The integration of grazing management with anthelmintic treatment to control Trichostrongylid infection in sheep

© 2000 Paul Gerard NivenA review of the relevant literature showed that worm control in sheep in the high rainfall areas of southern Australia is based almost exclusively on the use of anthelmintic chemicals. There are a number of potential strategies that can be integrated into worm control programs to reduce the reliance on anthelmintics. Grazing management is one that requires further investigation. The primary aim of grazing management is to optimise pasture utilisation and only a few grazing practices offer additional benefits for parasite control. A replicated field experiment was conducted over two years to compare the new strategy with the current standard strategy. In the new strategy, paddocks were prepared during the summer by intensively grazing them at about 2.5 times the standard stocking rate for one month (slightly longer than the pre-patent period of the parasite) after each of the two summer anthelmintic treatments. In the standard strategy, paddocks were continuously stocked at 15 wethers/ha for the entire preparation period, and the sheep received anthelmintic treatments at the same time as those wethers grazing the new strategy paddocks. Results from the experiment showed that the new strategy almost eliminated worm egg deposition on the pasture from November to April, whereas mean worm egg counts reached 250 eggs per gram for wethers grazing the paddocks prepared in the standard way. There were significantly fewer Trichostrongylus vitrinus in ‘tracer’ sheep grazing paddocks prepared using the ‘new’ strategy during June, July and August in both years. These counts were between 50 and 90% lower than for ‘tracers’ grazing the paddocks prepared using the standard strategy. Ostertagia circumcincta burdens of ‘tracer’ sheep grazing the paddocks prepared using the new strategy were reduced by between 20 and 75% for most ‘tracer’ grazing periods, when compared with ‘tracers’ grazing the standard paddocks. From April to November, the worm egg output from weaners that grazed paddocks prepared using the new strategy were significantly reduced to about half that of similar weaners grazing paddocks prepared using the standard strategy. The weaners grazing the new strategy paddocks grew significantly more clean wool (12%) and were significantly heavier (7%) at the end of the winter grazing period than the weaners grazing paddocks prepared in the standard way. The new strategy is a simple, reliable and effective way to prepare parasite ‘safe’ pastures for grazing by susceptible sheep in regions where two summer anthelmintic treatments are used to control O. circumcincta and Trichostrongylus spp