Development of equations to predict carcass weight, empty body gain, and retained energy of Zebu beef cattle.

The accurate supply of energy is essential to optimize livestock productivity and profitability. Furthermore, replacing empty BW gain (EBG) with carcass gain (CG) might be a suitable alternative to estimate the retained energy (RE) of beef cattle. Thus, this multi-analysis study was conducted aiming to estimate and validate new equations to predict carcass weight (CW), EBG, and RE of Zebu, beef crossbred, and dairy crossbred. A database composed by 1112 animals encompassing bulls, steers, heifers of different genetic groups (Zebu, beef crossbred, and dairy crossbred), and two types of slaughter plants (commercial and experimental) was used for generating the new CW equation. For the development of the EBG and RE equations, a database of 636 observations composed of bulls, steers, and heifers of different genetic groups (Zebu, beef crossbred, and dairy crossbred) was assembled. The validation of new equations was performed using independent databases composed by 137 observations (80 for CW and 57 for EBG and RE). The new approaches for EBG and RE validation also included data from our research group studies (Inside) and independent data from literature publications (Outside). Furthermore, the new RE equation was compared to the current model devised by the nutritional requirements, diet formulation, and performance prediction of Zebu and crossbred cattle (BR-CORTE, 2016). Validation analyses were performed by using the Model Evaluation System (MES; 3.1.13, College Station, US). The CW was accurately estimated by the new equation when using both commercial and experimental data. Also, the equations developed in this study accurately estimated EBG and RE using both inside and outside data. In conclusion, equations proposed in this study accurately and precisely estimated CW, EBG, and RE of Zebu beef cattle that composed validation data set. Therefore, we suggest the following equations to estimate CW, EBG, and RE of Zebu cattle: CW, kg = -11.0±1.56 + P + ((0.609±0.005 + G + B) × SBW); EBG (kg) = 0.044±0.017 + 1.47±0.026 × CG; RE (MJ/d) = 4.184 × (0.082±0.002 × EQEBW0.75 × CG0.777±0.039), where P = slaughter plant effect, if commercial = -10.98, if experimental =0; G = gender effect, if steer = 0, if bull = 0.008169 and if heifer = -0.00612; B = genotype effect, if Zebu = 0, if dairy crossbreds = -0.03301 and if beef crossbreds = -0.01595; SBW = shrunk BW; CG = carcass gain; EQEBW = equivalent empty BW.

Development of an equation to predict net protein requirements for the growth of Zebu beef cattle.

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.