When to Drench and When to WormTest for Sheep – Tasmania

Why check worm burdens in sheep?

Checking worm burdens with a WormTest is essential for correct and timely drenching decisions. The result is healthy sheep without unnecessary drenching. WormTests are the best basis for drenching decisions. Weight loss, scouring, a tail in the mob and deaths may mean that your sheep need drenching. If so, these signs occur well after substantial production losses (reduced weight gain and wool growth) from worms have already occurred in the mob. WormTests give early warning of significant production losses.

How are worm burdens tested?

Checking worm burdens throughout the year using WormTests is a critical part of the WormBoss worm control program.

Most WormTests are done through a laboratory. However, worm egg counts (but usually not larval cultures) can be done by producers if they have the equipment and skills. Ideally, producers should have their preparation and counting technique reviewed by an accredited laboratory and perform ongoing quality control checks, just like an accredited laboratory to ensure their results are correct.

Which mobs and how many should have a WormTest?

Testing all mobs is the ideal, especially prior to the second summer drench. To reduce the cost of testing you can WormTest at least one in every three mobs with a similar drenching history, paddock type and class of sheep. However, this approach does carry more risk that some mobs that aren’t monitored will have a high count.

If in doubt about how representative one mob is of another, test the other mob.

When should WormTests and drenches be routinely done?

Note: Some drenches are ‘strategic’, that is they are given routinely or when worm burdens are relatively low as a means to decrease the level of worm contamination of pastures in the next autumn and winter. ‘Therapeutic’ drenches are those given when worm burdens are high enough to cause production loss or illness.

Routine drench times in the high rainfall region of Tasmania

• Pre-lambing: all ewes should be treated with a short-acting drench. In wet, high worm-challenge seasons consider a long-acting product.
• Weaning: lambs being weaned should be treated with a short-acting drench.
• Weaners going onto fodder crops that have been prepared as low worm-risk, e.g. vegetable and cereal crop residues in later autumn: use a short-acting drench.
• Weaners going onto highly contaminated short-rotation ryegrass: these lambs may require a long-acting product such as a benzimidazole capsule (this has a short withholding period and is generally effective against Trichostrongylus vitrinus, which is common on irrigated pastures).

Routine WormTest times in the high rainfall region of Tasmania

WormTests can be done at any time, however there are certain routine times to WormTest:

Note: a larval culture (larval differentiation) is particularly useful in areas with non-seasonal annual rainfall or where barber’s pole worm are common on the property.

• Ewes at weaning: if staying on perennial pastures after weaning, drench at 200 epg (first summer drench).
• Ewes from January to the pre-lambing drench: WormTest each 6–8 weeks. Treat at 500 epg.
• Weaners from weaning until turnoff (May/June): WormTest at 3 weeks (summer) or 3–4 weeks (autumn) after short-acting treatments. Treat at 200 epg (this includes replacement ewe lambs that will be mated at 7 months).
• Ewes going onto fodder crops that are already low worm-risk and are being kept as low worm-risk paddocks for weaners: drench ewes at 150 epg.

Routine drench times in the medium and low rainfall and summer rainfall regions of Tasmania

• Pre-lambing: all ewes should be treated with a short-acting drench. In very high worm-challenge seasons consider a long-acting product; this will be indicated if more drenches than usual were required prior to this time and paddocks are highly contaminated.
• Marking: ewes in drier areas (less than 500 mm) may receive the first summer drench at marking. In high worm-challenge years ewes and lambs showing signs of parasitism may need a therapeutic drench.
• Weaning: lambs being weaned should be treated with a short-acting drench.
• Ewes that did not get the first summer drench at marking should be treated with a short-acting drench if they will remain on perennial pastures after weaning.
• Weaners going onto fodder crops that have been prepared as low worm-risk, e.g. vegetable and cereal crop residues in later autumn: use a short-acting drench.
• Weaners going onto highly contaminated short-rotation ryegrass: these lambs may require a long-acting product such as a benzimidazole capsule (which has a short withholding period and appears to be effective against T. vitrinus, common on irrigated pastures).

Routine WormTest times in the medium and low rainfall and summer rainfall regions of Tasmania

WormTests can be done at any time, however there are certain routine times to WormTest:

Note: a larval culture (larval differentiation) is particularly useful in areas with non-seasonal annual rainfall or where barber’s pole worm are common on the property.

