GENETIC EVALUATION OF HOLSTEIN BULLS FOR SOMATIC-CELLS IN MILK OF DAUGHTERS

Lactational means of somatic cell score from sample days of primiparou s DHI cows were analyzed to compare dairy records processing centers f or data properties, to examine effects of age at calving and month of calving on somatic cell score, and to calculate preliminary PTA of sir es for somatic cell scores. Five processing centers contributed data b ut differed substantially with respect to availability of records in p rogress, DIM on last sample day, and length of time that data were sub mitted. Highest lactational means of somatic cell score tended to be i n short lactations for young cows but in long lactations for older cow s. Regions of the country differed substantially for effect of month o f calving on lactational means of somatic cell score, but lactational means of somatic cell scores increased with age at calving for all dai ry records processing centers. Standardized yields of milk, fat, and p rotein were included as multiple traits with somatic cell score for an alysis. Heritability of somatic cell score was .08 to .16 for individu al processing centers and .10 across processing centers. Phenotypic co rrelations of somatic cell score with yield traits were negative, but genetic correlations were positive. Selection for decreased somatic ce ll score on a national basis should be possible and would seem advisab le to decrease mastitis infections. Response may be slow, however, bec ause of antagonistic genetic relationships with the yield traits, whic h have great economic importance.