Using Somatic Cell Score Evaluations for Management Decisions

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Abstract
Genetic evaluations for mean somatic cell scores for lactation enable producers to reduce increases in SCC that accompany genetic improvement for milk yield. Current progeny test designs will produce lower reliabilities for somatic cell score evaluations than for milk yield because of lower heritability. Genetic correlations of somatic cell scores between first and second lactation are lower than for milk yield. Thus, changes in evaluations of sires over time should be greater for somatic cell score than for milk yield. Absence of interaction of genotype and environment for somatic cell score indicates that the same bulls would be selected for use in herds with high and low somatic cell scores. Most evaluations will be between -.25 and .25, averaging .01 for Holsteins, which represents an increase of approximately 50% in geometric mean SCC and an 11% higher incidence of clinical mastitis for daughters of bulls with highest evaluations compared with daughters of bulls with lowest evaluations. Selection to reduce somatic cell score does not appear to be justified economically. Properly constructed indexes would reduce the projected increases in mastitis incidence by 20 to 25% with a 2% decline in genetic progress for milk yield. The greatest impact of genetic evaluations for somatic cell score would likely result from choice of sires of bulls to progeny test.