Heritability of Somatic Cell Count and Its Relationship with Milk Yield and Composition in Holsteins

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Abstract
The 85,271 observations of test-day somatic cell count were on 17,477 Holstein cows with identified sires in 664 herds on the Quebec Dairy Herd Analysis Service [Canada] during 1977. Somatic cell counts were transformed to a log (natural) scale and corrected for effects of month of test, age of sample at analysis, stage of lactation, season of calving and age of cow. Analysis was within lactation and age. Heritability of test-day somatic cell count ranged from 0.05 for .ltoreq. 2-yr-olds to 0.10 for mature (.gtoreq. 6-yr) cows and averaged 0.08 over all ages. Phenotypic correlations between somatic cell count and milk, fat and protein yields and fat and protein percentages averaged -0.13, -0.13, -0.11, -0.02 and 0.09, respectively, and increased in absolute size with advancing age of cow. Respective average genetic correlations were 0.14, 0.08, 0.18, -0.08 and 0.04.