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Understanding pasture contamination and its implications for management


Michael Gottstein, Teagasc Sheep Specialist, explains the importance of management in dealing with pasture contamination.

On sheep farms the parasite challenge on pasture is made up from a population of Nematodirus and other Strongyles larvae. In general Nematodirus only poses a risk to young lambs (up to about 12 weeks of age) because lambs acquire immunity to it after exposure. The other strongyle worms are what we typically are concerned about from the end of May onwards. There are a number of species but from a more simplistic control perspective we can further divide them into susceptible worms (i.e. those that an anthelmintic / wormer will kill) and resistant worms (i.e. those that will survive the dose).

On sheep farms the parasite challenge or burden that lambs are exposed to varies depending on how the pasture was managed previously as this influences the worm population. Consequently grazing management and drenching needs to take into account the type of parasite burden that different pasture types have.

James Doran, Teagasc Advisor caught up with Damian Costello, Teagasc Sheep Specialist at #Ploughing2022 to get an insight into what Anthelmintic Resistance is. Anthelmintic Resistance is a big problem globally and it's ever increasing in Ireland aswell. It's where on sheep farms the worms genetically develop the ability to survive a dose that would normally kill them - so you're dosing your lambs and your products are not working effectively.

In general we can divide the available pasture into 3 categories

Dirty pasture

This pasture under continual sheep grazing for a number of years, typically this pasture is contaminated with Nematodirus eggs, and susceptible and resistant Strongyles. The level of parasite challenge is typically high on these type of pastures

Cleaner pasture

This pasture has not been continuously grazed by sheep only occasionally during the season, it may be more typically grazed by cattle or cut for silage during the year (aftergrass). Reseeds may also fall into this category depend on how they were managed previously. The parasite challenge on this pasture is low.

Clean pasture

This pasture has not been grazed by sheep for two years previously by strict definition and may be entering the system from another enterprise. For example grazing that was previously only grazed by cattle or ground being reseeded having spent a number of years in tillage. There is no direct parasite challenge from this pasture.

Anthelminthic resistance and Pasture Management

In Ireland we have widespread issues with anthelminthic resistance with some resistant parasites on virtually every farm to one or more of the anthelminthic classes. To delay or slow down the development of anthelmintic resistant parasites we need to change our management approach on a number of aspects, how we manage pasture in relation to dosing is one of these.

From a lamb performance perspective cleaner or clean pasture will allow lambs to grow faster as they are being exposed to lower or non-existent worm burden. In the absence of this challenge they can potentially grow faster as they don’t have this challenge to deal with. So this is a bonus particularly during the post weaning period.

However there is a trade-off between maximising lamb performance and maintaining the efficacy of our drugs that help us to control worms. We need to manage pasture to help maintain a susceptible worm population i.e. keep a population of worms that the dosing products we use on farms are effective against. This is particularly important for clean and cleaner pastures.

Initially it may seem like a contradiction but let’s try and explain why this is important and what happens when we treat lambs with an anthelminthic (wormer). Every time we treat lambs with an anthelminthic or wormer we also logically expose the worm burden in the lambs to this product. Now ideally this would kill all the worms present, but as the population of worms are continually exposed to these treatments they begin to develop genetic resistance to them. As a result over time a greater number of the worms will develop the ability to survive the dose that would normally kill them.  This is called anthelmintic resistance and it is genetic, so the resistant worms pass those resistant genes  on to their offspring.

So when we treat sheep with an anthelminthic the susceptible worms are killed and the resistant worms survive, mate and lay eggs that will be passed out on pasture. This is giving the resistant worms a competitive advantage. Over time the population of worms on pasture will change as we continually dose lambs with ineffective products with an increasing number of resistant worms surviving as the susceptible ones are killed off.  This will dilute the worm population on pasture and tip the balance ever increasingly in the favour of the resistant worms. When the lambs subsequently graze this pasture they are picking up more and more resistant worms that won’t be killed off  by the wormer, eventually leading to treatment failure.

When we treat lambs depends on the type of pasture they are going in to

On dirty pasture population of worms in refugia (worms not exposed to the dose) typically 100 to 10000 greater than the exposed in lambs. So the worm population is much larger and the dilution effect is greater and  as a result the effect of resistant worms being passed out although still an issue out is much smaller. A dose and move to fresh grass will not have any negative impact.

On cleaner pasture the worm population is smaller so there is less susceptible worms to dilute out the resistant ones being passed out by lambs so the worm population on pasture can tip in the favour of resistant ones at a faster rate. In this situation it would be better to move the sheep first on to the pasture and then dose a week or so later.

On clean pasture this is a far bigger challenge as there is no worm population to dilute the resistant ones and by turning out freshly dosed stock on to it the only worm present will be resistant ones. Therefore every subsequent time sheep graze this pasture they will only pick up resistant worms and help spread them around the rest of the farm – thereby speeding up the development of resistant parasites on the farm. Grazing untreated sheep to “seed” the pasture with a mixture of susceptible and resistant worms is a must. Dosing and moving sheep onto clean pasture will contaminate the pasture with resistant worms making subsequent treatments more difficult

Visit us at The National Ploughing Championships

At the Teagasc Stand at this year’s National Ploughing Championships we will be demonstrating the different types of pasture and discussing what are the appropriate parasite control strategies for the different pasture types.

For more information please visit our webpage on anthelmintic resistance