowledge for co-innovation. Ecological intensification of beef cow–calf systems on native grasslands requires quantitative tools that link herbage condition, animal physiology, and management at the whole-farm level. We present and evaluate the animal dynamics module of PASpALuM (PAmpa Sustainable grAzing Livestock Management), a farm-scale model developed for high-diversity Pampas grasslands. The module simulates individual animal herbage intake, energy balance, body weight, body condition score (BCS), and reproduction as functions of herbage height, management practices, and weather. Herbage height and BCS are used as key, farmer-measurable state variables that integrate sward structure, intake regulation, metabolic status, and reproductive performance. Output is probabilistic as a result of stochasticity in animal input data and the parameters governing pregnancy. Model performance was evaluated using (i) experimental data on herbage intake and daily weight gain of heifers under contrasting herbage allowances during two single seasons (n=16), and (ii) on-farm data from six cow–calf farms to assess cow body weight, BCS and pregnancy, and herbage height during one mating season. Herbage height and mass were simulated by the herbage module presented in an accompanying paper. Simulated herbage intake showed strong agreement with the experimental data resulting in mean absolute difference (MAD) and RMSE values of 0.44 and 0.52 kg DM animal -1 day -1 , respectively. MSD was dominated by ‘lack of correlation’ (79%), whereas ‘squared bias’ (13%) and ‘non-unity slope’ (8%) were small, indicating limited systematic error and adequate representation of intake responses across pasture conditions. Daily weight gain was well reproduced (MAD and RMSE of 0.05 and 0.06 kg animal⁻ 1 day⁻ 1 , respectively). Modelling efficiency (EF) was 0.6 for herbage intake and daily weight gain indicating good model performance. MAD and RMSE values over 90 days for cow body weight, BCS, and herbage height were 22.6 and 25.4 kg, 0.35 and 0.40 units, and 1.3 and 1.4 cm, respectivel. EF was 0.6, 0.5 and 0.7, respectively. Pregnancy percentage was predicted within 95%-confidence intervals on five of six farms, with a MAD of 4.2 % points. The sensitivity of actual dry matter intake was greatest for sward height. The model includes the option to operate it with multiple paddocks. Together with the individual-animal-based approach and the use of easily measured state variables, herbage height and BCS, PASpALuM is distinct from other native grassland production system models. We discuss its scientific merits and validity domain, and its use for scaling the ecological intensification paradigm on cow-calf farms in the Pampas region.