Strategic anthelmintic treatment of young cattle during summer in a Mediterranean-type climatic environment [south-west Western Australia]

A worm-control program utilising treatment of young grazing cattle with fenbendazole on two occasions during summer was tested in the Mediterranean-type climatic environment of south-west Western Australia. The grazing system aimed to produce steers by introducing three-month-old weaned calves to pasture in mid-winter until they were sold in early summer. Comparisons were made of the numbers of worm eggs passed on to plots by treated and untreated animals during autumn, the performances of treated and untreated cattle and the performances of calves introduced to the plots in mid-winter. The "tracer" calf technique was used to determine the availability of infective larvae on one untreated and one treated plot for each of the two years of the experiment. Treated animals deposited fewer Ostertagia spp. eggs on to pasture during autumn than did untreated animals in one of the two years. In both years they deposited fewer eggs of worm species other than Ostertagia spp. Fewer intestinal worms were acquired by "tracer" calves grazing treatment plots than those grazing no-treatment plots in both years, but there were no differences in the number of abomasal worms acquired. The reduction in availability of infective larvae of intestinal worms was insufficient to prevent the occurrence of parasitic gastroenteritis in calves introduced to the plots in mid-winter. The fenbendazole treatments did not confer any immediate body-weight advantage on treated animals. On both treatment and no-treatment plots, there were few infective larvae available to grazing cattle during early autumn, there was a rapid attainment of peak availability in winter and then a decline to low availability by mid-spring. In one year, infective larvae of intestinal worms (almost exclusively Cooperia spp.) increased in availability again in late spring and early summer. A high proportion of retarded worms was never a feature of the worm counts of "tracer" calves. It was concluded that the treatments have been more effective had they been given during autumn