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Change pastures to reduce worm infection

Think about your pasture strategy. Photo: Malgré Tout

There are several ways to combat intestinal worms in horses. In addition to a good treatment strategy with your veterinarian, which involves taking fecal samples and treating based on a veterinary assessment of these, you can do a lot yourself to reduce worm infestation.

Read also: Deadly bloodworm: Your horse may show false negatives


With deworming treatments, you target the worms inside the horse, but since most worms are transmitted through the grass in the pasture, you can break the worms' life cycle with good pasture hygiene. This article provides insight into your options for reducing the risk of infection by moving horses between multiple pastures, also known as rotational grazing.

Martha Voss

Martha Voss is an equine agronomist and runs the independent consultancy and training business NENUC, offering courses, consultancy visits, and the development of educational games, among other services. Martha has more than 30 years of experience in teaching and research in the management and proper nutrition of horses.

Understand the life cycle of worms and remove the horse droppings

Most internal worms lay eggs in the horse's intestinal tract, and the eggs end up on the grass along with the horse droppings. Here, they develop into infectious larvae, which the horse ingests along with the grass. When horses graze in an area where they have been the previous year, a small population of larvae will have overwintered. With the arrival of spring, they awaken from hibernation and develop into the infectious L3 larval stage. If these are ingested by the horses and develop into sexually mature worms, which then expel eggs with the horse droppings, the infection on the paddock will multiply over the summer. The illustration shows how the number of infectious larvae increases throughout the summer and autumn. Adult horses will develop a certain level of immunity, allowing them to keep the worms in check to some degree. However, foals do not yet have a strong immune system, making them more susceptible to worm infections.

l3 larvae
Infections of bloodworms in mares and foals throughout the grazing season in a temperate climate. The X-axis shows the seasons, and the Y-axis displays the level of larvae in the grass and soil (L3) as well as eggs in the horse droppings (EPG). The figure is adapted from a diagram on page 18 of Eva Osterman Lind's doctoral thesis.

Since horses can infect themselves and other horses through their feces, the simplest measure to reduce the infection pressure is to take a wheelbarrow or tractor and collect the horse droppings a few times a week. If you have many horses and large pastures, however, this can also be a very cumbersome method, but you might use it on the closest pastures where the horses spend most of their time. Several smart machines can collect the droppings for you; you just need to go out and retrieve them. If this isn't practically feasible, here are a couple of other options to reduce the infection pressure in the pasture.

Rotate a pasture every year

The larvae of worms are equipped with a small nutrient store, allowing them to survive from the time they develop in the eggs until they are eaten by horses. This is where you need to intervene. Plan your pasture strategy, to let the larvae die of hunger or dry out in the sun. The strategy varies depending on the season. The recommended grass pasture management is that summer pastures rest in the winter, while the horses are moved to special winter pastures, or the pasture that will be rotated in the spring. When talking about worm infection, the latter strategy is the best, as the larvae die during the time it takes for the grass to grow after we harrow the soil, sow new grass, and let it grow strong enough for horses to graze on it.


Typically, grass seeds are sown along with, for example, oat grains, and then the oats essentially protect the small grass sprouts, because grains grow faster than grass. You can then choose to cut the oats when the panicles progress (before the kernels form) and make hay or wrap the crop, thus providing good winter feed. In the meantime, the infectious larvae have died of hunger. That means this pasture is free from worm infection when you put the horses out again. This strategy is often used in stud farms, where mares and their foals are set on clean pastures. Foals are very susceptible to worm infection, and therefore it is crucial to maintain as low an infection pressure as possible here.

This is also a way to ensure plenty of grass in the pastures, as grass and soil are damaged by horses in the winter, and so we ensure new and dense grass on the new fields. The best approach is then to have a system with 3-4 pastures, which you rotate between over the years.

Divide the pasture into smaller paddocks and switch between them over the summer

Another way to break the larvae's infection pressure in the summer is to move the horses between several paddocks, which are then allowed to rest between the horses' visits. This also means that the grass in the paddocks improves, as it has time to grow.

paddock rotation
The figure illustrates how the infection pressure from the paddock could appear if you move the horses to another paddock for a month or two. The dashed green line represents the level of the horses remaining in the same paddock.

If you mow the grass in paddock 1 with a harvester, you can remove the tussock grass where the horses have pooped. Tussock grass often grows tall and dense and is a perfect breeding ground for larvae. By cutting it, you expose the larvae to sun and air, causing them to dry out. If you're short on grass, you can advantageously fertilize it during the resting period to strengthen the grass's growth.

After 4-6 weeks, most of the larvae are gone, and then you can move the horses back to paddock 1. The effectiveness of the paddock rotation depends on several factors:

  1. The horses' excretion of eggs when they enter the paddock. Fecal samples with possible deworming before placement in the paddock limit the infection.
  2. Temperature and humidity – sun and heat dry out the larvae.
  3. Soil conditions – airy, dry, and calcareous soil inhibits the larvae...

You won’t completely get rid of the worms...

As an addition to fecal samples and deworming, paddock rotation will help reduce infection, allowing the horses to learn to protect themselves and live with the worms. However, there are still many challenges, and research into the worm problem is being conducted from many angles. If there are many horses in small areas, you may rely on dewormers, and have a good control strategy with your veterinarian. In this strategy, you can discuss different measures that could help reduce infection in the paddock:

  • Create a system so the paddocks only rest for 3-4 weeks. When you move the horses from the paddock, mow it and rake up the soil with, for example, a track harrow/rake. It shouldn’t be turned over completely, just aerated a bit.
  • There are various herbs that can support the horses’ own combat against worms. Perhaps you could sow a herb bed in the paddocks, or add these herbs as a supplement in their feed.

In next month’s article on worms, we will look at the possibilities of reducing worm infection by letting other animals graze the horse paddocks.


Eva Osterman Lind. Prevalence and Control of Strongyle Nematode Infections of Horses in Sweden. Doctoral Thesis. Swedish University of Agricultural Sciences Uppsala 2005.


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