Food efficiency in relation to estimated growth of body components in cattle

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Abstract
SUMMARY: A model is proposed to establish a functional relationship between food intake, weight gain and body composition during growth, based on the differential energy requirements for fat, fat-free tissues and contents of the digestive tract. If constant specific nutrient requirements are assumed for fat and fat-free tissues, these requirements can be directly estimated from changes in food intake and body composition during growth. By introducing an allometric function relating changes in fat-free tissue to changes in body weight, the model is extended to include situations where direct measures of body composition are not available. This enabled the model to be fitted to data on food intake, body weight and growth rate up to 2 years of age in Friesians and Jerseys fed ad libitum on a complete diet. The nonlinear model is contrasted with a linear relationship where food intake i s related to body weight and weight gain. The linear model was unable to account for the changes in food intake over the whole period of growth. The non-linear model allowed a gradual decline n i maintenance requirements per unit weight and indicated an increase in the net conversion coefficient of food into gain, consistent with an increase from early to late growth in proportion of fat deposited.For the period studied the Jerseys were less efficient in transforming food into weight gain than Friesians. On the non-linear model, about half of the difference was attributable to the higher metabolic rate per unit weight of Jerseys, the remainder to poorer utilization of nutrients for weight gain. If, as the model indicated the conversion coefficients of food into body constituents were the same in the two breeds, the difference in efficiency may be attributable to a higher proportion of fat in the overall gain of Jerseys. The pattern of estimated fat deposition also differed in the two breeds with Jerseys appearing to be less mature in fat percentage at all stages.