Genetic Studies of Dairy Production Early in Lactation

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Abstract
Heritabilities and genetic and phenotypic correlations are reported from a study of monthly productions of artificially-bred daughters in the first 6 months of lactation, using correction factors developed to correct records made in the same calendar month for stage of lactation simultaneously with age. Problems associated with estimating genetic correlations from daughter-dam pairs are discussed, and the interpretation of phenotypic correlations less than their genetic counterparts is also considered. Within-herd heritability of lactation yield is estimated as 0.12 from 4,465 daughter-dam pairs, and as 0.36 from 6,960 paternal half-sibs by 119 sires in 603 herds. Comparable estimates for monthly yields are 0.10 and 0.25 respectively. Genetic correlation estimates between monthly yields are mostly close to 1.00 from the paternal half-sib analysis but show wide variation in the daughter-dam analysis.

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