Beef cattle growth deceleration parameters and its correlations with growth, carcass and morphological composition traits

In this study were investigated two growth deceleration genetic parameters (gamma and g-ratio) to evaluate alternatives for sire selection for improvements in finishing precocity without promoting changes in adult size. Additionally, were estimated genetic correlations between these parameters and weights (YW: yearling weight, FW: final weight), carcass quality traits at yearling (LMA: loin muscle area, BF: back fat thickness, RF: rump fat thickness) and morphological composition traits at yearling (BS: body structure score, FS: finishing precocity score, MS: muscling score). Both gamma and g-ratio parameters were obtained by genetic evaluation of animals using a random regression model on linear spline polynomials. Subsequently, (co)variances components and genetic parameters were estimated for gamma and g-ratio parameters and other traits, by bivariate analyses through Bayesian method. All evaluated traits showed genetic variability. However, selection of sires for maternal ability may provide a slow genetic gain. For direct additive genetic effect, the weight at 365 days of age is a good indicator of the weight until 700 days of age. The stronger maternal genetic correlation estimated between weights at 120 and 205 days of age (0.99) indicates that only one of these traits would be necessary for maternal ability selection. Gamma parameter presented weak or null genetic correlation with YW, FW, BF, RF, FS, and MS. Genetic correlations between g-ratio and weight indicated that sire selection based on lower values for g-ratio could reduce breeding values for YW and FW. The g-ratio presented a positive and weak correlation with LMA, BF, and RF (ranging from 0.08 ± 0.02 to 0.15 ± 0.02). Weaker genetic correlation was estimated between g-ratio and BS (0.15 ± 0.02) compared to FS and MS (0.23 ± 0.02 and 0.23 ± 0.02, respectively). Gamma and g-ratio parameters are not, individually, satisfactory indicators of finishing precocity when it is assessed using BF, RF, FS, and MS. However, the inclusion of these growth deceleration parameters in selection indices, along with traits related to carcass quality and morphological composition, could be an alternative to gradually modify growth rate and improve body composition in Nelore cattle herds.