295 Development of a model to predict forage intake and growth of nursing beef calves

Abstract Models to predict intake and growth of preweaning calves have received less attention than postweaning calves. The objective was to develop a model to predict forage intake and growth of nursing beef calves. Data on milk intake, forage intake, and body weight (BW) of 51 Hereford calves from birth to 200 d of age were acquired from Boggs (1977). Milk and forage intake equations were those evaluated by Lancaster et. al. (2021; 10.1093/jas/skaa406). Forage digestibility was evaluated with a dataset on in vitro and in vivo measurements. Milk energy concentration was evaluated with a dataset on milk yield and composition. NASEM growth equation was evaluated using the dataset of Hildebrand et al. (2022; 10.1093/jas/skac064.321). Model precision and accuracy were evaluated using the concordance correlation coefficient (CCC) and mean bias (MB). The NASEM and WOOD milk yield equations predicted milk intake reasonably well (CCC = 0.796 and 0.820; MB = 4.29% and 1.31%, respectively); the WOOD equation was used in subsequent analyses. Forage intake prediction was imprecise (CCC = 0.348 to 0.613) and inaccurate (MB = 19.36 to 57.67%). Body weight prediction was moderately precise (CCC = 0.574 to 0.766) and moderately accurate (MB = 6.39 to 15.79%). With observed BW, forage intake prediction was moderately precise (CCC = 0.528 to 0.834) and highly to poorly accurate (MB = 0.81 to 27.56%). With observed forage intake, BW prediction by the NASEM growth equation resulted in CCC of 0.821 and MB of 9.69%. Published equations relating IVDMD to in vivo OMD were imprecise (CCC = 0.437 to 0.520) and moderately to highly accurate (MB = -9.46 to 7.67%). A new equation was developed: OMD, % = 7.7719 ± 4.3825 + 0.8937 ± 0.0772*IVDMD, % with RMSE = 2.803% and R2 = 0.9055. Milk energy concentration changed with days in milk (DIM; P < 0.05) and milk yield (P < 0.05): Energy, Mcal/kg = 0.7525 ± 0.0037 + 0.00069 ± 0.00016*DIM -0.00012 ± 0.00003*DIM*Yield, kg/d with RMSE = 0.031 and R2 = 0.527. Evaluating the NASEM growth equation in beef cattle resulted in CCC of 0.777 and MB of -13.56%. A new equation was developed: EBG, kg/d = 7.9279 ± 0.8254*(RE, Mcal/d/EBW, kg0.75) ^0.7083 ± 0.0385 with RMSE = 0.1127 and R2 = 0.6940. With observed forage intake, BW prediction with new equations resulted in CCC of 0.901 with MB of 1.58%. With the new equations, forage intake prediction by TED06, BAK76, TED09A, TED09B, and HOL82 resulted in CCC of 0.822, 0.700, 0.434, 0.686, and 0.380 with MB of -7.01, -5.08, 36.87, 13.24, and 10.96%, respectively. Body weight prediction resulted in CCC of 0.935, 0.938, 0.896, 0.926, and 0.886 with MB of -7.34, -6.21, -0.09, -5.35, and -9.41%. In conclusion, prediction of forage intake and BW in nursing beef calves were hindered by use of IVDMD, constant milk energy concentration, and the NASEM growth equation.