GENETIC-VARIATION IN LACTATION MEANS OF SOMATIC-CELL SCORES FOR 6 BREEDS OF DAIRY-CATTLE

Lactation means of somatic cell scores from sample days were used to e stimate the components of variation for additive genetic, permanent en vironmental, and herd-sire interaction effects. Data included records of 4314 Ayrshire, 7845 Brown Swiss, 18,115 Guernsey, 1,135,752 Holstei n, 67,862 Jersey, and 797 Milking Shorthorn cows from across the US. R ecords were preadjusted for length of lactation. Fixed effects of herd year, calving age, and calving month were included in animal models fo r estimation of variance components. Additive genetic estimates from R EML relative to a phenotypic variance of 1.00 were .07 for Ayrshires, .07 for Brown Swiss, .11 for Guernseys, .09 for Holsteins, .09 for Jer seys, and .08 for Milking Shorthorns; permanent environmental estimate s were .25, .26, .22, .21, .20, and .35; and herdsire interaction esti mates were .04, .02, .00, .02, .02, and .01. Effects of calving age we re similar for all regions of the US but differed for Jerseys and Hols teins. Effects of calving month were similar for all breeds. Cows calv ing during summer had the highest lactation means for somatic cell sco re from sample days. Impact of calving month was greatest in the South east.