Short communication: correlated selection responses in somatic cell count from selection against clinical mastitis.

Correlated selection responses in lactation mean somatic cell score (LSCS) were estimated for groups of cows selected for high protein yield and low mastitis frequency, respectively. Selection for increased milk production resulted in an unfavorable correlated response for LSCS, whereas direct selection against clinical mastitis resulted in a favorable correlated selection response. After 6 cow generations, the genetic difference between the high protein yield group and the low mastitis group was 0.3 units LSCS, equivalent to a difference in somatic cell count of approximately 15,000, assuming deviations from a population mean LSCS of 4.1.