Protein requirements for females of Nellore, Nellore x Angus and Nellore x Simmental fed on two forage: concentrate ratios

This study aimed to determine the protein requirements for females of Nellore, F-1 Nellore x Angus and F-1 Nellore x Simmental fed on two concentrate levels (30 and 50%). Sixty heifers from three genetic groups with 18 months of age were used: 20 Nellore, 20 Nellore x Angus and 20 Nellore x Simmental. Twelve heifers of the reference group (four of each genetic group) were slaughtered at the beginning of the experiment. Another 12 heifers (four of each genetic group) were fed on the level of maintenance and 36 heifers (12 animals of each genetic group) were kept in power system ad libitum with 30% (six of each group) or 50% (six of each group) dietary dry matter in concentrate. Heifers were randomly assigned to six treatments in a 3 x 2 factorial arrangement (three genetic groups and two diets) with six replicates per treatment. Nine more heifers (three from each genetic group) were used to estimate the apparent digestibility coefficients of food in a parallel experiment. A model was fitted according to the protein retained as function of the gain of empty body weight (EBW) and retained energy (RE) to calculate the protein net requirements. To estimate the metabolizable protein requirements for maintenance the consumption of metabolizable protein was contrasted with EBW. The joint use of the equation net protein gain (NPG) = 197.40 x EBWg - 11.14 x RE is recommended to predict the protein net requirements for weight gain. Protein and metabolizable protein net requirements for maintenance are 1.07 and 3.88 g/EBW0.75/day, respectively. The use efficiency of metabolizable protein for gain of all genetic groups is 37.04%.