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Effects of Body Weight, Frame Size and Rate of Gain on the Composition of Gain of Beef Steers¹

Oklahoma State University, Stillwater 74078

W. N. Garrett³

University of California, Davis 95616

Abstract

Energy concentration of gain (EG) is an inherent component in beef cattle feeding systems. The National Research Council (NRC) uses equations based on body weight, rate of gain and cattle type to predict EG and, in turn, to calculate dietary energy requirements. From EG, fat and protein deposition can be calculated directly. A dynamic computer growth model also can be used to estimate EG. In both the NRC and the computer model, EG increases from about 3 to 6 Mcal/kg as body weight increases from 200 to 500 kg if daily gain is 1 kg. Both NRC and the model predict EG of calves to be about .3 Mcal/kg greater than a previous NRC system. In contrast to the NRC, model-predicted EG of yearlings is lower at lighter and greater at heavier body weights. Rate of gain affects estimates of EG more for the dynamic model than for the NRC systems. When predicted EG was compared with observed EG for 46 pens of feedlot steers in comparative slaughter trials, NRC estimates exhibited a narrow range compared with observed values with correlation coefficients of r = .38 and r = .71 (previous NRC). Model estimates of EG were closer (r = .85). The NRC predictions of EG systematically erred with initial body composition, diet metabolizable energy and length of feeding period (P < .01) and with initial body weight and rate of gain (P < .05). No systematic errors in model-predicted EG were detected. Enhanced model sensitivity to compensatory growth and rate of gain should reduce both EG and body weight gain prediction errors. Over longer feeding periods, NRC errors may cancel out, but for shorter intervals, the dynamic model estimates are more precise and exhibit less bias.

¹ Journal Article No. 5239 of the Oklahoma Agric. Exp. Sta.

² Anim. Sci. Dept., Oklahoma State Univ.

³ Anim. Sci. Dept., Univ. of California.