How to prolong wormer efficacy with sheep

While it might seem a little strange, it is actually best to have a small susceptible worm population with livestock. If you go ham with wormer to try to kill all the parasites, you will likely only succeed in building a resistant population. That can be disastrous as there are no treatment options.

So, the better thing to do here is look to use wormers sparingly and prolong the efficacy. You can do different things to try to achieve that depending on the livestock. What we want to look at today is how to do it with sheep. There are four key action areas to look at.

Don’t dose all adult ewes

As ewes age they build their resistance to various parasites. By adulthood they will typically be immune to things like stomach worms. So, you generally don’t need to dose them at all. It can help to retain your susceptible worm population.

However, there are some ewes that can require wormer treatment. For example, if there are any that are mature but too thin, they may need worming to help with weight gain. More importantly, lactating yearlings may need a dose.

Treating nematodirus in lambs

One of the worst diseases with lambs is nematodirus. It can cause high mortality rates and stunt lamb growth. Even worse, it can strike quickly in certain conditions.

Farmers need to be on their toes here to tackle the worms that cause it. However, it is best to use a white drench rather than clear or yellow. White drenches can treat nematodirus effectively, and no resistance has been reported. It also has low efficacy for other stomach worms, meaning you can retain the susceptible population while killing those ones that cause nematodirus.

Be cautious about buying in

Farmers periodically buy in sheep to add to their herds. However, you need to be cautious here to ensure you aren’t bringing in a high number of resistant worms with them. The best strategy is to quarantine new animals first and do testing to check the worm load. You can then speak to a vet and do a quarantine drench with a suitable wormer if necessary to kill gut worms before you introduce the sheep.


Finally, it is important that farmers are aware of what will work best for their respective farms. There can be massive variations in worm loads, types, and more. So, one herd of sheep may have a higher burden of a type of worms than another. Climate conditions can have a big impact on this too.

The best thing to do here is frequent testing. Faecal egg count tests can clarify the worm load for herds. Always do a test when lambs are at the weaning stage. If the test says there is a high worm population, you may need to give them a dose.

Do you need any wormer?

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So, get in touch today if you need help with wormer or other crucial topics. We’d be happy to give you any assistance we can.