From “Sheep Husbandry Lectures”,
Prepared by Dr Allan Cunningham (CMG), Principal and DS Thompson, Instructor in Sheep Husbandry
Roseworthy Agricultural College, South Australia.
Circa 1947 – principally to assist returning Diggers taking up land.

Internal Parasites – Worms

General Principles – the life cycles of most worms are very similar. The female lays eggs which are passed out in the dung. Under suitable conditions of temperature and moisture these eggs develop into larvae in about one week, crawl up blades of grass and are ingested by the grazing animal. On being swallowed by the animal the larvae develop into mature worms in the stomach or intestines. A sheep can only become infested with worms by picking up larvae while grazing – there is no multiplication within the sheep.

Taken generally, larvae are fairly resistant to temperature and moisture conditions, but are susceptible to extreme heat or dryness, and many cannot stand extreme cold.

Heavy infestations in sheep result from:-

1. Contamination of pastures with faeces of infested sheep;
2. Moisture and temperature conditions suitable to development and survival of larvae and;
3. Susceptibility of sheep.

Many graziers consider that drenching is the only operation they have to perform to reduce losses from worms. This is far from the truth and there is no doubt that losses can be greatly reduced by adoption of appropriate management.

These managerial aspects aim to prevent infection and losses by:

1. Keeping pastures as clean as possible by rotational grazing and
2. Keeping the nutritive level of feed as high as possible.

This advice is still as good as it was in 1947 – so, why do the majority of producers in the Australian sheep industry still rely on drenches?

Particularly when the industry has tools such as worm egg counts, drench resistance tests, genetics to breed worm resistant sheep and advanced pasture management techniques?