Autores
M. P. M. Gama, H. T. Ventura, M. Alencar Pereira, L. El Faro, C. C. P. Paz

Resumo
In Brazil, the Guzerat breed is used for beef, milk production or as dual purpose breed. Progeny tests of dairy bulls are available for the breed (Programa Nacional de Melhoramento Genético do Guzerá para Leite–National Breeding Program of Dairy Guzerat Cattle), in addition to genetic evaluations of growth traits performed by the Brazilian Association of Zebu Breeders (Associação Brasileira de Criadores de Zebu-ABCZ). For this reason, beef bulls are used in dual purpose herds. To identify differences in the growth and milk production patterns of bulls of this breed, two-trait analyses were performed between cumulative milk production at 305 d (P305) and weight at 120 d (W120), weaning weight (WW), yearling weight (YW), post-weaning weight (PWW), and weight at 24 mo of age (W24). The data files contained 97,394, 65,181, 50,443, 40,425, and 31,279 records, respectively, obtained from the database maintained by ABCZ. The model used included the fixed effects of contemporary group and age of cow at calving (linear and quadratic covariate), and direct additive genetic, permanent environmental (maternal for weights; direct for milk) and residual effects as random effects. The variance components were estimated by the restricted maximum likelihood method using the WOMBAT program. The heritabilities estimated in the two-trait analyses were 0.24 (0.003), 0.14 (0.009), 0.16 (0.0012), 0.18 (0.0014), 0.21 (0.0017) and 0.22 (0.020) for P305, W120, WW, YW, PWW and W24, respectively. The genetic correlations between weights at different ages and P305 were positive but low, ranging from 0.27 (0.111) to 0.38 (0.099), indicating a low association between the breeding values of the traits. Selection for P305 resulted in a low correlated response to increase weights at the ages studied. Based on the breeding values estimated, bulls with a high milk production potential and bulls with a high beef production potential were identified by cluster analysis. The separation of bulls for each selection objective will potentiate the result of genetic selection of the breed.

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