ADJUSTMENT FACTORS AND GENETIC EVALUATION FOR SOMATIC-CELL SCORE AND RELATIONSHIPS WITH OTHER TRAITS OF CANADIAN HOLSTEINS

Test day SCC records were obtained from the Ontario DHI and converted to somatic cell score using a logarithmic transformation. Adjustment f actors for stage of lactation and calendar month were obtained for fir st and later lactations. Effects of stage of lactation were significan t and followed a systematic pattern. Seasonal effects were small. Sire estimated breeding values for lactation mean adjusted test day somati c cell scores were obtained from an animal model based on first, later , and all lactations, assuming a heritability of .11 and repeatability of .27. Mean accuracy of estimated breeding values for sires with at least 30 daughters was .64 for first lactation, .76 for second to fift h, and .86 for all lactations. Genetic trends for somatic cell score w ere not significant. The correlation of breeding values estimated from first lactations with estimates based on later lactations was .62 for sires with at least 50 daughters, which resulted in an approximate ge netic correlation of .72. Correlations of sire estimated breeding valu es for somatic cell score from first lactation with estimated breeding values for milk, fat, protein, fat percentage, protein percentage, an d milking speed were .12, .05, .11, -.09, -.02, and .20. Coffelations between sire estimated breeding values for somatic cell score and type traits were generally small, but favorable with mammary system, -.13, and fore udder, -.16, and unfavorable with dairy character, .24. Soma tic cell score should be considered as an auxiliary trait in dairy cat tle breeding programs.