Genetic evaluation of Holstein bulls for somatic cells in milk of daughters.

Lactational means of somatic cell score from sample days of primiparous DHI cows were analyzed to compare dairy records processing centers for data properties, to examine effects of age at calving and month of calving on somatic cell score, and to calculate preliminary PTA of sires for somatic cell scores. Five processing centers contributed data but differed substantially with respect to availability of records in progress, DIM on last sample day, and length of time that data were submitted. Highest lactational means of somatic cell score tended to be in short lactations for young cows but in long lactations for older cows. Regions of the country differed substantially for effect of month of calving on lactational means of somatic cell score, but lactational means of somatic cell scores increased with age at calving for all dairy records processing centers. Standardized yields of milk, fat, and protein were included as multiple traits with somatic cell score for analysis. Heritability of somatic cell score was .08 to .16 for individual processing centers and .10 across processing centers. Phenotypic correlations of somatic cell score with yield traits were negative, but genetic correlations were positive. Selection for decreased somatic cell score on a national basis should be possible and would seem advisable to decrease mastitis infections. Response may be slow, however, because of antagonistic genetic relationships with the yield traits, which have great economic importance.