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Worm tolerant sheep: breeding our way to reduced reliance on wormers

Infection with gastrointestinal roundworms causes significant disease in lambs, costing UK sheep producers ~£42M per year in wormer treatments and lost production.

Worms have evolved resistance to many of the wormers currently available (anthelmintic resistance), meaning that other control strategies are needed to reduce the impact of worms on lambs.

A new project, Breeding for tolerance to worms for the future of sheep farming in England”, has recently been funded through Defra’s Farming Innovation Programme, managed by Innovate UK. The project aims to tackle the problem of roundworm by promoting tolerance of sheep to worms.

Tolerance is the ability of an animal to maintain its performance even while its parasite burden increases. This is distinct from resistance to worms, which is the ability of the animal to reduce its parasite burden. Thus, tolerant animals continue to grow well, despite a high worm burden.

By breeding from rams with genetic tolerance to worms, farms could benefit by introducing worm tolerant genetics into their flock.

This pilot project is based at Trefranck Farm in Cornwall using a mob of Romney ram lambs with sires from New Zealand genetic lines. We have recently completed data collection: dung samples were taken from lambs every two weeks from July to November and were analysed for worm burden with faecal egg counts. Lambs were also weighed every two weeks.

Tolerance is measured as the relationship between egg count and weight gain, such that more tolerant animals maintain weight gain as worm egg count increases and less tolerant animals lose weight more rapidly as egg count increases. Early analysis suggests that we are seeing variation between both individual lambs and sire lines in their tolerance slopes.

It is hoped that selection of the most tolerant sire lines will enable production of animals that are more able to maintain their performance despite worm infection.

Matt Smith of Trefranck Farm said:

“As sheep breeders, we are well aware of the challenges being faced on farm. With this first trial, we hope to identify sheep that are more tolerant to worms, to inform our breeding programmes, helping to reduce the sheep industry’s reliance on wormers and still remain profitable without compromising welfare.”

Progress of the project can be tracked through the Twitter hashtag #WormTolerantSheep and online through the CIEL website ( ). Final results will be disseminated to sheep industry members at events at Trefranck Farm in May 2023.

Project team

Matt and Pippa Smith, Trefranck Farm – Matt and Pippa have been farming sheep at Trefranck for several years, and currently run a flock of 800 ewes. Matt is passionate about bringing an efficient, sustainable grass-based sheep farming system to the UK.

Moredun Research Institute – World-class scientific research institute focussed on improving animal health & welfare through the prevention and control of infectious diseases of livestock.

Castle Veterinary Group – Based in Cornwall, Castle Veterinary Group are experts in sheep veterinary health and research.

CIEL – The UK Agri-Tech Centre for livestock innovation and a leading membership organisation, uniquely positioned at the heart of a collaborative network of expertise linking industry, academia and government.

This Project is funded by the Defra Farming Innovation Programme Research Starter Competition that is managed by Innovate UK.


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