Estimates of Genetic Parameters of Some Functions of Part Lactation Milk Records

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Abstract
Estimates of repeatability of monthly milk records and functions of monthly milk records were obtained from the within herd regression of a 2d, record on the preceding record from the records of 2358 Holstein cows in 226 New York herds. Highest repeatability estimates of monthly records were for the 5th, 6th, and 7th months of lactation. The estimates of repeatability of cumulative production increased with advance in the stage of lactation. Repeatability estimates of bimonthly records were nearly as large as estimates for complete yield. Heritability estimates and estimates of genetic correlations obtained from a paternal half-sib analysis of the records of 1526 daughters of 145 sires used in artificial insemination followed the same patterns as the estimates of repeatability. These results indicate that temporary environmental conditions are more important during 1st, and last stages of production than during the middle stages when genetic and permanent environmental differences between cows provide relatively more control over production. The high genetic correlations between some functions of monthly records and total yield indicate that genetic progress by selection of sires on the basis of these functions may be nearly as rapid as by selection on the basis of total yield. If generation time can be decreased or selection intensity increased by the use of part records, then genetic progress may be more rapid with selection based on part records than with selection on complete yield.