SELECTION ON SOMATIC-CELL SCORE TO IMPROVE RESISTANCE TO MASTITIS IN THE UNITED-STATES

Increased genetic susceptibility to mastitis has accompanied the rapid genetic increase in milk yield, and genetic selection for mastitis re sistance should be considered. Somatic cell score is recommended as an indicator trait to achieve genetic improvement for mastitis resistanc e. Heritability of somatic cell score is around 10%, and genetic corre lation between somatic cell score and clinical mastitis is around .6 t o .8. Selection for lower somatic cell score is consistent with the go al of maximizing genetic improvement for total economic merit and shou ld be included in breeding programs. National genetic evaluations for somatic cells scores will use the same animal models and methods as ar e currently used for milk yield traits. Reliabilities of PTA for somat ic cell scores will be smaller than for yield traits because of lower heritability and availability of records from fewer cows. Several form s are proposed for reporting genetic evaluations of somatic cells to p roducers, and advantages and disadvantages are discussed. Using somati c cell scores for breeding decisions would marginally decrease genetic gain for milk yield and increase total economic merit. Optimal select ion indexes would slow the rate of increase in mastitis, rather than d ecrease its incidence.