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Oxalate Pastures: How Do They Affect Horses?

Tess Lawrence - Thursday, August 21, 2014

A horses grazing activities make up a major part of their diet, yet all too often horse owners fall into the trap of disregarding the nutritional composition of the pastures available when considering the horses overall requirements. Unfortunately, educational articles and talk are generally based around hard feed, as opposed to pastures. This can cause a misunderstanding that 'grass is just grass', instead of recognising that pastures have greatly varying nutritional compositions, and that some can actually be detrimental to your horse. Such is the case for oxalate pastures. This particular type of pasture is often hardy and persistent, and can be found in most corners of the globe. It's effects can be significant if not managed correctly. It can be a big contributor to the condition Osteodystrophia fibrosa (Big Head) and can also compromise the integrity of essential, weight bearing bones.

Why are oxalate pastures bad for horses?

Oxalate pastures are full of - you guessed it - oxalates. When oxalates are present in higher amounts than calcium, it creates an calcium oxalate compound in the grass that essentially ties up calcium. What this means is that it effectively inhibits the horse from absorbing any calcium from the grass during digestion.

Horses uses their well developed large intestine to digest nutrients from the food they eat. This is unlike ruminant animals such as cows and sheep, who digest much of their food through their multiple stomachs. As horses only have one stomach, their ability to digest low quality food is minimal. When feeding on oxalate pastures, unavailable calcium passes straight out through their manure. Over time, the body reabsorbs calcium from existing sources (bones), and this is where the problems start! Initially, the horses body will reabsorb calcium from non weight bearing bones in the face and skull- which is how Big Head is caused. Once weakened, the nasal contents are able to push the bones out, creating hard lumps and swelling.

Over time, if a horse is grazing solely on oxalate pastures and is not being supplemented, the body will start leeching calcium from weight bearing bones as well. This can lead to poor performance, stiffness and weakened bone composition.

Typically, the ideal 2:1 calcium to phosphorus ratio is reversed or altered in oxalate pastures too. 

How do I know if my horse is grazing on oxalate pastures?

The following group of grasses are the most common in the oxalate category. The most conclusive way you can find out if your pasture is oxalate in nature is to get a pasture and soil test. Stance recommends testing grasses at Dairy One in the USA. Click here to check out their services.

It should be noted that many of these oxalate pastures have been introduced as they are good livestock grazing. However, as mentioned previously, the digestive process for cows and sheep is greatly different than that of a horse, which is why the pasture suitable for one may not be suitable for the other! 

Please also note that pastures may vary even from paddock to paddock on the same property!

Pennisetum Clandestinum, commonly known as Kikuyu grass, is native to arid East African regions. It can now be found in most areas of the world and is known for its ability to survive in harsh climates and take over other types of pastures. Kikuyu grass is characterised by its thick roots and good ground coverage. The calcium availability of Kikuyu grass for horses is only 20%.

Cenchrus Ciliaris, or buffel grass, is one of the highest level oxalate pastures, with only 17% calcium availability. It can be recognised by its long seeded head and often forms in clumps.

Narok Setaria is another highly oxalate grass. The oxalate content in this type of grass is so much higher than the calcium content that the calcium availability is 0%. Setaria is a long, wispy grass with a long seeded head.

Megathyrsus Maximus, or Panic grasses are another type of oxalate pasture. The calcium absorption for horses comes in at 42% which is higher than some other grasses, however this number is still in the risky territory for horses. It can be recognised by their long, split heads and their tendency to grow around the base of trees. 

Other types of pastures that may be hazardous to horses are Guinea and Pangola grasses.

Some suitable grasses for horses include Rhodes, Flinders Grass and Bluegrass. 

How can I help my horse if they are grazing on oxalate pastures?

If your horse has no choice but to graze on an oxalate pasture, there are some things to do to make sure they are still getting some calcium that they can absorb in their diet. A good form of available calcium is lucerne hay, or oaten chaff. Additionally, supplementing with a good quality, balanced vitamin and mineral supplement such as VitaStance (in Australia). This supplement contains a good source of both calcium and phosphorus, which is vital for remineralising bones. Outside of Australia, the supplement and other roughage you choose to feed should have a 2:1 calcium to phosphorus ratio. 

Ideally, feed your horse in a yard, or somewhere away from the source of oxalate grass. And of course, if you can at any stage, remove the horse from the oxalate pasture altogether!

The key to keeping your horse as nutritionally balanced as possible, no matter what pasture they are on, is education. Find out what pasture your horse primarily eats, then supplement based on what they are lacking. 

Having a spare paddock so that you may rotate your horse/s is also ideal. Horses are selective grazers and will often 'eat down' certain areas of their paddocks where their preferred grass is, and leave unpalatable grasses. By having your paddocks rotated, it enables you to let the preferred grasses grow back and prosper, instead of weeds potentially taking over bare patches of ground.

Additionally, good hygiene and manure spreading or disposing practices are highly efficient and can boost your paddocks pasture productivity!

Happy Riding (and grazing)!

Tess- Media and Communications Manager

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CoolStance copra is a unique horse feed because it has low Non Structural Carbohydrate (NSC), and yet has a high digestible energy content.
Learn More



PowerStance is a unique powdered coconut oil supplement.  PowerStance delivers the secret ingredient from CoolStance as a powder.
Learn More