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Pre-lambing—drench routinely or WormTest first?

Figure 1. Ewes lose immunity to worms at and after lambing. Source: Deb Maxwell.
Figure 1. Ewes lose immunity to worms at and after lambing. Source: Deb Maxwell.

As sheep mature into adults, their immunity to worms also develops and matures. Their ultimate level of worm-resistance depends on their genetics and their exposure to worms. But at lambing and during lactation, that immunity temporarily declines.

As a result, lactating ewes can develop much higher worm burdens than when they are dry or pregnant, which in turn leads to a worm-contaminated lambing paddock and lambs becoming infected. As lambs and weaners have little immunity to worms, their growth and even survival rates can be affected.

There are also further consequences with flow-on contamination to other sheep and paddocks on the property when sheep are later moved into and out of those lambing paddocks.

To combat this, a pre-lambing drench may be warranted. But in which regions should drenches be routinely given before lambing, and where should a WormTest be done first? And are there any places where routine pre-lambing worm testing is not indicated?

What are the regional and state recommendations?

A summary of drenching versus testing recommendations is provided below, as well as a link to the WormBoss worm control program for each region.

In Tasmania and the summer rainfall tablelands and slopes region of northern New South Wales and southern Queensland, the WormBoss regional experts recommend that a pre-lambing drench is mandatory. A pre-lambing WormTest is not required, instead, routinely give an effective, combination, short-acting drench. In wet, high worm-challenge seasons, a long-acting product could be warranted if the lambing paddocks are already heavily contaminated with worm larvae.

In Victoria, South Australia and the other moderate to high rainfall areas of New South Wales and Queensland, a WormTest should routinely be carried out first to determine whether a pre-lambing drench is required.

In Western Australia, WormTest late-lambing ewes (lambing later than mid-June), but in early-lambing ewes, if a drench was given in March–April this acts as a pre-lambing drench, so no further WormTest or pre-lambing drench is required.

In the pastoral region, a pre-lambing WormTest is not specifically indicated, especially in the drier areas, but if a drench before or during lambing has been required occasionally in the past on your property, a routine pre-lambing WormTest would be useful.

Timing the pre-lambing drench?

When a pre-lambing drench is recommended, it is ideally given within the three weeks before the start of lambing. Giving a drench sooner than this—particularly in the higher rainfall areas—allows more time for the ewes to acquire a substantial burden of worms before lambing starts. If ewes are to be moved to a specific lambing paddock, also try to time this to within the three weeks before lambing and give the pre-lambing drench at the move.

If WormTests are used, allow enough time for samples to be posted to the laboratory and results to come back; usually about a week. If a larval culture is also being done, allow more than a week as the culturing process itself takes seven days.

Regional DDGs—threshold worm egg counts  

The threshold worm egg count for drenching pre-lambing ewes varies across regions as a result of different climates, different worms and how important a strategic approach to drenching is—that is, suppressing the seasonal build up of worms by drenching ewes at levels lower than they would normally require a drench.

Use your regional Drench Decision Guide (DDG) to decide whether drenching is recommended.

The DDGs can be found here or a link is at the top of each regional Program summary page.

What drench should be used?

Effective, short-acting, combination drench products are the best first option to consider. Don’t worry about “rotating” from what was used last time these sheep were drenched.

But what does “effective” really mean? Many drenches appear to work—the sheep continue to live or stop showing signs of worms—but they may be leaving behind many drench-resistant worms. As the numbers of these increase in the worm population on your property, your sheep will suffer further productivity losses, and the time before you next need to drench will gradually become shorter. Both these problems are hard to see because you won’t have a similar “control group” of sheep to objectively compare production with and the time to next drench is confounded by week-to-week seasonal differences—no two years are the same.

WormBoss defines an effective drench as one that will decrease the worm egg count in the drenched sheep by 98%. On many properties, the older, single-active drench products may not achieve that, though it will vary according to the worm type. Therefore, combination drenches are recommended; fewer worms are able to resist combinations of different drench groups given at the same time.

As the drench-resistance profile on each farm can be quite different to that of a neighbour, DrenchTests and/or DrenchChecks are needed to know what works for you.

Avoid using long-acting products where possible. Reserve these for particularly high-risk times of the year or specific situations.

In particular at lambing, moxidectin-based long acting products have greater potential for development of drench resistance because it transfers through the milk to lambs, leaving them with a sub-optimal dose that can result in selection of drench-resistant worms.

What else can make a difference to worm control at lambing?

Decreasing the exposure of ewes to worms during lambing can have a substantial impact.

In the summer rainfall areas where barber’s pole worm is the greatest threat, prepared low worm-risk paddocks combined with a pre-lambing drench maintains low exposure and sets the scene for lower across-farm worm burdens for the rest of the summer and autumn.

In winter rainfall areas, the dry summer combined with smart-grazing can be used to prepare lambing paddocks, but these are probably best reserved for last year’s lambs—when they are weaners—as they will be very susceptible to worms in their first winter.

The alternative to specially prepared paddocks for ewes in the southern half of Australia is to monitor worm egg counts of sheep in the lambing paddocks in the months before lambing and use the paddocks that had the lower worm egg counts to lamb down the most susceptible lambing ewes, for instance, twin-bearing or maiden ewes.

The WormBoss worm control programs provide procedural, region-specific information on how to apply these proven and practical strategies for worm control.

The WormBoss Drench Decision Guides help you to make your day-to-day drenching decisions.