By vet Shona Mouncey
To help maximise growth rates in young stock, whether it be calves or lambs, it is essential to control parasite burdens.
Gastrointestinal worms are one of the main challenges to grazing youngstock over the summer months. As temperatures rise and animals get older, if pastures are contaminated the number of larvae ingested from the grass increases and multiplies inside the calf/lamb which in turn leads to further contamination of the pasture.
There are several factors when it comes to effective control of gut worms: use of faecal egg counts; effective use of wormers; pasture rotation; preserving susceptible worms.
The use of faecal egg counts is essential to avoid unnecessary treatments. Many traditional methods include worming at set intervals or the use of long-acting treatments, without knowing what the worm burden is. By taking muck samples from untreated animals (at least 3 weeks since last treatment) and looking for worm eggs, you can know whether at that point in time treatment is necessary or not. Many places can offer this service in house meaning you can have an answer the same day you take the samples.
If treatment is needed, which wormer should you use? It is widely agreed wormers should be used in rotation, but also that you need to make sure they work against the worm population on your farm. This is done by taking pre and post treatment muck samples and looking at how much the faecal egg count has reduced. For sheep flocks, currently funding is available to do this through the Animal Health and Welfare Pathway. Please speak to your vet for further information.
Carrying out a pasture risk assessment can help to reduce the dependence on wormers. Over the summer months, high risk pastures include those grazed by ewes and lambs earlier in the year, whereas those grazed by cattle or used for silage will be lower risk. In spring, pastures are high risk if grazed by lambs the previous spring or by store lambs in the autumn. In spring the only pasture that will be low risk is where no sheep have been grazed the year before or silage was taken after it was grazed by sheep the previous year.
Wormer resistance is when an accurate dose of wormer doesn't kill all the worms within the animals treated. For a wormer to be classed as effective the faecal egg count must reduce by over 95%. It makes sense to want to kill as many worms as possible with an effective treatment but to ensure resistance to wormers doesn't speed up, we must preserve some susceptible parasites on the pasture to dilute any resistant worms present. There are two ways to do this. The first is by leaving 10-20% of the group untreated so that those animals maintain a mixed population (of resistant and susceptible worms) and in turn pass those eggs onto the pasture to dilute any resistant worms present. For example, single lambs which are growing well could be left untreated. The second is returning treated animals to the pasture they were already on to pass out any parasite eggs present, before moving them onto another field. The traditional strategy of dose and move selects heavily for resistance and so should be avoided.