If you manage your own pasture, you're also managing the worm burden on it. Most of a horse's parasite load actually comes from the grass, not from the horse itself. 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 — small and large strongyles — 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.
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.
Larvae survive best in a moist, temperate environment. Warm, dry summer spells and hard frost kill off some of them; mild, wet weather conditions let the pressure build up instead. The high season for infection therefore usually falls in late spring, summer, and early fall.
Removing Manure: The Biggest Lever
No single measure does as much as regularly removing manure from the pasture. It takes out the eggs and larvae before they spread across the grass.
A workable rhythm for most pastures:
- Remove manure at least twice a week, more often during the infection season if possible.
- On a small pasture with several horses, daily removal pays off, since the pressure per hectare is highest there.
- Don't compost manure on or along the pasture, where rainwater washes it back in; haul it away or compost it at a distance, where the heat of a properly working compost pile kills the larvae.
Dragging manure (harrowing the pasture) actually spreads the larvae over a larger area. That's only a sound approach at a hot, dry time, followed by leaving the paddock empty for several weeks afterward so the sun kills off the larvae. On a moist pasture that stays in use, harrowing increases the infection risk rather than helping.
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 | An empty paddock lets larvae die off before you graze it again |
| Grazing alongside cattle or sheep | They ingest horse-worm larvae without the larvae developing inside them — a natural "vacuum cleaner" |
| Not grazing too short | Larvae mostly sit low on the blade; on grass grazed bare, the horse eats relatively more larvae |
How long a paddock needs to sit empty to be effective depends heavily on the weather and can range from a few weeks to several months. Rest during a cold spell or a scorching hot one helps more than rest during mild, damp weather.
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.
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. In practice, it often turns out that a portion of a herd sheds most of the eggs, 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 above a threshold set by the vet.
- Specific parasites such as tapeworm and bot fly larvae are difficult or impossible to detect with a standard fecal egg count; a targeted treatment at a fixed point in the year is often advised for these, regardless of the result.
- Young horses (foals, yearlings, two-year-olds) have their own risk profile — including roundworms — and need a different schedule than adult animals.
Which product, threshold value, and timing suit your horses depends on age, grazing, and region. Have that schedule drawn up by your vet; don't work out your own dosage or interval. Current guidelines are available from your vet and from the KNHS.
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 before treatment and again a few weeks after: 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. 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 twice a week is the most powerful and cheapest measure.
- 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 every so often whether the product still works.
See also: Daily Horse Care: The Basic Routine · Veterinary Health Care: Vaccinations, Deworming, and Dental Care · Glossary: horse-world terms