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Summer drenching

Should you drench sheep and cattle in a drought?

District Vet, Bruce Watt

October 2019

For over four decades the ‘first summer drench’ has been a cornerstone of worm control in sheep in south eastern Australia.

This strategy was underpinned by research conducted by the late Dr Norm Anderson in western Victoria over forty years ago. Dr Anderson placed previously worm free lambs onto pastures for a month before replacing them with the next group of lambs. He then killed the lambs and counted all the worms inside them. He found that these lambs picked up plenty of worms each month from April until mid-October. However, after this they picked up fewer worms. The climate in western Victoria is different from here but the general principles apply at least for the winter worm species.

As few worms survive on the pasture over the summer, the aim is to drench all sheep (with perhaps a second summer drench in late summer or early autumn) ensuring low worm burdens on pastures in the following autumn and winter.

Summer drenching has been highly effective but has been criticised for contributing to the development of drench resistance. Parasite control in sheep is always a compromise between effective control to prevent disease and production loss in the short term and managing drench resistance in the long term. Each producer has a different set of priorities as well as a different farm and environment, making local expert advice invaluable.

In drought years it is especially important to consider the consequences of drenching. With fewer worms surviving on pastures, the tiny number of worms surviving an effective drench or two will be the parents of a new, more resistant worm population.

My first suggestion is to conduct a worm egg count test before drenching. As Wormboss (a wonderful resource on parasite control that I recommend to you) states, ‘in very dry or drought years do a Worm Test before (the first summer drench) … as even this drench may be unnecessary and may cause increased selection for drench resistance.’

Alternatively, after worm egg count testing, you may be able to leave at least some mobs or some sheep within mobs undrenched. The aim is to have the progenitors of next year’s worms to be from your general worm population, not highly selected worms.

Local animal health advice is again important here because Haemonchus (Barber’s Pole worms) can build up rapidly on summer pastures following rain and leaving sheep or mobs undrenched can be hazardous if Haemonchus is a risk.

Even though we sometimes find low egg counts in lambs at weaning, in my opinion they should always be drenched. This is because they may be harbouring immature worms, because little weaners need all the help they can get to survive the summer and autumn and because we cannot risk contaminating the weaning paddock.

There are some important differences in worm control in cattle. We know that at least on the Central Tablelands, young cattle, from weaning for about the next six months, suffer disease and weight loss from internal parasites but that mature cows usually don’t need drenching. So mature cows provide a ‘refuge’ for undrenched worms, helping to manage resistance.