Technical note: a dynamic model to predict the composition of fat-free matter gains in cattle.

Composition of empty BW (EBW) was described in terms of ether-extractable lipid (FAT) and fat-free matter (FFM), and the terms dEBW, dFAT, and dFFM were used to represent daily gains in these components. The dFFM comprised protein, water, and ash, and a model was developed to predict the composition of dFFM. The conceptual approach used in model development was based on experimental data that showed as cattle grew from birth to maturity: 1) the water content of FFM decreased and the protein and ash content increased; 2) the protein content of FFM increased at a decreasing rate; and 3) the protein-to-ash ratio in the fat-free DM was approximately constant. These results suggest that the protein content of dFFM would be high at birth and decrease at a decreasing rate as the animal grows. The protein content of dFFM was predicted as a function of the fraction of dEBW that was dFFM, FAT content of EBW, and dFFM. A fixed protein-to-ash ratio of 4.1:1 was used to calculate the quantity of ash, and water was obtained as a residual. Growth and body composition of Hereford x Angus steers from birth to 500 kg BW were simulated with a previously published model using the experimental growth data as input, and the model under discussion was used to predict the composition of dFFM. Predicted response curves of the EBW components over the growth period were similar in shape to observed data. Predicted curvilinearity in response of protein weight against FFM weight for Hereford x Angus steers was similar to observed data. The standard error about the regression of predicted on observed protein weight was 1.87 kg, and the average bias of the model was to underpredict protein weight by 0.64%. Compared with using a constant value for the protein fraction of dFFM, the model provided more accurate predictions of dEBW in an independent evaluation data set.