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Heritabilities of Measures of Somatic Cell Count per Lactation

Dairy Herd Analysis Service and Department of Animal Science, Macdonald College of McGill University, Ste. Anne de Bellevue, Quebec, Canada H9X 1C0

ABSTRACT

Measures per lactation of somatic cell count were developed from monthly test-day observations for 3,966 Holstein cows on the official test option of the Quebec Dairy Herd Analysis Service. Cows were daughters of 99 sires that had five or more daughters in two or more herds. Two lactation periods were considered. One was based on all somatic cell counts regardless of stage of lactation, and the other included only cell counts after 45 days postpartum. Data were processed both on the original (1000 cells/ml) and log (natural) transformed scales. Lactation measures of somatic cell count considered were arithmetic, geometric, harmonic, and weighted (by milk yield) means, median, range, coefficient of variation, ratio of high to low test, and percent tests over 300, 400, and 500 thousand cells/ml for complete and partial lactations and days to highest test, cell count on first test, regression on days in milk, and intercept at day 45 postpartum for partial lactations. Heritabilities of measures were estimated from sire and error variances obtained by iterative minimum norm quadratic unbiased estimation. Heritabilities ranged from 0 to .14 with the highest for averages of log transformed cell counts. Heritabilities of measures on partial lactations, from day 45 postpartum, were generally lower than corresponding measures on entire lactations.

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