• Ewes in January (include a larval culture in the summer rainfall region): WormTest to see whether the second summer drench is required. If the ewes are grazing perennial pastures, treat with a short-acting drench if the count exceeds 100–150 epg.
• Weaners in January (include a larval culture in the high rainfall region): WormTest to see whether the second summer drench is required. Treat with a short-acting drench if the count exceeds 100–150 epg.
• Ewes after January through winter: WormTest each 6–8 weeks and treat with a short-acting drench if the count exceeds 500 epg.
• Weaners after January until turnoff (May/June) or until spring for store weaners: WormTest at 3 weeks (summer) or 3–4 weeks (autumn) after short-acting treatments. Treat re-stocker store weaners with a short-acting drench if the count exceeds 300 epg.
• Finisher weaners and ewes to be mated at 7 months should be treated with a short-acting drench if the count exceeds 200 epg.

If you are in the summer rainfall region and if worm egg counts are unusually high and/or sheep show signs of barber’s pole worm (anaemia, bottle jaw, death), use a drench that is effective against barber’s pole worm.

Only in very high worm-challenge years may a long-acting product be required in early winter for weaners. This will be indicated if more drenches than usual were required prior to this time and autumn/winter weaner paddocks could not be kept low worm-risk.

When are other WormTests done and drenches given?

The timing of WormTests and drenches will vary between farms and seasons. Use the Drench Decision Guide (see next page) to weigh up important factors when deciding when to drench or WormTest on your property. These factors include signs of worms, time since last drench, the persistence of the last drench, WormTest results, recent rainfall, and condition of sheep and pastures.

If drenching is done for other reasons (such as an early drench before holidays or harvesting), use the Drench Decision Guide to decide when to drench or WormTest again.

Barber’s pole worm in this region is usually sporadic and short-lived. If summer and/or autumn are unusually wet, check worm egg counts each 4–6 weeks through to early winter to identify unusual increases in barber’s pole worms before they cause production loss and deaths. If worm egg counts exceed 1000 epg (or a little lower if sheep are in poor condition), drench with a short-acting drench effective against barber’s pole worm or closantel (generally effective in this region). Test again in 4–5 weeks.

If your property faces a significant barber’s pole worm risk for several months each year seek professional advice regarding an effective program, which may include the Barbervax^® vaccine.

What samples should be collected for WormTests?

Sheep do not need to be yarded for a WormTest. Collect warm fresh dung from the paddock (but make sure that ewe and lamb samples are not mixed).

WormTest kits can be obtained from laboratories or resellers in your area. Follow the instructions provided in the kit or talk to an adviser on the best method to use. As a guide, collect 20 individual samples from mobs up to 400 sheep, and 20–40 samples from larger mobs. The laboratory will then ‘bulk’ these samples using an identical amount of dung from each sample.

Avoid delays in transit (when worm eggs can hatch) by collecting and posting early in the week. Also ensure samples are kept cool before sending.

If you do your own worm egg counts, a ‘bulk’ test is easier than counting individual samples. Fewer bags or trays are needed and more sheep in the mob can be sampled.

The following fact sheet is on the WormBoss website: ‘Checking a mob of sheep for worms with a WormTest’.

The WormBoss Drench Decision Guide

The Drench Decision Guide helps to simplify decisions on whether and when to drench. There is a version of the Drench Decision Guide for each WormBoss region. It considers

• whether signs of worms are present
• the class of sheep
• the WormTest results
• the condition of the sheep
• the condition of the pasture
• the likely worm contamination of the paddock The Drench Decision Guide will recommend
• whether to drench now
• whether to use a persistent drench
• when to WormTest again

Results from the Drench Decision Guide can be applied to mobs without a WormTest providing they are the same class, and have the same drenching and paddock histories. If in doubt, WormTest the mob.

How to use the Drench Decision Guide

You can use the Drench Decision Guide at any time, whether you are contemplating drenching a mob now or in coming weeks. Not all situations require a WormTest: the Drench Decision Guide will recommend when these should be done.

Each Drench Decision Guide is available as a separate 2-page printable version, or can be used directly online.

Using the print version:

1. Start on the page that shows the ‘Drench Decision Guide Questions’.
2. Read Question 1.
3. Follow the ‘go to’ information on the right for the answer that applies to your sheep.
4. Only go to the question or recommendation to which you are directed by your answer.
5. When you are directed to a letter, this is the final recommendation, and is shown on the next ‘Recommendations’ page.
6. Also, read the important information in the green boxes.

Using the web version:

1. Agree to the terms of use and press ‘Start’
2. Select an answer for the first question and you will automatically be taken to the next appropriate question.
3. Select an answer for each question and you will automatically be taken to the Recommendation, where your choices and Recommended Actions will be shown, as well as other important information.

Both printable and interactive versions of the region’s DDG are available here: