Genetic study of visual scores and hip height at different ages in Nelore cattle

The objective of the present study was to evaluate the genetic variability of visual scores used as selection criteria in Nelore cattle, as well as their associations with yearling hip height (YH) and mature hip height (MH), to determine whether the selection considered would change the hip height of animals. (Co)variance components were obtained by two-trait animal model using Gibbs sampling, including YH or MH in each analysis. Breeding values for YH and MH were used to obtain the response to selection. The posterior mean of the heritability estimates for conformation, finishing precocity (or body condition) and muscling at weaning (WC: 0.21 +/- 0.02; WP: 0.21 +/- 0.02; and WM: 0.18 +/- 0.02 respectively) were lower than those obtained for the same traits at yearling (YC: 0.34 +/- 0.02; YP: 0.37 +/- 0.02; and YM: 0.38 +/- 0.02 respectively). Heritability estimates for YH and MH were 0.28 +/- 0.01 and 0.33 +/- 0.03 respectively. The results suggested that these traits should respond to selection process, but in different magnitudes. Positive and high genetic correlations were estimated between WC and YH and YC and YH (0.80 +/- 0.03 and 0.76 +/- 0.03 respectively), and lower values between these scores and MH (0.48 +/- 0.09 and 0.36 +/- 0.07 respectively). Weak genetic associations were obtained between finishing precocity score and YH and muscling score and YH (ranging from 0.05 +/- 0.05 to 0.18 +/- 0.06). Moreover, negative and favourable genetic associations between each of WP, WM, YP and YM, and MH (ranging from -0.21 +/- 0.07 to -0.12 +/- 0.09) were estimated. Correlated responses are expected to be unfavourable for hip height, measured at yearling and at maturity, when animals better genetically evaluated for conformation in relation to finishing precocity and muscling scores are selected. Genetic changes obtained for YH and MH were null (-0.02 cm/year and 0.03 cm/year respectively). It can be explained, in part, by smaller weights in the selection index for conformation rather than finishing precocity and muscling, as applied in the studied herds. However, in herds of beef cattle not evaluated and selected for finishing precocity and muscling scores or that have cows with larger size than acceptable, the adoption of mature hip height as one of the selection criteria can be one alternative for obtaining females with a desirable size.