Problematic Pasture Grasses for Horses

November 6, 2025

Grass is a natural and important part of a horse's diet, but pasture can vary considerably in its nutritional content and the compounds it contains.

Some grasses, weeds and pasture plants can contain higher levels of oxalates, potassium, naturally occurring pigments, mycotoxins or other compounds that may cause problems for susceptible horses.

Understanding what is growing in your horse's paddock can therefore be an important part of managing their diet and wellbeing.

Compounds in Pasture That May Affect Horses

Fluorescing Pigments

Some pasture plants contain naturally occurring pigments that can contribute to photosensitivity in susceptible horses.

Plants such as clover, lucerne, St John's wort, buttercup and plantain may be associated with photosensitivity. Affected horses can become unusually sensitive to sunlight, with exposed areas of skin becoming red, sore or inflamed.

If a horse develops unexplained skin irritation or sensitivity to sunlight, the pasture and other potential causes should be investigated.

Oxalates

Oxalates occur naturally in a number of pasture grasses, including kikuyu, setaria, couch, signal, buffel, buffalo, pangola, para grass and guinea grass.

Oxalates can bind with calcium in the digestive tract and reduce the amount of calcium available to the horse. Long-term exposure to high-oxalate pasture can therefore contribute to calcium-related problems in susceptible horses.

Signs associated with prolonged exposure can include facial swelling and skeletal or muscular problems.

If your horse is grazing predominantly oxalate-containing grasses, the overall diet and mineral intake should be carefully considered.

Potassium

Potassium is an essential mineral, but some grasses and feeds can contain relatively high levels.

Lucerne, clover, kikuyu and ryegrass can contribute significant amounts of potassium to the diet. Some horses appear particularly sensitive to diets high in potassium, especially when several high-potassium feeds or pasture types are combined.

If a horse develops changes in behaviour, sensitivity or general wellbeing, it can be worthwhile reviewing the entire diet rather than looking at a single feed or grass in isolation.

GrazeEzy is one of the nutritional products available from Greenpet for horses affected by pasture.

Mycotoxins

Mycotoxins are substances produced by certain fungi. They can occur in pasture, hay and other feedstuffs when suitable conditions allow fungal growth.

Potential sources can include ryegrass, tall fescue, paspalum, phalaris, couch, clover and kikuyu, although the presence and level of mycotoxins can vary considerably depending on growing and storage conditions.

Some horses may appear more sensitive than others, with possible changes in behaviour or general wellbeing.

Good pasture and hay management, together with careful observation of your horse, can help identify whether feed may be contributing to a problem.

ToxDefy is available as nutritional support for horses where dietary toxins are a concern.

Clover and Other Legumes

Clover and other legumes, including lucerne, contain naturally occurring compounds that can influence horses differently depending on the species, variety and individual animal.

Some horses appear particularly sensitive to clover-rich pasture or diets containing significant amounts of legumes.

If reproductive, behavioural or other unusual changes occur, it is worth reviewing the horse's pasture and overall diet with an experienced equine nutrition or animal-health practitioner.

Which Grasses Are Suitable for Horses?

There is no single list of grasses that will be suitable for every horse.

Some grasses commonly used for horse pasture or hay include:

Rhodes grass
Brown top
Cocksfoot
Prairie grass
Yorkshire fog
Timothy
Creeping bluegrass
Queensland bluegrass
Redgrass

However, even grasses generally considered suitable can vary considerably in their nutritional content.

Growth stage, weather conditions, soil fertility, rainfall, drought, frost and other environmental factors can all influence the composition of pasture.

This is why the individual horse and the whole diet need to be considered rather than relying solely on the name of the grass.

Know What Is Growing in Your Horse's Paddock

Pasture is much more than simply 'grass'. Different species can provide very different levels of fibre, protein, sugars, minerals and other naturally occurring compounds.

Knowing which grasses and plants are present in your paddocks can help you make better feeding and management decisions, particularly if your horse develops unexplained changes in behaviour, skin sensitivity, soundness or general wellbeing.

If you are concerned that pasture may be affecting your horse, consider the whole picture, including pasture species, hay, supplementary feeds, minerals and seasonal conditions.