Relationship between milk somatic-cell counts in the first lactation and clinical mastitis occurrence in the second lactation of French Holstein cows.

The relationship between 10 cell-level descriptors in the first lactation and the risk of clinical mastitis in the second lactation was studied in 10205 Holstein cows from 1254 commercial herds using logistic regression. These 10 cell descriptors were based on the mean cell level over the whole first lactation, on the cell level at the beginning or at the end of the first lactation or on the proportion of cell counts below or above given thresholds. Separate analyses were also performed for subsets of herds defined according to their clinical mastitis incidence risk and their lactation mean somatic-cell count (SCC). Odds of clinical mastitis in second lactation was highest (lowest) for high- (low-) yielding cows. All cell descriptors provided consistent results. Lower (higher) mean cell level, higher (lower) proportion of low SCC values, and lower (higher) proportion of high SCC values in the first lactation were associated with a lower (higher) risk of clinical mastitis in the second lactation. The same trends were observed in all subgroups of herds. Cows with the lowest mean SCC in the first lactation had the lowest risk for clinical mastitis in the second lactation. This suggests that selection for decreased SCC may effectively reduce clinical mastitis incidence and that the breeding goal should favor cows with the lowest observed SCC.