25 Adjusting the use of body weight on dry matter intake predictive equations in feedlot finishing cattle.

Abstract The Nutrient Requirements of Beef Cattle published by the National Academies of Science, Engineering, and Medicine in 2016 set the equation to estimate dry matter intake (DMI) as a function of a linear effect of the metabolic body weight (BW) of the animal and a quadratic effect of the net energy concentration of the diet. When predicting feed intake relative to BW (doi: 10.2527/jas.2014-7557), this last one has a linear effect on DMI as well. Current finishing feedlot systems tend to finish cattle at a heavier BW, and concerns that neither equation can accurately predict DMI have arisen. We hypothesize that BW must be a quadratic function in the model for the prediction of DMI. To test our hypothesis, we used datasets from our labs from the last ten years, which include 28 d (±3) BW and DMI measurements. A total of 23 experiments were used, where the lighter BW was 207 kg and the heavier 803 kg. The animals were fed high-grain diets (1.85 – 2.11 Mcal/kg) for at least 161 d and a maximum of 284 d. Some experiments had individual fed animals, but some had pen fed animals. For those with pen fed animals, the average of the pen was used as a single datapoint Data were analyzed using mixed models (SAS 9.4) to evaluate the relationship between DMI (kg/d) and BW (kg) or DMI as a percentage of BW (DMI%BW) and BW (kg). A linear and a quadratic model were compared. The linear model included BW as a fixed effect, while the quadratic model included both BW and BW2 as fixed effects. Random effects included experiment and pen within the experiment. Model fit was evaluated using the Likelihood Ratio Test (LRT; SAS, 9.4). The linear model estimates for DMI = 4.1376 + 0.01332BW and DMI%BW = 3.06 – 0.00175 BW (P < 0.01 for intercept and slope for both models); the quadratic model estimate for DMI = - 0.2116 (P = 0.62) + 0.03191 BW -0.00002 BW2(P ≤ 0.01 for linear and quadratic slopes) and DMI%BW 2.84 + -0.00082 BW- 9.53x10-7 BW2 (P ≤ 0.01 for intercept and linear and quadratic slope). The quadratic model provided a better fit to the data than the linear model, as evidenced by the LRT, for both DMI and DMI%BW (P < 0.01). Despite the equations for DMI estimate using the net energy concentration of the diet, we consider that this provides preliminary data to reassess the use of BW or metabolic BW as a quadratic variable for a better estimate of DMI.