An experiment was conducted to evaluate: (1) the effects of ensiling maize or sorghum grains after reconstitution on readily soluble fraction (a), potentially degradable fraction in the rumen (b) and rate constant for degradation of b (c) of dry matter (DM), organic matter (OM) and starch (STA); and (2) an appropriate incubation time for in situ or in vitro procedures to estimate in vivo digestibility. Four rumen-cannulated Nellore bulls (body weight = 262 ± 19.6 kg) distributed in a 4 × 4 Latin square were used. Diets were based on dry ground maize (DGM); or dry ground sorghum (DGS); or reconstituted ground maize silage; or reconstituted ground sorghum silage. In vitro and in situ incubations of the individual grains and diets were simultaneously performed with in vivo digestibility. In general, reconstituted grains and diets based on reconstituted grains presented greater (P < 0.05) fraction a and lower (P < 0.05) fraction b of DM, OM and STA compared to dry grains and diets based on dry grain. However, the magnitude of response of the reconstitution and ensiling process on DM and OM degradability parameter was greater for maize than that for sorghum. Moreover, no differences (P > 0.05) were observed between DGM- and DGS-based diets for c estimates. The results suggest that the reconstitution process promotes grains protein matrix breakdown increasing STA availability. The incubation times required for in vivo digestibility estimations of DM, OM and STA are 24 h for in situ and 36 h for in vitro procedures.

The accurate estimation of protein requirements for beef cattle is a key factor in increasing livestock profitability and decreasing the environmental impacts of excessive N excretion due to mismatching between assumed requirements and diet formulation. A meta-analysis was conducted to evaluate and validate a new equation to predict the net protein requirements for growth (NPg) of Zebu beef cattle. For the development of the new approach, a database of 552 observations comprised of bulls, steers, and heifers of different genetic groups (Zebu, beef crossbreed, and dairy crossbreed) was assembled. The new approach was evaluated and compared to current models devised by the international nutrient requirements system committees (Agricultural Research Council, 1980; Beef Cattle Nutrient Requirements Model, 2016; BR-CORTE, 2016) to predict NPg. The model evaluation was performed through the model evaluation system (version 3.1.16) using an independent data set (n = 177 observations). An equation was considered the best estimator of NPg if the following conditions were met: (1) the intercept and slope of the regression between ordinary residues and/or predicted NPg values must have been equal to zero and one, respectively; and (2) the greatest concordance correlation coefficient (CCC) and determination coefficient (R), and lowest mean squared error of prediction (MSEP) were attained. Based on the regression models of the observed v. predicted NPg of Zebu beef cattle, both the new approach and that of the ARC (1980) correctly estimated NPg, since the intercept and slope were not different (P > 0.05) from zero and one, respectively. Additionally, the new approach’s determination coefficient was the greatest and the closest to one. The fact that the new model achieved a higher CCC and lower MSEP than the existing models indicated its superior reproducibility and accuracy. The equations proposed by BR-CORTE (2016) and the BCNRM (2016) did not correctly estimate NPg in that the intercept and slope were different (P < 0.01) from zero and one, respectively. Thus, the equations proposed by the new approach and the ARC (1980) accurately and precisely estimated NPg and are recommended for Zebu cattle. Furthermore, the inclusion of equivalent empty BW (EQEBW) in the new approach improves the estimation of NPg. We suggest the use of the following equation to calculate NPg for Zebu beef cattle: NPg = 176.01 × EBG – 0.381 × EQEBW0.75 × EBG1.035 (R = 0.80 and CCC = 0.75); where NPg = net protein requirements for growth, EBG = empty body gain, and EQEBW = equivalent empty BW.

An experiment was conducted to evaluate whether a reduction in dietary crude protein (CP) and genotype affects animal performance, feed efficiency and carcass traits of beef cattle at different stages of the feedlot. Twenty-two Nellore and 22 crossbred F1 Angus × Nellore (Nellore: 8 months, initial body weight (iBW) = 212.7 kg; Crossbred: 8 months, iBW = 226.1 kg) were used in this experiment. The experiment was conducted in a complete randomized design with six replicates, in a 2 × 3 factorial design. The factors were two genotypes (Nellore and Crossbred) and three CP concentrations (100, 120 and 140 g/kg dry matter [DM]). The experimental period lasted 224 day, being divided into two stages (Backgrounding = 112 day, and Finishing = 112 day). At the end of the experiment, all animals were slaughtered to evaluate their carcass characteristics. Similar patterns were observed during backgrounding and finishing phases on intake and average daily gain (ADG) regarding genotype. Intakes of DM, organic matter, CP, neutral detergent fibre, non-fibre carbohydrates and total digestible nutrients, ADG (kg/day) and feed efficiency were greater in Crossbred animals than Nellore in both phases. During the backgrounding phase, dietary CP did not influence DM intake. However, a linear effect of CP on ADG was observed in the backgrounding phase. During finishing, Nellore cattle fed 100 g CP/kg DM presented lower ADG than Nellore fed 120 and 140 g CP/kg DM. Crossbred animals were heavier than Nellore at the beginning and end of the trial. Crossbred animals also gained more carcass, retained more CP and fat, and had a greater subcutaneous fat thickness during the experiment compared with Nellore. There was a quadratic effect of dietary CP on retained CP and dressing. The current study found that crossbred animals (F1 Angus × Nellore) not only had greater performance, but also better carcass traits compared with Nellore, representing an option for increasing productivity. Also, it is possible to adjust diets according to phase. During the backgrounding stage, 140 g CP/kg DM should be used, being reduced to 120 g CP/kg DM during the finishing stage of feedlot for Nellore and to 100 g CP/kg DM for Crossbred, without affecting performance adversely.