Milk records were subdivided into low, medium, and high groups by herd-year averages. Mean milk yields and standard deviations of three production levels on the basis of ME milk yields of all cows freshening in the same herd-year were 7,284.68kg - 9,333.28Kg and 1,370.30 - 1,686.56, respectively and therefore showed the heterogeneity among three production levels of milk yield. When milk records were transformed to logarithms, average milk yields, "standard deviations, and coefficients of variation for production levels were stable among sub-groups. In case of transforming to logarithms, residual variance component of low group increased markedly and this result indicated that the ranks of genetic evaluations for bull-sires and bull-dams could be reversed. REML estimates of heritability for low, medium, and high yield groups were 0.189, 0.234, 0.254, respectively, and multiple trait evaluation was necessary. Proportions of sires selected from groups by herd-year production levels corresponding to top 5% based on PTAM were 18.2%, 36.4%, 45.5% for low, medium, and high groups. In case of log transformation, proportions of sires selected from groups corresponding to top 5% of PTAM were 38.9%, 61.1%, 0.0% for low, medium, and high groups, respectively. High genetic and rank correlations among sub-groups indicated that most of heterogeneity was due to environmental differences in Korean dairy herds.

K. J Lee, K. D Park

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 17. Genetics and breeding of dairy and beef cattle, swine and horses, , 46–49, 1994
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