Abstract
Cows and heifers from 22 herds were used in a trial to determine the effect on milk yield of anthelmintic treatment given at calving. Alternate animals were treated with either fenbendazole suspension or a placebo. Cows given fenbendazole showed a mean reduction in milk yield of 221 kg in the subsequent lactation compared with the placebo-treated cows, a difference which was statistically significant. In heifers no significant effect of the treatment was found on milk yield. When looking at the pooled data from both cows and heifers, the reduction in milk yield of the fenbendazole-treated animals was 151 kg, which also was statistically significant. The results indicate that anthelmintic treatment of dairy cows cannot be generally recommended in Norway.