327 - Parameter Updates in the Davis Growth Model for Growth, Intake, and Composition of Modern Angus-cross Cattle

Abstract: The Davis Growth Model is a dynamic model used to estimate body composition and growth in beef cattle by simulating total body DNA and protein turnover. Since model development and parametrization in the 1980s, the model has been re-structured to account for previous rate of protein accretion and prior nutrition. Original parameter estimates for alpha (maintenance constant, where net energy for maintenance is alpha x shrunk body weight^0.75; 0.08406) and K2 (protein synthesis rate constant; 0.0444) may not fit modern, heavier, more productive cattle. Angus-cross steers (n = 24; initial body weight 402 ± 12 kg) were fed using automated feed weight stations in group pens (8 steers/pen). Steers were fed a high energy ration twice daily, at 10% greater than the intake from the previous day. Steers were measured for hip height, body weight (BW), and ultrasonic measurements for ribeye area and back fat every 28 d until harvest (d 128). These measurements were used to calculate empty BW, empty body fat, and empty percent body fat for alpha and K2 estimates. Parameter estimates (± SD) were 0.12661 (0.01587) and 0.04919 (0.00220) for alpha and K2, respectively. Parameter estimates in the current study reflect a large increase in apparent maintenance. Maintenance requirements were moderately correlated with back fat thickness and feed conversion efficiency. Protein turnover had very strong correlations with ribeye area and feed conversion efficiency. Average feed conversion efficiency in the current study was 0.17. To evaluate parameter estimates, the model was re-run using the new values for alpha and K2. On average, the model underpredicted final empty body weight by 0.08 kg and empty body fat percent by 0.01%-units. Considering the relatively large variation among observed alpha values and the magnitude of increase compared with those previously reported, further research is required to determine an accurate alpha.