Heritabilities of Measures of Somatic Cell Count per Lactation

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Abstract
Measures per lactation of somatic cell count were developed from monthly test-day observations for 3966 Holstein cows on the official test option of the Quebec [Canada] Dairy Herd Analysis Service. Cows were daughters of 99 sires that had 5 or more daughters in 2 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 1st 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 0-0.14 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.