Variance components and genetic trend for somatic cell count in Norwegian Cattle

First-lactation mean somatic cell score (LSCS) in Norwegian Cattle, sampled during 1978–1995 for 1.3 million cows from 2043 sires, were used in genetic analyses. Variance components and genetic trends were estimated for four linear sire models with alternative definitions of contemporary group effects. Model validation demonstrated necessity to separate environmental time trends from genetic trends by modelling contemporary groups as fixed. A model with fixed effect of herd×year was chosen, because it was the simplest and had less significant bias. The heritability estimate for LSCS using this model was 0.11. The resulting genetic trend for all relevant progeny tested bulls was approximately flat, but bull sires born 1984, onwards, showed consistently favourable selection differential for LSCS, most likely due to indirect selection resulting from selection on clinical mastitis.