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Paternal half-sib heritability estimates oi' milk yield, number of quarters infected with mastitis organisms, and number <>f quarters showing abnormal mill; were obtained from an analysis of milking and mastitis data of 2,865 Holstein cows. The within-herd heritability estimate for daily milk yield was 0.353 and that for the number of quarters infected with Streptococcus agalactiae 0.196. The heritability estimates for the number of quarters infected with organisms other than Streptococcus agalactiae, number of quarters infected, and number of quarters showing abnormal milk were all below 0.1.
Within-herd correlation coefficients were calculated among daily milk yield, milking data, age, and mastitis indicators. Age, stripping' time, and machine-on time were positively correlated with daily milk yield. Older cows had longer machine-stripping times and machine-on times than younger cows. Older cows also showed higher values fro' all indicators of' mastitis.