Genetic Analysis of California Mastitis Test Records. I. Coded Tests

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Abstract
A sample of 15,965 Holstein lactation records having California Mastitis Test (CMT) scores for at least the first 9 mo. of standard Dairy Herd Improvement production testing were studied for the effects of sire, parity and year and month of calving. CMT readings of negative or trace were coded normal while readings of 1, 2 or 3 were coded elevated. The frequency of elevated tests increased as parity increased. Effect of year of calving and a trend toward higher percentages of elevated coded test scores were significant. There was no distinct trend in elevated coded test score with month of calving. Sire effect on the incidence of elevation in the coded score was important. Heritability of 1st and 2nd lactation monthly coded CMT scores from a paternal half-sister analysis within herd, year and season for sires with at least 25 daughters ranged from 0.11 .+-. 0.04 to 0.48 .+-. 0.07. Genetic correlations bewteen the 3rd test and following tests of 1st and 2nd lactations ranged from 0.25 .+-. 0.15 to 0.58 .+-. 0.18.