Genetic and Phenotypic Relationships Between Lactation Cell Counts and Milk Yield and Composition of Holstein Cows

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Abstract
Monthly somatic cell count data were collected between Feb. 1977 and Feb. 1982 for Holstein cows in 928 herds enrolled on the Quebec Dairy Herd Analysis Service [Canada]. The geometric mean of the long monthly cell counts was calculated for each lactation. Official lactation records for 305-day milk, fat and protein yields, and fat and protein percents were obtained for same cows. There were 18,189 cows in 1st lactation representing 257 sires, 13,225 in 2nd lactation representing 206 sires and 8683 in 3rd lactation representing 151 sires. Heritabilities of yield traits and protein percent increased across 3 lactations. Heritability of fat percent was similar in 1st and 3rd lactations but decreased slightly in 2nd lactations. Heritability of lactation cell count was small, being least in 2nd lactations. Genetic correlations between lactation cell count and yield traits were positive in 1st lactations, small and negative in 2nd lactations and small and positive in 3rd lactations. Genetic correlations between lactation cell count and fat and protein contents were small in the 3 lactations. Phenotypic correlations between lactation cell count and production traits were small in each of the 3 lactations. Genetic correlations between yield traits in early lactation and lactation cell count in a subsequent lactation were positive. The genetic correlation between protein percent in an early lactation and cell count in a later lactation was large between 1st and 2nd lactations, decreased between 2nd and 3rd lactation, and small between 1st and 3rd lactations. Genetic correlations were small and negative for fat percent.
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