Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/1998
Publication Date: N/A
Interpretive Summary: Genetic evaluations can be more accurate if yields on test day are analyzed instead of lactation yields because the effect of the environment on a specific test day can be accounted for. A genetic evaluation system based on a test-day statistical model requires information on how the test-day yields are related to one another over the course of lactation. Milk, fat and protein yields from 37 Pennsylvania and Wisconsin herds were used to estimate correlations and heritabilities for four stages of lactation using test days nearest the center of each lactation stage. These estimates of heritabilities and correlations will be used as the basis for a multiple-trait genetic evaluation system that allows information from milk yield to improve the accuracy of estimates of genetic merit for fat yield even when information on fat yield is not recorded for some test days. Development and implementation of a multiple- trait, test-day evaluation system will provide dairy breeders with genetic evaluations that allow more accurate selection for desired traits.
Technical Abstract: (Co)variance components for milk, fat, and protein yields during first lactation were calculated from test day data from 23,029 cows representing 37 large herds in Pennsylvania and Wisconsin. To minimize the number of lactations with missing values, four stages of lactation of 75 days each were defined, and the test day nearest the center of each interval was used. A canonical transformation was used to estimate variance components. A lactation stage was progressively added and observations with missing values were deleted for four analyses; 17,190 observations were available for the final analysis of lactations with test days in all lactation stages. Estimates were similar across analyses, which indicated little effect by selection. Heritabilities usually increased with stage of lactation, were highest for milk yield, and averaged .15. Phenotypic and genetic correlations between milk and protein yields were higher than between milk and fat yields. Within each trait, the genetic correlation declined from about .9 for adjacent lactation stages to about .75 between lactation stage 1 and lactation stage 4. When traits were from the same lactation stage, the genetic correlation averaged .4 between milk and fat yields, .79 between milk and protein yields, and .53 between fat and protein yields. Phenotypic correlations were >.9 between milk and protein yields and around .65 between milk and fat yields and between fat and protein yields for the same lactation stage.