Selection Practiced Among Dairy Cows. I. Single Lactation Traits

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Abstract
Efficiency of the selection for milk and fat practices among Holstein cows was measured by comparing the actual selection with that which might have been possible if all removals had been made strictly on yield. The correlation rYP is the efficiency of selection where P is an implicit normally distributed variable upon which actual selections are made and Y is yield. The selection differential and the reduction of variance in the selected population were used to estimate the efficiency of selection for 34,000 lactations made by cows in 464 herds on test continuously from 1957 to 1962. Phenotypic selection (cows surviving to start another lactation) and realized selection (cows represented by daughters) were investigated. Estimates of the efficiency of intraherd-year-season-lactatlon phenotypic selection, as measured by the selection differential of actual milk deviations, were 40, 32, 27, and 24% for the 1st 4 lactations, respectively. The corresponding efficiencies for M. E. milk deviations from the herd-mate average were 43, 34, 27, and 25%. The realized standardized selection differentials of actual milk deviations for the 4 lactations were .16, .14, .15, and.U, respectively. The corresponding phenotypic differentials were .12, .11, .11, and .10, respectively. Selection pressures on milk and fat deviations were similar.