The net energy content for maintenance (NEm) and gain (NEg) of the napiergrass, under rotational grazing, as well as the changes on protein and fat concentrations in body and in weight gain of 36 ¾ Gyr-Holstein steers, grazing the forage, during the rainy season were determined. Half of steers had grazing time restricted to four hours daily, to limit energy intake to a level close to the maintenance level, and half had free access to the pasture. Six steers were slaughtered at begining of the trial, to determine initial body composition, and six (three from each treatment), at each subsequent 35 day. Prediction equations of the changes of body composition, with increase of body weight, were fitted by regression of log10 body content of protein and fat on log10 empty body weight. By differentiation of those equations, the prediction equations of composition of weight gain were obtained. The energy concentrations were estimated from protein and fat content and respective calorie equivalents. By regression of retained energy (RE) on metabolizable energy (ME) intake, the efficiency of ME utilization for gain, as the regression coeficient, and the ME requirement for maintenance (level of ME intake for RE = 0) were estimated. The efficiency of utilization of ME for maintenance (Km) was obtained by the ratio: Km = NEm/MEm. The NEm and NEg of napiergrass were determined, respectively, as the products of Km and Kg to ME content of the forage, and the values 1.02 and 0.59 Mcal/kg dry matter, respectively, were obtained. The protein concentrations in the body and in body weight gain remained in higher levels and the fat energy concentrations remained in lower levels than those reported, in Brazil, for zebu and crossbred steers showing higher weight gains.
body composition; grazing; napiergrass; net energy content; rainy season; steers
