‘Worm resistance as vital as any other sheep trait’

Breeding for worm resistance is as important as breeding for any other heritable trait, believes Wiltshire sheep grazier, Tim White.

“It’s logical to keep animals that are resistant to worms as they are the genetically superior animals – it’s the same principal as keeping those that have had twins,” he says.

Breeding for worm resistance forms part of an overall strategy to reduce the need for anthelmintics among his flock of 900 ewes. Worm management also includes rotational grazing and selective worming based on faecal egg counts (FEC).

Where possible, sheep are also grazed on forage varieties with natural anthelmintic properties such as chicory and bird’s foot trefoil.

“I also move ewes and newborn lambs on to clean grazing after lambing. This means when lambs graze they slowly build up natural worm immunity.

“Very often I don’t have to worm them at all, and even at high levels of worm burden not all animals will need treating.”

With the flock grazed on various rented sites across a 100 mile radius in Wiltshire, breeding an easy care animal, which doesn’t require regular worming is crucial. His background working as a shepherd for a recorded flock also means he has always recognised the benefits of breeding from the best animals and using wormers responsibly.

Sheep breed

Mr White is one of a group of breeders who have worked to develop the Exlana open-composite breed; a low input, performance recorded, commercial wool-shedding sheep.

“I’ve been sampling for about eight years and do FEC on 300-400 individual lambs at 20 weeks of age. At £6 a sample, it’s not cheap, but as a breed, we believe it’s an investment worth making.”
Tim White, Wiltshire sheep grazier

In the early stages of breed development, worm resistance was one of the main priorities for the Exlana. There are about 5,200 recorded ewes in the Exlana flock with more than 800 individual FEC samples taken every year.

All of the seven breeders are members of Sheep Improved Genetics (SIG) with data recorded through Signet.

This allows the formulation of a FEC Estimated Breeding Value (EBV), enabling animals to be selected for worm resistance.

Faecal egg sampling

“I’ve been sampling for about eight years and do FEC on 300-400 individual lambs at 20 weeks of age,” says Mr White. “At £6 a sample, it’s not cheap, but as a breed, we believe it’s an investment worth making.”

Recording has highlighted the big differences between lambs, with some producing 50 eggs/g and others, 1,000 eggs/g. The open- composite nature of the breed also means Mr White has been able to introduce new genetics from breeds known for worm resistance, such as the Barbados Blackbelly.

For Mr White, breeding from the top 10% of animals for EBVs, including FEC EBV, will help reduce wormer use in his flock in the long-term and within the Exlana breed as a whole.

“The moment I have to touch [treat] a ewe, it is put to a terminal sire,” he explains. “I also only breed from rams which have never had any form of treatment – including worming.”

FEC EBVs

FEC EBVs are used to select rams resistant to worms, which means they will produce daughters that are more resistant. “If a ewe sheds less eggs pre- and post-lambing, lambs aren’t exposed to as big a challenge and they will also inherit some resistance,” he says.

To ensure stock are only wormed when necessary, group FECs are also carried out on lambs every 10-14 days. This is done in the field, which enables fast analysis. This regular sampling also ensures problems are picked up early.

Mr White encourages all commercial producers to use worm resistance as part of their culling policy. “Even if you are not doing individual FECs, you can select by eye. Culling animals that have high dag scores or need regular worming can only help,” he says.

Mr White believes he is worming less than when he started breeding for worm resistance, but it will take time for big differences to be seen. He also believes improvements have been down to improved grazing management.

At 0.25, worm resistance has a good heritability, with a flock with greater variability and high sample numbers likely to see big improvements. In fact, studies in New Zealand and Australia have shown selection for increased resistance is possible and leads to reduced anthelmintic requirements.

The number of UK breeders recording FEC to select on FEC EBVs remains relatively small. Lleyns, Romneys, Texels, Suffolks and some composite breeds are the main breeds recording for this trait through Signet.

Signet manager, Sam Boon believes FEC EBVs could create a unique selling point for some flocks, however it is important to commit fully to the concept.

“The cost of sampling can put people off. But if it’s of interest, you need to really get behind it, sample in big numbers and use it as a strong marketing point.”

Mr Boon says the more FEC samples taken the better, but suggests a minimum of about 30-40 lambs to increase the reliability of the figures.

He sees FEC EBVs playing a bigger role in maternal breeds where replacements are being kept and worm resistance can be bred into the flock. To get maximum effect he also explains the importance of selecting for both resistance and resilience.

“Resistance is measuring worm eggs that are put on to pasture. So you can select resistant animals with low FEC using FEC EBVs. But you also want to test lambs under reasonably high worm challenge and identify differences between them to select for resilience.”

Resilient lambs are able to thrive even when there is a high worm challenge.

“Most flocks don’t like to expose lambs to high worm challenge which is another reason why they don’t select for it,” he says.

He believes it is more likely slightly larger flocks, selling from home could choose to market themselves as producing worm resistant sheep.

The fact these flocks are more likely to sell stock as shearlings means they may be more receptive to exposing lambs to high worm challenges.