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Strategic versus tactical drenching

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What are strategic and tactical drenches?

Strategic drench: a drench given at a critical time to reduce worm larval contamination of a pasture for the benefit of the whole mob or herd rather than just for the treated animals, and/or a drench given to sheep or goats, irrespective of worm egg count at times when they are expected to be most susceptible to worm infection.

Tactical or therapeutic drench: a drench given when sheep or goats are suffering from the effects of worms and is best based on worm egg count (or for individual goats, use the three signs outlined in the WormBoss Australian Smallholders Program and DDG).

Note: Details of when/how to use strategic drenches in each sheep producing region are in WormBoss Your Program.

Principles for choosing and using drenches

The choice of drench recommended by WormBoss will be governed by what is effective on your property against the target worms and how drench resistance can be managed.

Use all 3 principles for choosing drenches where possible.
They are equally important and greatly slow the development of drench resistance.

  1. Use drenches most effective on your property. Drenches that reduce worm egg count by at least 98% are preferred. 
  1. Use an effective combination of two or more drench groups, either in a multi-active product or using more than one product concurrently (up the race with one and then the other) to combine different drench groups. 
  1. Use short-acting treatments and restrict the use of persistent products for specific purposes and high worm-risk times of year

Rotating drench groups gives only a small benefit and only when stock are also rotationally grazed.

Note: Choose a drench with an appropriate withholding period (WHP) and export slaughter interval (ESI) for your situation; these can be found in the Drenches section of WormBoss.

When using drenches follow these principles where possible:

  1. Avoid unnecessary drenching by referring to regional WormBoss Drench Decision Guides.
  2. Calibrate drench guns to ensure the correct dose is delivered.
  3. Calculate the dose based on the heaviest animal in the mob.
  4. Follow the label instructions to ensure correct dose and use of treatments.

When are strategic drenches given?

The timing and purpose of strategic drenches depends on the region and the class of sheep or goat. Their use is closely associated with times:

  • When development of eggs to infective larvae on pasture is likely to be extremely low (such as the ‘summer drench’ to reduce pasture contamination in winter rainfall regions), or high (such as an autumn treatment to reduce pasture contamination in summer rainfall regions); and/or
  • When sheep or goats are most susceptible to worms (such as lambs or kids at weaning, or ewes pre-lambing or does pre-kidding).

Strategic drenches are given regardless of the average worm egg count of the mob.

Weaners facing their first winter in southern Australia are often in poor condition from a dry summer and then face cold and wet conditions combined with increased worm challenge during winter. A low worm-risk paddock has large benefit for the health and production of weaners and can be prepared using Smart grazing—winter rainfall.

Lambing ewes or kidding does in summer rainfall regions may face infection from barber’s pole worm at a time when they are highly susceptible to infection. A low worm-risk spring-lambing or kidding paddock has large benefits for the health and production of ewes or does and their lambs or kids and can be prepared using Smart grazing—summer rainfall.

There are seven common strategic drenches; not all are used in every region. The WormBoss programs outline which strategic drenches to use in each region.

1. Smart grazing for temperate winter rainfall regions

This is the summer grazing of a paddock by sheep or dry adult goats that have just received an effective drench (known to reduce worm egg count by at least 98%) as part of the preparation of that paddock as a low worm-risk paddock for later use by weaners.

The sheep or goats are drenched regardless of their worm egg count so that they do not contribute any worm eggs onto the pasture. The animals can use the paddock for up to 30 days after a short-acting drench. Weaners should receive a strategic drench before entering the prepared paddock when used in winter. This will prevent initial contamination and allow the paddock to be used for a longer time before worms build up.

2. Smart grazing for summer rainfall regions

This is the summer and autumn grazing of a paddock by sheep or dry adult goats that have just received an effective drench (known to reduce worm egg count by at least 98%) as part of the preparation of that paddock as a low worm-risk paddock for later use by lambing or kidding females. 

The sheep or goats are drenched regardless of their worm egg count so that they do not contribute any worm eggs onto the pasture. The sheep or goats can use the paddock for up to 2–3 weeks (shorter period when heavy barber’s pole worm challenge is suspected) after a short-acting drench. Ewes or does should receive a strategic drench before entering the prepared paddock when used in spring. This will prevent initial contamination and allow the paddock to be used for a longer time before worms build up.

3. Preparing lambing and weaning paddocks—summer rainfall regions

This method involves allowing time for most of the eggs and larvae on the pasture to die and is most successful in reducing pasture larvae for barber’s pole worm and black scour worm.

Prevent contamination of the paddock with worm eggs in the months (2–6 months; dependent on daily temperatures) before the paddock is required for use as a lambing/kidding or weaning paddock.  Prevent contamination by any combination of these:

  • Spelling (including cropping and haymaking)
  • Grazing with sheep or goats only in the 0–21 days after they receive a drench shown to be effective (in a DrenchTest) on your property
  • Grazing with cattle

Whether the paddock is for lambing ewes, kidding does or for weaners, the method of preparation is the same. However, the length of preparation will vary according to the time of the year the paddock first needs to be used (the hotter the weather during preapration, the shorter the time needed).

