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Trends in Genetic and Phenotypic Variances for Milk Production

Department of Animal Science, Cornell University, Ithaca, MY 14853

ABSTRACT

Residual standard deviations estimated separately for each year of first freshening from first lactation milk records of artificially sired Holstein cows increased from 1960 to 1982, especially after 1976. The pattern on the square root scale was similar. On the logarithmic scale, residual standard deviations were smallest in the middle of the time period. Heritabilities estimated from paternal half-sib correlations were greater than .30 on all scales until about 1976 when estimates began to go below .20. Estimates were similar for untransformed and transformed records. When records were divided into four herd production groups within each year, the same patterns over time for residual standard deviations were observed as for the combined data for each year. On the untransformed scale, the largest standard deviations were associated with high herd production and the smallest with low herd production. On the square root scale, standard deviations were similar for all herd production groups. On the log scale, residual standard deviations were smallest with high production and largest with low production—the reverse of the untransformed scale, although differences were smaller. Heritability was highest with middle-production groups and smallest with low-production groups. Data available for each year ranged from 1,400 and 2,849 to 6,821 and 58,082 records, respectively, of daughters of sampling and proved bulls with from 115 to 513 sampling bulls.

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