Standardization of lactation means of somatic cell scores for calculation of genetic evaluations.

Sample day records of means of somatic cell scores were analyzed to develop adjustments to standardize records for length of lactation. Estimates for effect of DIM were from a model that included random lactation and cow effects for lactations to date of 753,929 Holsteins and 21,842 Jerseys calving in Pennsylvania or Wisconsin from 1987 to 1991. Lactation, cow, and residual variances were estimated using REML. Lactation and cow variances relative to a phenotypic variance of 1.00 were .57 and .31, respectively, for Holsteins and .52 and .35, respectively, for Jerseys. Estimates of effect of DIM were used to compute additive adjustments. Final lactation mean of somatic cell score at < or = 305 DIM for 1,857,532 Holsteins and 113,998 Jerseys from all participating states were standardized for lactation length and analyzed to determine the national effects of calving age and the regional effects of calving month. Multiplicative adjustments were developed for calving age and additive adjustments for calving month. Sample day records of lactation means of somatic cell scores were used to estimate weights based on number of somatic cell sample days to account for the lower accuracy of short records for genetic evaluation.