(Co)variance components for milk, fat, and protein yields during first lactation were calculated from test-day data from 17,190 cows representing 37 large herds in Pennsylvania and Wisconsin. Initially, four lactation stages of 75 days each were defined, and the test day nearest the midpoint of each interval was used. This approach minimized the number of lactations with missing values. A canonical transformation was used to estimate variance components, which required that all observations with missing values be deleted. Preliminary analysis showed little effect from this selection. Heritabilities usually increased with lactation stage, were highest for milk, and averaged .15. Phenotypic and genetic correlations between milk and protein yields were higher than between milk and fat yields. For each yield trait, the genetic correlation declined from about .90 for adjacent lactation stages to about .75 between lactation stages 1 and 4. When yield traits were from the same lactation stage, the genetic correlation averaged .39 between milk and fat, .78 between milk and protein, and .53 between fat and protein. Phenotypic correlations within lactation stage were >.90 between milk and protein and around .65 between milk and fat and between fat and protein. Estimates for the four 75-day lactation stages were extended to provide estimates for twelve 25-day stages (36 traits) using (co)variance functions, which allowed denser coverage of the lactation.
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