4. Summer drenches—temperate winter rainfall regions 

The first summer drench is generally given to all sheep and goat mobs once the pastures are haying off, usually in November or early December. Weaning of spring-born lambs and kids usually coincides with the first summer drench and all lambs and kids should be drenched at this time.

The decision about the second summer drench, given in January or February, is based on the results of a WormTest and is not routinely needed in some areas or in all years. However, when the second summer drench is required, it is very important because contamination with worm eggs in late summer and autumn is a strong determinant of the peak availability of worm larvae the following winter (hence worm problems will occur if it is not given).

5. Summer-autumn drenches—Mediterranean climatic regions

Summer drenches are recommended only for lambs, kids and yearlings/hoggets (last season’s lambs or kids), due to the high risk of selection for drench resistance when worms survive in environments where no larvae remain on the pasture. A single drench in early summer is given once the pasture has dried off, or as lambs, kids and hoggets go onto crop stubbles.

Strategic drenches to adult sheep or goats are not recommended for summer, but are best given in mid-autumn. Delaying drenching until late March or early April provides a source of worms that have not been selected as survivors of drenching and this slows the development of drench resistance. However, the autumn drench is recommended to ensure that these sheep or goats do not carry excessive worm burdens that may lead to winter worm problems.

6. Pre-lambing ewes and pre-kidding does

This strategic treatment is only recommended routinely for the following regions:

  • Tasmania
  • Regions in Queensland and the northern half of NSW (summer rainfall)

The immunity of ewes or does is depressed at lambing or kidding and during lactation. This results in ewes or does developing higher worm burdens than normal with a subsequent increase in contamination of the paddocks they graze with worm eggs.

A strategic drench is typically given to pre-lambing ewes or pre-kidding does in some regions and, in combination with low worm-risk lambing or kidding paddocks, will allow ewes or does and their lambs or kids to graze the paddock through to weaning with a relatively low build-up of worms.

7. Lambs and kids being weaned

Lambs or kids may not have developed a strong level of immunity to worms by weaning and so are susceptible to infection, especially when combined with the stress of separation from their mothers. In all but the very dry pastoral (rangeland) region, a weaning drench is given regardless of the worm egg count of the lambs or kids.

The practical use of the strategies described above can be found in the WormBoss regional programs.

When should tactical (also called therapeutic) drenches be given?

These drenches are best administered once the production loss being suffered by sheep or goats starts to outweigh the cost to drench these animals. When some animals in a mob start to show signs of worms, the mob or herd is usually experiencing a level of production loss far greater than the cost of drenching. As such, using the first signs of worms to trigger drenching is a poor choice for cost-effective worm control.

Worm egg counts are the best indicator of when drenching should occur. The recommended worm egg count thresholds for tactical (or therapeutic) drenching are based on treatments being cost-effective and timed to prevent serious production loss. Consult the Decision Decision Guide for your region.

Goat owners with few goats who are able to monitor each individual regularly and also treat on an individual basis should use the 3 signs system found in the WormBoss Australia smallholder program and Drench Decision Guide.


Choosing drenches

Use all 3 principles where possible.
They are equally important and greatly slow the development of drench resistance.

  1. Use drenches most effective on your property. Drenches that reduce worm egg count by at least 98% are preferred. The more effective a drench is the fewer drench-resistant worms will remain in the animals after treatment. If drench effectiveness is unknown, conduct a DrenchCheck after drenching.
  1. Use an effective combination of two or more drench groups, either in a multi-active product or using more than one product concurrently (up the race with one and then the other) to combine different drench groups. The higher the efficacy of each drench group and the more drench groups included in the combination, the greater the benefit for slowing drench resistance. The chance of a worm being resistant to all active ingredients in a combination is much lower than for each individual active on its own. For goats, be aware of what drench groups are registered or permissible with a veterinarian’s prescription.
  1. Use short-acting treatments and restrict the use of persistent products for specific purposes and high worm-risk times of year. Persistent products provide a long time during which ingested resistant larvae can survive and reproduce. There is little need to use mid-length or long-acting treatments if animals are being moved to low worm-risk paddocks.

A small benefit can be gained by rotating drench groups providing you also rotationally graze stock across the property so that paddocks are exposed to sheep that have received different drenches. However, if you set-stock, drench rotation will not slow the development of drench resistance.

While not affecting resistance, it is essential to choose a drench with an appropriate withholding period (WHP) and export slaughter interval (ESI) according to the time left before the animals may go to slaughter, or their milk may be used for human consumption.

Search for drenches based on the worms or other parasites targeted, drench group or active and product name.