Effective grazing management (pasture planning) reduces the exposure of sheep to worms, and hence the likelihood of worm disease or significant production loss. There are three methods for minimising the worm risk of pastures:
- Allow time for most of the eggs and larvae on the pasture to die
- Avoid paddocks heavily contaminated with worm larvae
- Reduce contamination of paddocks with worm eggs
There is a risk that drenching sheep onto low-worm pastures may increase drench resistance levels in the same way that summer drenching does.
Which sheep are most susceptible to worms?
- Lambs in the months after weaning, before they develop a significant level of worm immunity. However, lambs do need to take in some worm larvae to develop their immunity; this occurs while sheep are on green pasture. Later-drop lambs (July and later) and lambs kept on low-worm pastures take longer to develop an immunity to worms than earlier-born lambs.
- Hoggets are also relatively worm-susceptible, especially if late-born.
- Lambing ewes are susceptible to worms as their worm immunity is temporarily reduced for several weeks from the time of lambing. This can contribute to the seasonal increase in worm numbers and later infection of lambs. A particular risk is for autumn-lambing ewes in barber’s pole worm areas.
The following practices to prepare or choose low worm-risk paddocks are most important in the South-West Medium to High Rainfall zone, especially in areas of higher rainfall where worm problems are common.
Preparing low worm-risk paddocks
The main need for low-worm pastures in WA is in winter and spring for weaners and lambing ewes, as in summer the hot, dry conditions prevent larval survival, and pasture growth is generally not sufficient to support significant larval development until May or later.
In the 3–4 months during autumn to winter, before you want to use a low worm-risk paddock, prevent contamination with sheep worm eggs by:
- spelling these paddocks from sheep (graze with cattle or use for crops, hay or new pasture establishment).
- or (not in coastal areas where barber’s pole worm is a concern) graze with sheep that have a tested low worm egg count (less than 200 epg).
Spelling for 3–4 months in spring or autumn results in about 90% or more worm larvae dying. Less than two months is not enough for a low worm-risk pasture; four months is only required if spelling includes winter months, when larvae take longer to die.
Avoid grazing the most susceptible sheep in the highest worm-risk paddocks
Where possible, avoid grazing weaners and lambing ewes (especially maidens, twin-bearing ewes or those in poorer condition) in high worm-risk paddocks. Paddocks most likely to carry significant numbers of worm larvae in winter or spring include those grazed by:
- Wormy sheep: where worm problems have occurred, or sheep have significant worm egg counts, sheep not given summer or autumn drenches, or where ineffective products have been used (this will rarely be visible in terms of signs of worms).
- Young sheep (two years or less of age). These are likely to have higher counts than older animals, but whether they have a significant count at a particular time can only be shown by a worm egg count.
Tools