Data from 17.968 records from 2,130 first lactations of Holstein cows calved between 1988 and 1991, daughters of 136 sires, were analyzed. The data set was divided into ten subsets based on days in milk. Traits studied were test-day milk yield (Ml to M10) and 305d milk yield (M305). These traits were adjusted for some environmental effects: class of age at calving, interval from calving to first test-day and herd-vear-season. Restricted Maximum Likelihood estimates of (co)variances components were obtained from two-trait analyses under a sire model. Estimates of heritabilities for M l to M10 ranged from 0.04 to 0.32 and the largest value was found in the second half of the lactation (M5 to M7). Heritability estimate for P305 was 0.32. Genetic correlation estimates between individual test-day and M305 ranged from 0.78 to 1.00. The results suggested that test-day yields, mainly in mid lactation, can be used in genetic evaluation instead of 305d milk yield, because estimates of heritability have practically the same magnitude. Moreover, early selection can reduce generation interva

S. G Machado, M. AR de Freitas, C. H Gadini

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 23: Opening: plenary; technology; transfer; beef cattle; dairy cattle; pigs., , 427–430, 1998
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