We evaluated genetic change in milk yield up to 305 days, lactation length and daily milk yield in Gyr cattle selected for milk production. The data were obtained from the records of a herd raised in the municipality of Cajuru, SP, Brazil. Two statistical models were utilized to estimate repeatability and genetic change. Month and year of calving, bull, within-bull cow, cow's age at calving, and lactation length had significant effects. A total of 3997 lactations corresponding to the period from 1962 to 1987 were analysed by methods A and B. Method A is based on repeated observations of the same animal, and method B is based on the principle of the use of sires over successive years. The genetic change estimated for milk yield during the period analysed was 18.5 kg/year. When adjusted for lactation length, the estimate was 12.9 kg/year. By analysing the data containing information about the first lactation only for each cow, genetic change for milk yield was estimated by method B. The estimate thus obtained was -0.45 kg/year.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XIV. Dairy cattle genetics and breeding, adaptation and conservation., , 199–202, 1990
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