Strategic use of anthelmintics to prevent parasitic gastroenteritis in cow-calf herds in California.

On the basis of the hypothesis that the peak numbers of infective nematode third-stage larvae (L3) on herbage in winter months results from fall contamination of pastures, 2 methods to reduce fall contamination were tested. In trial 1, morantal sustained-release boluses were administered to 15 fall-calving cows on Sept 7, 1982. Fifteen untreated cows (controls) were placed on separate pastures. Numbers of L3 on herbage during the winter and spring were assessed by use of worm-free tracer calves. In trial 2, 19 cattle due to calve in the fall were administered 200 micrograms of invermectin/kg of body weight, SC, on Sept 2, 1983. Also, 17 cattle similarly were given a placebo injection and served as control animals. Treated cattle were placed on the pasture used by control cattle in trial 1 and control cattle on the pasture used by treated cattle in trial 1. Worm-free tracer calves were again used to assess numbers of L3 on herbage. In trial 1, tracer calves grazing the control animal pasture from January 14 to 28 acquired 37 times as many nematodes as did those grazing the treated animal pasture. In trial 2, the greatest difference observed was a 10-fold increase of nematodes in calves grazing control animal pastures, compared with worm numbers in tracer calves grazing the treated animal pasture.