Genetic evaluation of somatic cell scores for United States dairy cattle.

Increases in milk yield from genetic selection may be accompanied by correlated increases in genetic susceptibility to clinical mastitis and somatic cells. Unlike clinical mastitis, somatic cell scores can be easily determined and recorded and are related to milk loss from subclinical mastitis. Selection against high somatic cell scores should decrease incidence of clinical mastitis and provide direct economic benefits through higher milk quality premiums. Genetic evaluation for lactation means of linear somatic cell scores has been implemented by USDA and parallels that for yield traits. Because additive genetics accounts for only about 10% of differences in somatic cell scores among cows, more information is needed for the same degree of confidence in genetic estimates as for yield. Only 80% of DHIA cows currently have somatic cell records. Thus, reliabilities of somatic cell evaluations are smaller than those for yield traits. Most progress in selection for lower somatic cell scores will come through sires of cows considered as bull dams. Somatic cell evaluations may best be reported through an economic index with a small amount of emphasis on somatic cell score relative to yield traits. Greater emphasis on somatic cell scores would decrease genetic gain in yield traits, which are economically more important.