Evaluation of NRC (2000) model energy requirement and DMI equation accuracy and precision for wintering beef cows in western Canada

Three years of winter feeding trials using 90 Angus cows (15 pens of six cows) fed typical western Canadian wintering diets formulated to stage of pregnancy were used to evaluate National Research Council (NRC 2000) energy requirement and dry matter intake (DMI) equation accuracy and precision. Data collection included pen DMI, individual cow weights, body condition scores, calving dates and weights, and daily environmental temperature. Diet energy density was estimated from nutrient analysis of composited weekly feed samples. Equation evaluations compared observed and predicted DMI and conceptus corrected average daily gain (ADG) for the second and third trimesters using regression, means comparison, concordance correlation coefficient (CCC), and total deviation index (TDI) methods. Across all 3 yr, second trimester DMI was over-predicted (P < 0.01) with low precision (CCC = 0.24, TDI 90 = n/a) using actual environmental conditions, but not (P = 0.34) when assuming thermal neutral (TN) conditions, although precision remained low (CCC = 0.25, TDI 90 = 1.91 kg d -1 ). Third trimester DMI over the 3 yr was also over-predicted (P < 0.01) with low precision (CCC = 0.12, TDI 90 = 1.57 kg d -1 ) using actual environmental conditions, but was largely under-predicted (P < 0.01) with lower precision (CCC = -0.01, TDI 90 = 2.34 kg d -1 ) when assuming TN conditions. Across all 3 yr, second trimester ADG was largely under-predicted (P < 0.01) with low precision (CCC = 0.50, TDI 90 = 0.58 kg) using actual environmental conditions, but over-predicted (P < 0.01) with similar precision (CCC = 0.51, TDI 90 = 0.50 kg) when assuming TN conditions. Third trimester ADG predictions using actual environmental conditions were inaccurate (P < 0.01) with low precision (CCC = 0.20, TDI 90 = 0.38 kg) using actual conditions and lower precision (CCC = -0.01, TDI 90 = n/a) when assuming TN conditions where ADG was over-predicted (P < 0.01). These results indicate a lack of accuracy and precision with the current NRC (2000) model energy requirement and DMI equations that was not addressed by assuming TN conditions. Future research should be targeted at alternate DMI equations and refinements to maintenance and gain requirements.