Genetic and environmental correlations between test-day somatic cell count and milk yield traits

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Australian Holstein cattle data were analysed to estimate genetic and environmental correlations between test-day yield (milk, protein, fat) and somatic cell count (SCC) using a random regression sire model. The estimated correlations were also compared to those from multi-trait and repeatability models. Estimates obtained from different models showed a similar trend in which genetic correlations between yield and log SCC were positive at the beginning and negative at the end of the first lactation. In the second and third lactations genetic correlations were near zero at the beginning of the lactation but negative at the end. However, environmental correlations were always negative particularly when the two traits are recorded on the same day or on days close to each other and the correlation values become stronger with parity. The increase in the strength of negative environmental correlations between yield traits and SCC over parities shows that older cows are more likely to suffer prolonged elevation of SCC or even permanent udder damage that depresses their milk yield. Genetically at the beginning of the first lactation the fact that high milk yield tends to be positively associated with high SCC may suggest that high producers are more susceptible to mastitis than cows with average or low production. On the other hand in the second half of the first parity and in later parities the negative genetic correlation between yield traits and SCC suggests that mastitis cause high SCC and udder damage resulting in reduced milk yield. The current results suggest that selection for whole lactation milk yield over the lifetime of the cow should not result in an increase in SCC. In practice, however, selection is largely on yield traits in the first lactation, thus the use of animals with high breeding value for SCC should be avoided.