Genetic Analysis of California Mastitis Test Records. II. Score for Resistance of Elevated Tests

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Abstract
A lactation score for rating individual cows for their apparent resistance to elevation of California Mastitis Test is described. The score uses the first 9 mo. California Mastitis Tests (CMT) weighted by the number and position of elevated coded tests. CMT readings of negative and trace were coded normal and 1, 2 or 3 elevated. A cumulative lactation score of 21 was assigned to lactations without elevation in coded test and a score of 0 was assigned to lactations with all 9 tests elevated. The number and position of the elevated coded tests influenced the 305-day milk yield and the position of persistency. Differences were significant among sire progeny groups for the cumulative lactation score. Heritabilities for the cumulative score were 0.48 .+-. 0.07, 0.36 .+-. 0.08, 0.46 .+-. 0.15 and 0.23 .+-. 0.12 for 1st, 2nd, 3rd and 4th or later lactation groups. Selection for a high cumulative lactation score should reduce the occurrence of elevated coded test scores. The genetic correlation between 305-day milk yield and cumulative score was -0.31 .+-. 0.13 for 1st lactation records.