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Mathematical Models of Postweaning Growth, Feed Intake and Carcass Composition of Beef Cattle. I. Empirical Predictive Model of Voluntary Feed Intake from Weaning to Slaughter^1,2,

South Dakota State University, Brookings 57006

Abstract

Data from the literature were analyzed to develop a mathematical model to estimate voluntary feed intake (VFI) of cattle varying in age and breed and fed rations varying in energy density and/ or crude fiber content of diet.

A mathematical equation was developed to estimate VFI regulated by physiological demand for energy (VFI_P) where VFI_P was varied with degree of maturity of cattle and energy density of diets. Regression analysis indicated that kcal metabolizable energy per kilogram metabolic weight (kcal ME/kg W^.73) decreased as degree of maturity of cattle increased. It was interpreted that the decrease in energy intake per kilogram metabolic weight was due to decrease in physiological demand for energy with aging.

Another mathematical equation was derived to estimate VFI controlled by gastrointestinal tract (GIT) distension (VFI^d) based on the rate of disappearance of digesta from GIT. The rate of disappearance varied with rate of digestion, rate of excretion of undigested material, dry matter digestibility of feeds and gut fill in dry matter.

Partial mean retention time (PMRT) developed in this study was defined as mean retention time of feed particles from one feeding excreted together within a certain range of time. Rate of excretion varying with percent crude fiber of diets and retention time of food was estimated from an equation fitted to the Gompertz curve.

Because of the interaction between breeds and diets on feed intake, breed differences in feed intake were expressed as breed differences in VFI_P (BDVFI_P) and breed differences in VFI_d (BDVFI_d). BDVFI_P was expressed as a ratio to the mean of Angus and Hereford breeds, and BDVFI_d was expressed as a ratio to the mean of several breeds involved in this study.

A computer program to estimate true VFI for various breeds of cattle with different degrees of maturity fed varying diets is available.

¹ Published with the approval of the Director of the South Dakota Agricultural Experiement Station as Pub. No. 1477 of the Journal Series.

² The cooperation of Dr. James Bond, ARS, USDA, Beltsville, MDand Dr. Gerald Smith, ARS, USDA, Clay Center, NE in providing data is gratefully acknowledged.

³ Current address: Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Can.

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