Advice on developing a worm control plan for sheep

Having a worm control plan is often overlooked, but with anthelmintics resistance on the rise and becoming a real threat to lamb production, it is a fundamental tool for achieving good rates of daily liveweight gain.

Many sheep farmers will already be familiar with strategic worming as an aid to control a worm burden.

However, there are also many management considerations that can reduce reliance on anthelmintics.

See also: 6 tips on how to carry out faecal egg counts

Mature ewes

While ewes generally develop a good level of immunity against worms, a drop in immunity around lambing time is enough to drastically increase their worm burden – and, therefore, the number of eggs on pasture.

This is particularly the case in ewes that are under nutritional stress, as they are less able to withstand a challenge from internal parasites.

Worming mature ewes should be considered as part of a complete worm control plan.

Strategic worming for ewes in poor body condition or ewes carrying triplets can help reduce a spike in worm eggs on pasture in early spring.

Grazing lambs

Grazing lambs are naïve and at risk of heavy worm burdens.

From about 12 weeks of age, they can be weaned and moved to less heavily contaminated pasture.

Silage aftermath is an ideal option as it is clean pasture with very few worm eggs.

Herbal leys have been shown to have some anthelmintic properties, however, the exact mechanism for reducing worm burden in not completely understood.

Sainfoin, chicory and bird’s-foot trefoil contain tannins that have been shown to have this effect.

Their anthelmintic properties are believed to be due to a reduction in egg hatching, larval development and mobility of larval stages, as well as an indirect effect of increasing host resilience.

Faecal egg counts

Anthelmintics resistance is predominantly driven by overuse.

Throughout the grazing season, faecal egg counts (FEC) should be taken every three to four weeks to establish the worm burden in lambs and determine the need for treatment.

A good faecal sample should include at least 10 individual samples, be taken at random, and must use fresh faeces.

Additionally, FECs can be used to calculate worm resistance.

This is done by taking a sample before and after treatment with anthelmintics (timing will vary depending on the product used).

Targeted treatment

If treatment is necessary, leaving some lambs untreated reduces worm exposure to anthelmintics.

It is thought that leaving the strongest 10% of lambs untreated can delay the development of anthelmintics resistance.

Alternatively, targeted selective treatment (TST) can be used to identify lambs that do not need worming.

TST uses specific individual markers, such as growth rate and dag score, as well as considering the flock’s FEC.

The vet can work with the farmer to put these steps in place as part of an effective worm control plan tailored to the specific needs of the flock.