Forget routine drenching, only the leanest ewes should be wormed around lambing time to help reduce the incidence of anthelmintic resistance.

Instead, sheep farmers need to tailor their worm control better by making more use of diagnostic tests and only worming a percentage of their flock.

Speaking at a SCOPS press briefing, Stephen Dawson, secretary general of AMTRA, said every farm is different as is every year, therefore, flockmasters need to tailor their drenching according to the weather and the individual parasites that are likely to appear at certain times.

Instead of using the same products at the same time, every year, he urged producers to think of a sustainable parasite control programme.

With growing risk of anthelmintic resistance, producers need to reduce the number of ewes treated to safeguard wormers which in turn will save time and money.

Fit, mature ewes are able to keep worms under control using their immune system. However, if they are under nutritional strain around lambing, their immune system weakens allowing worms in the ewe’s gut to produce a lot more eggs. This is known as the peri-parturient rise (PPR) and these eggs, if deposited on pasture, will develop into larvae and add to the challenge to lambs later in the season.

Lesley Stubbings of SCOPS says: “The rationale behind treating a ewe is that it removes the worms in her gut and hence the number of worm eggs contaminating the pasture. The problem is that this can be very selective for resistance. The good news is we know not all ewes exhibit the PPR so, if we can identify these and only treat the ones likely to put out the most eggs, we can reach a practical solution.”

Historically, sheep farmers were told the higher the litter size the more likely the ewe was to have a high PPR. However, more recent evidence shows it is body condition* (BCS) and the degree of nutritional strain the ewe is under is most likely to determine the PPR. This means well fed ewes carrying twins, or even triplets that are holding their condition, do not need to be treated. Lean ewes carrying singles however, should be.

“We can use BCS rather than litter size to determine the need to treat. Flocks that have adopted this approach have significantly reduced the number of ewes they treat around lambing.

“To reduce the risk of selecting heavily for resistance it has generally been recommended to leave 10-20% of ewes in each grazing mob untreated. However, with mounting evidence that fit, well fed ewes do not need a treatment, we can now view this figure as a minimum,” Ms Stubbings says.

Independent UK research published in 2018, carried out by APHA** and funded by VMD, found there was no advantage in blanket worming ewes at lambing. There was no difference in faecal egg counts or growth rates of lambs reared on ewes that were wormed compared to those that were not treated at lambing.

SCOPS adds that it is very important that a proportion of ewes is left untreated if a persistent worming product is used. To minimise the risk of resistance developing, SCOPS and Zoetis have agreed guidelines for the use of moxidectin in ewes around lambing.

*As a guide, this is BCS>3 for lowland ewes.

**Learmount et al Vet Parasitol. 2018 Apr 15; 253:55-59.