Relationship Between Lactation Measures of Somatic Cell Concentration and Milk Yield

Data were from herds on Dairy Herd Improvement in Wisconsin which were regularly tested for somatic cells. Lactation cell counts were taken as the average of either actual or natural log of individual tests during the lactation. Lactation milk yield was regressed on somatic cell concentration, among other concomitant variables, for each of the first four lactations. The amount of data ranged from 4,912 cows in 346 herds for lactation 1 to 1,759 cows in 202 herds for lactation 4. Yield loss per unit increase in average loge cell count was 135 + 20 kg in first lactation and 270 + 30 kg for all other lactations. These relationships were linear, indicating that loss per unit increase in actual cell count is greatest when cell count is low. Linearity of the regression of yield on average log cell count makes this more desirable than actual cell count as a potential trait for indirect selection for genetic resistance to mastitis. The effect on yield in a given lactation of increased average log cell concentration in the previous lactation was not statistically significant. However, the magnitude of estimates for carry-over effects were as much as 50% of concurrent effects. This suggests the possibility of an important amount of carry-over.