Genetic and Phenotypic Statistics of Lactation Cell Counts in Different Lactations of Holstein Cows

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Abstract
Between Feb. 1977 and Feb. 1982, 680,246 monthly test-day observations of somatic cell count were taken for Holstein cows having completed 79,124 lactations in 941 herds on the Quebec [Canada] Dairy Herd Analysis Service. Data were transformed to natural log scale, and analyses were separate within 5 parity groups. Two lactational measures of cell count, geometric mean and weighted (by test-day milk yield) mean of the log of monthly cell counts, were calculated for each lactation. Maximum likelihood, iterative Minimum Norm Quadratic Unbiased Estimation and multivariate Restricted Maximum Likelihood procedures were used for estimation of genetic and phenotypic parameters. Repeatabilities of log test-day cell counts and log of test-day cell counts corrected for milk yield were the same at each parity, 0.36, 0.41, 0.42, 0.42 and 0.42 in 1st, 2nd, 3rd, 4th and 5th and later parities. Repeatabilities of lactational measures of cell count were 0.33 between 1st and 2nd parities, 0.40 between 2nd and 3rd parities, 0.13 between 1st and 3rd parities and 0.27 between 1st, 2nd and 3rd parities. Heritabilities of measures of lactational cell count were small from 0.06-0.14 in the 5 parities. Genetic correlations between measures of lactational cell count in different parities were close to unity 0.90-0.97.