If you manage your own pasture, you're also managing the worm burden on it. Horses pick up worms mainly through the grass. With a steady rhythm of removing manure, smart pasture layout, and targeted deworming, you keep that pressure low, often with less dewormer than most people think.
Why Grass Is the Source
The most important worms in adult horses are the small strongyles, a type of intestinal worm. They spend part of their cycle outside the horse. Eggs reach the pasture through manure, develop there into larvae, and crawl up onto the blades of grass. The horse ingests them while grazing, and the cycle repeats. Large strongyles (large bloodworms) are now found in the Netherlands only occasionally, in extensively or semi-wild kept horses and in imported horses.
That explains two things. First: a horse with a low worm burden can still pick up a high one on a dirty pasture. Second: your biggest gain lies in interrupting the outdoor phase. Larvae you remove from the pasture before they're eaten don't need to be fought later with a dewormer.
Weather determines how fast the pressure builds. In warm, moist weather, eggs develop into infective larvae within a week. In a summer that isn't too hot, those larvae can stay alive on the pasture for up to about three months, and some even survive the winter. As a result, infection pressure builds from June and peaks at the end of the grazing season.
Removing Manure: The Foundation
Regularly removing manure from the pasture is one of the most important measures. It takes out the eggs and larvae before they spread across the grass.
A workable rhythm for most pastures:
- Remove manure at least once a week in spring and twice a week in summer. That is the minimum: removing it more often lowers the pressure further. The American Merck Veterinary Manual even mentions every one to three days, so the advice differs.
- On a small pasture with several horses, removing manure more often pays off, since the pressure per hectare is highest there. Daily removal also helps against tapeworms.
- Don't compost manure on or along the pasture, where rainwater washes it back in; haul it away or compost it at a distance. Horse manure may only go back onto the land once it is fully composted: a core temperature of 70 degrees Celsius or higher throughout the pile for at least a week, turned regularly. Manure from foals and yearlings should never go onto a horse pasture.
Dragging manure (harrowing the pasture) helps, according to the Merck Veterinary Manual, only in hot, dry weather: the harrow breaks up the manure piles and exposes the larvae to the sun. Harrowing therefore doesn't replace removing manure. In other weather, you may mostly spread the larvae over a larger area.
Pasture Rotation and Stocking Density
The more horses on the less land, the faster the worm pressure builds up. Space and rotation work in your favor:
| Measure | What It Does |
|---|---|
| Lower stocking density per hectare | Less manure per area, so fewer larvae in the grass |
| Rotating paddocks | Put foals and yearlings on the cleanest paddocks, where no young horses grazed the year before. After a year without horses, the infection pressure is much lower; after two years, nearly all small strongyle larvae are dead |
| Grazing alongside cattle or sheep | They ingest horse-worm larvae without new ones developing for the horse, a natural "vacuum cleaner". With cattle, do watch for liver fluke |
| Not grazing too short, and mowing regularly | With enough grass, horses don't eat close to the manure spots, where infection is highest; on grass grazed bare, they do. Mowing (for hay, for example) removes larvae from the grass tips; this sometimes conflicts with the advice to keep a pasture short for a horse with a weight problem or laminitis risk (see Turnout and Outdoor Living for Horses). In that case, discuss with the vet which risk weighs heavier |
A short rest doesn't make a paddock safe. If you only move horses between pastures without also removing manure, the Leidraad Paard en Parasieten (Guideline Horse and Parasites) of the KNMvD (the Royal Dutch Veterinary Association) and Royal GD (the Dutch animal health service), a handbook for veterinarians, says you have to rotate every week to every two or three weeks, depending on the weather. Eggs need two to three weeks to become infective in spring, and a little under a week in summer.
Deworming Based on Fecal Egg Counts
In recent years, the advice from vets and parasitologists has shifted from fixed calendar-based deworming to targeted deworming based on fecal egg counts. The reason is resistance: the more often and more routinely you deworm, the faster worms become insensitive to the available products, and the number of effective products is limited. The Guideline calls a fixed schedule of at least four times a year outdated.
With a fecal egg count, a laboratory determines the number of worm eggs per gram of manure (the EPG value). Based on that, you only deworm the horses that genuinely stand out above the rest. There aren't many of those: only about a third of adult horses shed so many eggs (more than 200 EPG) that treatment is worthwhile, while other horses barely contribute.
In broad terms, that approach looks like this:
- During the grazing season, periodic fecal egg counts per horse; only treat at more than 200 EPG in an individual sample (more than 100 EPG in a pooled sample from several horses). In the first year, the Guideline advises three counts: before the grazing season, in summer, and in September. If the result stays low, counts can be done less often afterward.
- Tapeworm and bot flies are difficult or impossible to detect with a standard fecal egg count. Tapeworm is shown with a saliva test; treat only when an infection is proven or strongly suspected. Bot flies usually need no treatment: removing the eggs from the legs helps.
- Young horses (foals, yearlings, two-year-olds) have their own risk profile, including roundworms, and need a different schedule than adult animals.
Which product and timing suit your horses depends on age, grazing, and the situation at your yard. Have that schedule drawn up by your vet; don't work out your own dosage or interval. How fecal egg counts and deworming work through the year is covered in Deworming Horses: When, How Often, and Why Fecal Egg Counts Are Smarter.
Checking for Resistance
Because resistance undermines the whole system, it's worth checking every so often whether a product still works. You can do that with a fecal egg count on the day of treatment and again fourteen days later (the FECRT, fecal egg count reduction test): if the egg count doesn't drop enough, the product may no longer work on your yard, and you discuss an alternative with the vet. The Guideline advises doing this once every two years. That way you avoid continuing to deworm with something that no longer has any effect.
In Short
- The worm pressure sits mainly in the grass; that's also where your biggest gain lies.
- Removing manure, at least once a week in spring and twice a week in summer, is one of the most important measures.
- Lower stocking density, pasture rotation, and grazing alongside cattle or sheep reduce the pressure further.
- Deworm based on fecal egg counts, not routinely on the calendar: that keeps the products effective.
- Have the schedule, threshold values, and product choice drawn up by your vet, and check once every two years whether the product still works.
Sources (verified September 2, 2026, manure removal, pasture rotation, and deworming November 2, 2026): KNMvD and Royal GD: Leidraad Paard en Parasieten, extended version October 21, 2024 · Merck Veterinary Manual: Parasite Control in Horses
See also: Daily Horse Care: The Basic Routine · Veterinary Health Care: Vaccinations, Deworming, and Dental Care · Glossary: horse-world terms