Anthelmintic treatment of dairy cows and its effect on milk production

The results of more than 80 experiments on gastrointestinal parasitism and the impact of anthelmintic treatment on milk production in dairy cattle wer e reviewed. Abattoir surveys of culled dairy cows, faecal egg counts in mil king cows, and serological tests and worm counts in cull cows in milk produ ction studies were collated to assess the level of parasitism in dairy herd s. The studies were divided into four general categories: induced infection s in previously uninfected cattle; naturally infected cattle treated in mid -lactation; naturally infected cattle treated one to three times during the dry period and/or just before or just after parturition; and naturally inf ected cattle treated repeatedly from early lactation or given strategic tre atments throughout the year. In most studies, the milk production of anthel mintic-treated cattle was compared with that of untreated controls. The ant helmintics investigated included members of the organophosphate, benzimidaz ole, imidazothiazole and macrocyclic lactone groups. The number of experime nts in which the medicated (or uninfected) group had a higher milk yield wa s compared with the number of experiments in which the control (or infected ) group had a higher yield. Overall, the studies demonstrated that grazing dairy cattle are likely to be infected with gastrointestinal nematode paras ites, usually Ostertagia ostertagi and Cooperia species. These infections m ay be present as inhibited larvae, and a periparturient or spring rise is a ssociated with their emergence. There is, at present, no reliable means of determining whether a cow or a herd may be parasitised subclinically at a l evel sufficient to interfere with milk production. In 70 of 87 experiments (80 per cent) there was an increase in milk production (P<0.001) after anth elmintic treatment, with a median increase of 0.63 kg/day. In each of the f our trial categories, a majority of the studies showed that anthelmintic tr eatment increased milk production. The yield of milk fat by the medicated c ows was greater than by the controls in 26 of the 35 experiments in which t hat variable was studied (P<0.01).