Sheep farmers advised to time their Group 4 and 5 wormers right this season
9th August 2025
The Sustainable Control of Parasites in Sheep (SCOPS) is advising farmers on the importance of using Group 4-AD (monepantel) and Group 5-SI (derquantel) wormer actives in lambs as a mid-to-late season dose to help manage resistance and support optimal growth rates.
However, with variable weather conditions across the country this summer, it is crucial not to rely on a fixed calendar date for administering these products.
Instead, decisions should be based on treatments administered to date, parasite burdens observed in recent weeks or months, and overall flock performance.
Nerys Wright, technical adviser at SCOPS, said that the variation in worm burdens across the UK is “huge”.
“Some farmers have yet to administer any treatments, yet lambs are finishing well. That means the timing of the mid-to-late season dose will look very different this year, and it’s more important than ever to tailor your approach,” Ms Wright added.
‘Timing is everything’
The SCOPS expert said it is important to understand that the Group 4 and 5 wormers are largely aimed at clearing out worms that have survived earlier treatments with Group 1 (BZ), Group 2 (LV), and Group 3 (ML) actives. Therefore, if farmers have not needed to treat their lambs this season, their flock may not be ready for a Group 4/5 break dose.
“These actives play a key role in slowing resistance development and clearing out persistent worm burdens, but timing is everything,” Ms Wright continued.
Farmers should discuss the best time to use the Group 4-AD and 5-SI with their vet or adviser, based on the treatments already given in the flock this season.
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