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Understanding pasture contamination and its implications for management


The current parasite challenge on pasture is made up of Nematodirus and other Strongyles larvae. While Nematodirus does not pose a large risk now don’t be surprised if it shows up in faecal egg count results. Ciaran Lynch, Teagasc Sheep Specialist explains this and talks about pasture contamination

In general Nematodirus does not pose a large risk currently as lambs have acquired immunity to it after previous exposure earlier in spring, but it is still present on pasture so don’t be surprised if it shows up in faecal egg count results. The other strongyle worms are what we typically are concerned about, at this stage of the season. They are from 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. It helps to better understand this as it has an influence on lamb performance but also how we manage that parasite challenge and deal with the development of anthelmintic resistance.

In general we can divide the available pasture on farms into 3 categories as follows:

Dirty pasture

This pasture is under continuous 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 pastures.

Cleaner pasture

This pasture has not been continuously grazed by sheep.  It was only occasionally grazed by sheep 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 2 years previously by strict definition and may be entering the system from another enterprise. Full reseeds following a year’s break from sheep grazing would also fall into this category.  There is no direct parasite challenge from this pasture

To find out more about pasture contamination watch the following short clip:

Unfortunately we know in Ireland we have widespread issues with anthelmintic resistance, with some resistant parasites on virtually every farm, to one or more of the anthelmintic classes. To delay this 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.

Consider how we manage pasture

However we also need to consider how we 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.

What happens when we treat lambs with an anthelmintic (wormer)?

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 anthelmintic (wormer). Every time we treat lambs with an anthelmintic 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 proportion of them are able to survive the dose.  The genetic resistance the worms have developed varies depending on which anthelmintic class they are exposed to.

So when we treat sheep with an anthelmintic that the worms have begun to develop resistance to, all the susceptible ones will be killed and only the resistant ones will survive, mate and lay eggs that will be passed out on pasture. This is giving the resistant worms a competitive advantage over the susceptible ones on pasture. Over time the population of worms on pasture will change as we continually dose lambs with ineffective products, with an increasing number of resistant ones present 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 effectively by the wormer.

So why is this important in the context of the type of pasture we are grazing and what effect has turning out freshly dosed lambs onto it have?

On dirty pasture the population of worms in refugia (worms not exposed to the dose) are typically 100 to 10000 greater than the worms 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, is smaller.

On cleaner pasture the worm population is smaller so there is less of the susceptible worms to dilute out the resistant ones being passed out by the lambs. In this case the worm population on pasture can tip in the favour of resistant ones at a faster rate.

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 onto it, the only worms present will be resistant ones. Therefore every subsequent time that 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.

To counteract this we need to consider ‘seeding’ or exposing this pasture to susceptible worms by either delaying dosing when turning onto this ground and allowing lambs with a low worm burden to graze it initially, or follow them with un-dosed sheep.  Even in subsequent rotations try to ensure lambs graze this clean pasture after grazing on dirty pasture and not directly following a dose

To find out more watch the following clip on the Effect of dosing

Hear more about why we need to reconsider dosing and moving by listening to OviCast podcast episode below: 

The Teagasc Sheep Specialists issue an article on a topic of interest to sheep farmers every second Tuesday and from time to time here on Teagasc Daily.  Find more on Teagasc Sheep here