This research aimed to estimate the maintenance requirements of net energy (NEm) and metabolizable energy (MEm), and the changes in body and gain composition of confined adults non-pregnant and nonlactant beef cows. Thirty six cows were used, 12 were Nellore (NEL), 12 F1 Brown Swiss-Nellore (BSN) and 12 F1 Red Angus-Nellore (RAN). The comparative slaughter method was utilized to estimate the changes of animal body chemical composition during the experimental period. Four animals from each breed group were randomly allocated to the treatments: initial slaughter (IS), restrict feeding (RF) and ad libitum feeding (AL). The RF animals received levels of energy and protein 15% above maintenance during the experimental period, while the AL group was fed ad libitum, to gain 1.1 kg liveweight (LW) daily. The IS animals were slaughtered at the beginning of the experimental period, and those from RF and AL treatments were slaughtered 120 days latter. The proportions of muscle, adipose tissue and bone beyond body content of protein, fat and energy were estimated in all slaughtered animals. Linear and non linear models were fitted to estimate the NEm and MEm requirements. There was no difference among breed groups for NEm and MEm requirements expressed fast body weight (kcal/kg^0.75 FBW). When the linear and the non linear models were fitted, it was observed values of NEm requirements of 80.7 and 83.6 kcal/kg^0.75 FBW, and the MEm requirements of 109.9 and 128.0 kcal/kg^0.75 FBW, respectively. Breed groups did not differ with respect to protein content per unit of empty body weight (EBW) and of empty body weight gain (EBG). However, NEL animals showed higher fat and energy concentrations per kg EBW and EBG, at a given live weight.