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Genetic parameters were estimated for the body weights, daily gains and body measurements of young bulls at central rearing stations. The data included 5512 black-and-white bulls and 1186 Holstein-Friesian (HF) and 1/2 HF bulls bom during 1981 - 1984 and 3959 black-and-white, 2098 HF and 1/2 HF bom during 1985 - 1987. The values of genetic parameters estimated in both periods were similar. Heritabilities of body weights at 120 and 360 days and average daily gains were between 0,21 to 0,29; estimates of heritabilities for heart girth and height at withers were slightly higher. The highest genetic correlations were found between body weight at 360 days and average daily gain. For bulls bom between 1981 and 1984 the BLUP method was used to evaluate predicted differences (1/2 of breeding value) in body weight at 360 days and average daily gains of young bulls. Averages of predicted differences calculated for each rearing station yielded small genetic differences among them. In addition, the predicted differences were evaluated for bulls’ sires based on their sons performances.
To examine the infuencęs of environmental factors on the performance of 13 734 young bulls (Black-and-White, HF and 1/2 HF) bom between 1981 - 1987 and tested at central rearing stations, the fixed effects of station, month and year of birth were included in the linear model. The F-tests showed highly significant effects of stations on all traits, Le., body weights at 120 and 360 days, heart girth and height at withers. Differences among years and months were not always significant Estimates of least squares constants showed seasonal differences: young bulls bom in the second half of a year usually performed better than those bom during winter or early spring.
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