Effect of intake level on the body composition and net energy

Three comparative slaughter experiments with individually fed Nellore bulls (31 animals) and steers (66 animals) were used to determine their NEm and NEg and the effect of level of intake on their composition of gain. Bulls (experiment 1) were divided into two intake levels (60 g DM/kg0.75 of shrunk body weight (SBW) and ad libitum of a diet containing 2.3 Mcal ME/kg) and three slaughter groups, which were based on days on treatment (100, 150, and 190 days, and 130, 180, and 200 days, respectively for older and younger animal subgroups). For experiments 2 and 3, steers were allocated to three intake levels (55 and 70 g DM/kg0.75 SBW, and ad libitum of a diet containing 2.2 Mcal ME/kg). In both trials, three slaughter groups within each intake level were set as when animals of the ad libitum treatment reached 400, 440, and 480 kg SBW on average for the first, second, and third group, respectively. For all experiments, initial body composition was determined with equations developed from a base line slaughter group, using SBW, empty body weight (EBW), fat (EBF), and protein (EBP) as variables. For bulls, there were no differences (P > 0.05) between treatments in EBW percentage of protein and water, and fat and protein in the gain (FIG and PIG, respectively). In contrast, in steers level of intake affected EBW percentage of protein and fat (P < 0.05), but FIG and PIG were not different (P > 0.05). A nonlinear equation with the pooled data was developed to predict retained energy (RE) using maturity degree (u = actual/mature weight, which was estimated to be 350 and 550 kg of SBW at 25% EBF for steers and bulls, respectively); RE = 0.00949*EWG0.9425*EBWexp(0.0924∗u). The NEm was similar for bulls and steers, and averaged 76 kcal/kg0.75 EBW. However, the efficiency of conversion of metabolizable to NEm was greater for steers than bulls (70.6 and 65.2%, respectively), indicating that bulls had an 8.6% greater ME requirement for maintenance than steers (P < 0.05). Our analyses do not support the hypothesis that Nellore, a Bos indicus species, has a lower net energy requirement for maintenance than Bos taurus breeds.