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Part Lactations. II. Genetic and Phenotypic Studies of Monthly Milk Fat Yield

New Zealand Dairy Board, Wellington, and Department of Animal Husbandry, Cornell University, Ithaca, New York

ABSTRACT

Monthly milk fat yields have been analyzed to obtain estimates of heritability, repeatability, genetic and phenotypic correlations. Generally speaking, the heritability estimates decrease and the repeatability estimates increase as lactation progresses. The differences between cows other than additive genetic effects, therefore, appear to affect production more in late lactation than in early lactation. Estimates of the genetic correlation of monthly yield with lactation yield show approximately the same trend over the lactation as the heritability estimates—an increase up to the third month and then a general decline. This indicates that in addition to environment playing an increasingly more important role in determining production as lactation progresses, the sets of genes determining total yield and end-of-lactation yield are not altogether the same. Genetic correlations estimated among monthly yields are higher among early months of lactation than among the later months, and also higher than the correlations between early and late months. The estimated sampling errors of these estimates are very high, in the order of 0.20, indicating that considerably more data than the available 2,500 daughter-dam pairs are needed for satisfactory estimation of these parameters.