Heritability of Mastitis Score in Pacific Northwest Dairy Herds

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Abstract
Dairy Herd Improvement Association records of Holsteins in the Pacific Northwest were used to estimate heritabilities and correlations between milk yield, fat yield, fat percentage and mastitis score. Effects of sire and lactation number were important for all traits except lactation number for fat percentage. Average mastitis score as well as frequency of elevated tests increased with parity. Paternal half-sister analysis showed that heritabilities of milk yield, fat yield and fat percentage decreased with age, averaging 0.33, 0.28 and 0.47 for 1st records and 0.29, 0.20 and 0.33 for last records. Heritability of mastitis score increased slightly with age, averaging 0.10 for 1st records and 0.11 for last records. Milk yield and fat yield showed small genetic correlations of 0.05 and 0.07 with mastitis score. Pearson and Spearman correlation coefficients were computed between regressed least square estimates for sires derived for 6 independent sets of daughters. Pearson correlations between mastitis scores of 1st and last lactations were 0.19 and 0.34, corresponding average Spearman correlations were 0.11 and 0.29. These low correlations between sire rankings in different data sets correspond closely with the low heritabilities for mastitis score.