Genetic analysis of California mastitis test records. II. Score for resistance to elevated tests.

A lactation score for rating individual cows for their apparent resistance to elevation of California Mastitis test is described. The score uses the first nine monthly California Mastitis Tests weighted by the number and position of elevated coded tests. California mastitis test 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 zero was assigned to lactations with all nine tests elevated. The number and position of the elevated coded tests influenced the 305-day milk yield, and the position of elevated coded tests influenced lactation persistency. Differences were significant among sire progeny groups for the cumulative lactation score. Heritabilities for the cumulative lactation score were .48 +/- .07, .36 +/- .08, .46 +/- .15, and .23 +/- .12 for first, second, third, and fourth 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 -.31 +/- .13 for first lactation records.