The objective of this work was to evaluate body composition and net requirements of protein and energy for weight gain of Nellore and crossbred F1 Limousin-Nellore steers finished on Mombasa grass pastures. Mombasa grass (Panicum maximum) pastures were managed under intermitent stocking during the rainy season, with a forage offer of 8% of the body weight (BW) based on the dry matter of green leaves. Twenty-four steers were used: 12 were Nellore (386 kg SBW) and 12 were F1 Limousin-Nellore (397 kg SBW). Changes in body composition in a 104-day experimental period were evaluated according to the comparative slaughter method. After slaughter, all body components were weighed and representative samples of carcass (HH section) and non-carcass tissues were collected, processed and chemically analyzed to determine body contents of protein, fat and energy in each animal. Linearized allometric equations were fitted to describe the pattern of growth of body components as a function of empty body weight of the animals. The obtained equations were derived to predict net requirements of protein and energy for weight gain. There were no differences between Nellore and crossbreed steers regarding to protein deposition. Nellore animals tended to a greater deposition of energy on weight compared to crossbred animals. It is possible to obtain well finished carcass in Nellore and crossbred steers with a greater weight on tropical pastures with an adequate supply of leaves.