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

Dairy Herd Analysis Service and Department of Animal Science, Macdonald College of McGill University, Ste. Anne de Bellevue, Quebec, Canada H9X 1CO

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 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 .05 for < 2 yr olds to .10 for mature (> 6 yr) cows, and averaged .08 over all ages. Phenotypic correlations between somatic cell count and milk, fat, and protein yield, and fat and protein percent averaged —.13, —.13, —.11, —.02, and .09 and increased in absolute size with advancing age of cow. Respective average genetic correlations were .14, .08, .18, —.08, and .04.

¹ Department of Animal & Poultry Science, University of Guelph, Guelph, Ontario N1G 2W1.

² Department of Dairy Science, University of Wisconsin, Madison, WI 53706.