Production loss due to intestinal worms remains one of the biggest issues on sheep farms with even a low to moderate worm burden reducing lamb growth rates by up to 50% – without obvious clinical signs.

Add in the increased resistance to drugs and worm control is proving a huge challenge. Hence a successful worm control strategy needs to be considered.

Farmers are, therefore, being advised to work closely with their vet, RAMA or advisor and ideally as a collaboration among all of them, to develop a worm control plan that is specific to the farm and flock.

In doing so, Matt Colston, from Elanco, said there are three important areas to include when planning: measuring worm burdens, determining how well the wormers are working on the farm, and administering the right active at the right time in the right way.

Measuring worm burdens

Timing is essential when it comes to worming and to worm effectively it is important to know when worms are on the pasture. However, this varies every year.

Treatment timing can be calculated using the results of routine faecal egg counts (FEC).

“FEC results give vital information on whether worming is needed or not, and there is no point worming if there are no or very few worms present,” said Mr Colston.

Worming lambs based on FEC results will ensure wormers are only used when most needed, avoiding unnecessary treatments that can increase the risk or speed of wormer resistance developing. This therefore helps maintain the effectiveness of wormers for years to come.

“This targeted approach not only minimises the cost of purchasing unnecessary drenches, but also reduces the labour involved in handling stock, and maximises the time animals spend grazing, promoting better growth and productivity,” he added.

Changing weather patterns have made the worm life cycle less predictable, but in general, worms can be present from spring to late autumn, even early winter. Regular FEC monitoring helps confirm, rather than guess or rely on historical data, as to when lambs are at risk.

The timing and regularity of testing depends on the farm, but over the summer, farmers are advised to collect faecal samples from lambs at least every three to four weeks.

“There’s no exact science to this, so work with your vet or adviser to develop a plan that suits your farm and takes weather conditions into consideration,” he said.

When collecting dung samples, it is crucial these are fresh, and should be from a minimum of 15 animals in a mob, to give an accurate representation of the worm burden present.

“If the FEC indicates treatment is required, then it’s advised to consider which wormer group to give, based on the history of the farm, the timing of the treatment and the number of treatments that have been given that season.

“Your vet, RAMA, or adviser can help support in choosing the most appropriate wormer for your flock.”

Determining wormer efficacy

Every farm will get different worms at different times of year, some of which may be resistant to certain wormers and susceptible to others.

Detecting resistance, or knowing whether the wormer used has been effective, as early as possible can help maintain effective control and avoid production losses.

It is essential farmers and advisers work together to build a picture of resistance levels on each farm, which can take time.

Mr Colston said the best way to check if a treatment had been effective was to carry out a FEC before worming and again seven days post-drenching if using a yellow wormer, or 14 days after using a white, clear, orange or purple wormer.

Choosing the right active at the right time

Choosing the right active ingredient is essential to effectively manage parasitic worms and minimise resistance development.

The UK sheep sector is lucky to have five groups of broad-spectrum wormers, but it is crucial to understand when is best to use them on each farm, ideally taking into account of what works on each farm.

“Earlier in the season, it’s generally recommended to use a Group 1 wormer to prevent Nematodirus and then a Group 2 or 3 wormer as the season continues, but FECs should be used to determine the need for and timing of these treatments,” said Mr Colston.

“As the level of challenge and worm burdens increase in lambs towards the end of the grazing season, it’s important to consider the pasture risk. For instance, I’d avoid grazing lambs in high-challenge areas previously grazed by lambs, and use a different newer active to effectively remove worm burdens.”

He added that to remove worms that may have survived earlier treatments, farmers are urged to treat lambs with a mid to late-season break dose, using a Group 4 wormer, like Zolvix. This will help them reach growth potential and help preserve efficacy of the Group 1, 2 and 3’s for longer.

However, it was crucial not to move lambs directly onto fields with low worm burdens immediately after treatment. Instead, he advised leaving them on their old pasture for a few days before relocating, helping ensure resistant worm eggs were not deposited on ‘clean’ pasture.

“Regardless of your current wormer resistance status, it’s important to integrate this into your regime now. It is also advised to use a Group 4 wormer to treat all incoming stock to prevent resistant worms from being brought onto the farm.

“Considering these key areas and working with advisers will go a long way to addressing the current worm challenge and help slow the development of resistance, contributing to the long-term sustainability of UK sheep production.