Selection Practiced Among Dairy Cows. II. Total Production Over a Sequence of Lactations

Abstract

Phenotypic selection among dairy cows was investigated with respect to milk yield in 1 or more lactation records simultaneously. An average index was constructed such that the efficiency of selection practiced on more than one record was maximized in retrospect. Lactation records made by Holstein cows in 464 New York herds tested continuously under the DHIA system at least during the period 1957 to 1962 were studied. The phenotypic selection differentials expressed in standard deviation units for the M.E. milk yield deviation (deviation from the intra-herd-year-season-lactation subclass mean) and the conventional deviation from herd-mates for the 1st to the 4th lactations were .13, .12, .11, .11, and .12, .11, .11, .10 for all cows, respectively. Generally, differentials were progressively lower for the earlier lactations when cows had at least 2, 3, and 4 lactation records. The standardized selection differentials of the herd-mate deviations for cows with at least 4 records were .02, .08, .08, and .11 for the 1st to the 4th lactations. Relative weights in the indices were largest for the most recent lactation record. A negative weight was obtained for the 1st-lactation deviations of the 2 yield deviations when cows had at least 4 records. The inclusion of earlier yield deviations in an index increased the efficiency of the description of the selection process by .01, .01, and .03 with the addition of 1, 2, and 3 records, respectively. Efficiencies of phenotypic selection for the M. E. deviations were .40, .37, .31, and .26 when all records were considered in an index and selection was practiced after the 1st to the 4th lactations, respectively.