Genetic and environmental relationships among somatic cell count, bacterial infection, and clinical mastitis.

Incidence of bacterial infection in 9784 lactations of 7763 cows in 31 herds, SCC in 32,448 lactations of 19,764 cows from 54 herds, and incidence of first parity mastitis recorded in the first lactations of 148,143 cows in 828 herds were analyzed. Bacterial infection was analyzed dichotomously by both threshold and linear models. The effects of parity, season, stage of lactation, and parity by stage of lactation interaction on SCC were estimated. Heritability of mean lactation log SCC--corrected for the effects of parity, season, and stage of lactation--varied from .13 to .27 for all parities in different data sets. Heritability of bacterial infection was .04 for the threshold model and .02 for the linear models. Heritability of field-recorded mastitis was .01. The genetic correlation between bacterial infection and SCC was near unity, but the genetic correlation between SCC and mastitis was .3. Selection for lowered SCC should reduce incidence of bacterial infection by 2% per unit of selection intensity.