Pasture Management for Parasite Control in Horses

Good pasture management is a vital part in maintaining a healthy horse. Some misguided owners think that turning a horse out to grass will provide adequate nourishment. Sadly, all too often the grass is on poor quality land and has been over grazed.

Horses need quality leys seeded with nutritional grass mixtures, well fertilised and kept free of noxious weeds. Soil types and location determine the type of herbage and grasses that will flourish on your land.

Turning your horse out and forgetting him, for days on end, is sure to result in a sick depressed horse. Leaving a horse alone in a field is unnatural, as horses are herd animals needing companionship to thrive. A companion animal need not be another horse. Donkeys, goats and small ponies all make good companions for horses.

Parasite infestations are a problem on all horse grazing. Good hygiene practices are essential to keep pasture healthy. Keep fields clean and safe with routine measures such as rotation, faeces removal and fence mending. Simply turning your horse out into a weed and parasite infested, bare field is a sure way to own a sick or dead horse. Worm infestations can lead to colic, bloat, ulcers, lung and even heart problems.

Fungal spores and bacteria thrive in mud harbouring nasties such as mud fever. Rain water dripping from trees can cause rain scald. Horses need shelter, water, mud free areas and clean feed buckets. Please show you care about your horse’s welfare give some thought to the field your horse lives in.

The life cycles of most horse worms start with the larvae or eggs being ingested from the grass. They mature through several larvael stages into adult egg laying worms in the horse. The eggs are passed out of the horse in the faeces onto the pasture. The eggs then hatch into fresh larvae. This revolving cycle means new infestations constantly occur. It is almost impossible to completely clear pasture of all types of worm. Re-infestation can be drastically reduced by adopting the following sensible control measures.

The removal of all horse manure is an essential chore, it should be done at least twice weekly. Preferably, the faeces will be completely removed from the pasture to be composted. Horse manure left in a heap in a corner of the field provides a breeding nursery for worms. Care should be taken to add layers of agricultural lime between the layers of horse faeces to hasten decomposition and destroy the worm larvae. Otherwise, you will be providing a perfect breeding ground for worm and fly larvae, especially pesky stable flies. Stable flies are the intermediate hosts to small stomach worms Habronema spp.. Horses are infected by stomach worm larvae that emerge from the flies, whilst the flies feed around the horse’s lips. The infective larvae are swallowed, taking up residence in the stomach mucosa where they cause lesions and ulceration. Horse flies are often the cause of persistent summer sores and blisters round the muzzle due to the bacteria they carry.


Harrowing is only useful during the dry spells with hot temperatures, in mid summer (July & August over 25 degrees Celsius). This will expose the worm larvae to the hot sun, temperatures over 25 degrees Celsius are fatal to worm larvae. Warm, wet spells in Spring and Autumn favour worm eggs hatching. Worm larvae survive low temperatures in winter by hibernating until Spring when they become infective. Harrowing at any other time of year, is a futile waste of time as you will be spreading the infective larvae across the land.

Past History & Companion Animals

Your pasture may have been contaminated by other animals? Consideration should be given to other grazers past and present of your pasture. Ruminants such as cattle, sheep and goats are infected by the same small stomach worm of horses – Trichostrongylus axei. These worms cause a chronic catarrhal gastritis of the stomach, diminishing the horse’s ability to digest and absorb feed. Repeated infestations can cause painful lesions leading to stomach ulceration. Horses become afraid to eat anticipating the discomfort. Foals are particularly susceptible to stomach worms.

Donkeys are host to lungworms Dictyocaulus arnfield. Up to 70% of donkeys carry lungworms often showing no clinical signs of infestation. Lungworms can over winter in pasture unaffected by low temperatures. Grazing horses ingest infective larvae that migrate to the lungs via the lymphatic system, and the pulmonary arterial blood supply. Larvae travel from the alveoli to the bronchi and bronchioles where they mature. The eggs are coughed up then swallowed and expelled in the faeces. Once a pasture has been contaminated with lungworm, they emerge every summer. Many horses who are hypersensitive to dust from hay and straw have suffered lung damage as foals due to lungworm. Foals are particularly susceptible to this pest with long lasting lung damage.

All companion animals should be treated for worms. New animals bring added risks of further infestations. They should be dewormed before introduction to the pasture.

Rotational Grazing & Pasture Contamination

A high density of horses per acre increases the worm count. Over grazing forces horses to graze on rough patches of grass that they usually leave for dunging. Inevitably, this leads to re-infestation and high worm counts.

Rotational grazing will help to control the build up of parasites. The amount of acreage per horse/pony is dependent on the amount of supplementary fed provided and whether winter grazing is provided. A pony would need one and a half to four acres, an average adult horse three and a half to five acres, split into three or four separate paddocks. Many horses particularly those in livery stables have considerably less than three acres. As a result fields become contaminated with high levels of parasite infestation.

Parasitic infestation can be naturally controlled through a combination of good pasture management and sensible regime of natural horse wormer application.