Responses to selection against clinical mastitis in the Norwegian cattle population

The genetic trend for clinical mastitis was estimated for Norwegian Cattle, a population in which mastitis resistance has been included in the breedin g programme since 1978. More than 1.2 million first-lactation daughters, wi th clinical mastitis records registered from 1978 to 1995, bred by 2043 sir es, were analysed with a linear sire model. For bulls born from 1974 to 199 0 a flat genetic trend for clinical mastitis was found. However, bull sires born from 1983 onwards, selected on a breeding goal with increased weight on mastitis relative to milk production, showed consistently larger and fav ourable selection differentials for mastitis relative to bull sires born ea rlier, which had an average selection differential of approximately zero. T his will influence genetic trend in the population through their sons, whic h were born from 1990 onwards.