Effects of anthelmintics on dairy cow yields

Abstract

In the first of two trials with Friesian crossbred cows, monthly drenching starting about 9 weeks after calving increased milkfat yield per cow by almost 10 kg over the last 6 months of lactation. In the second trial, in which drenching began about 6 weeks after calving, there was no response in fat production from the treatment group as a whole, or from 2 and 3 year olds, but fortnightly drenching increased fat yield of mature cows by about 15 kg/cow over the lactation. Responses were most pronounced during the second half of lactation (December-May) and were due largely to a significant increase in milk yield. No effects on live weight were observed. There were no clinical symptoms of lielminthiasis at any stage, and the responses are attributed to small variations in immunity associated with seasonal changes in the availability of infective larvae on pasture.