Avaliação genética de touros usando produções em lactações completas ou parciais projetadas

Abstract

Records of 4,495 lactations from 2,254 cows, daughters of 1,618 dams and 145 sires of the Gyr breed were used to study the viability of using production from projected partial lactations, on the evaluation of sire's genetic merit. After estimating milk yield up to 90, 150, 210, 240 and 305 days of lactation, 10, 30, 50 or 70% of those were extended. Extension were done from 91, 151, 211 or 241 days in milk to the observed lactation length or to 305 days. Genetic evaluations were done by MTDFREML package, using an animal model. For all traits, models included the fixed effects of herd-year, season of calving and age of calving as linear e quadratic, and as random effects, animal, permanent environment and error. Heritability estimates for milk yield up to 305 days (P305d) was 0.1729 (0.047), and for the estimated production (PE) by extending a percentage of lactations, ranged from 0.1675 (0.048) to 0.2680 (0.056). Genetic and phenotypic correlations estimates betweenP305dandPEranged from 0.9410 (0.466) to 1 (0.589) and from 0.7307 to 0.9958, respectively. Rank and values correlations estimated from the results of genetic evaluations of 145 sires, using PE and P305d ranged from 0.7982 to 0.9981 (rank correlations) and from 0.8329 to 0.9988 (values correlations). Genetic evaluations using PE were statistically equal (P>0.05) by the Friedman test to the ones using P305d. Selecting the best 5% sires based on theirs predicted transmitting ability (PTA) for PE milk yield resulted in the selection of 41,65% to 100% of the same sires as they were selected using PTA based on P305d. On the hypothesis of culling 5% of sires of smaller PTA,the coincidence ranged from 70,41 to 100%. Based on the results, the conclusions were: the quadratic log function used in this study can be used to extend lactations to do sire's genetic evaluation; considering the small change in the sire's rank when PEs are used, it is suggested to extend 10% of lactations from 91 days in milk, as the same as to extend 10% to 70% of lactations from 211days in milk; production estimated by extending lactations were highly correlated with P305d; heritabilities of PE extending 10% of lactations from 91,151 and 211days in milk or PE extending 10, 30, 50 or 70% of lactations from 241 days in milk were similar to the ones estimated using P305d.