Warm season grasses are being increasingly used in equine diets. This is because of their potential for lower energy and non-structural carbohydrate (NSC) content in comparison to cool season grasses.
These potentially lower energy, lower NSC characteristics make them useful forages for managing ‘metabolic’ horses. Those horses need careful regulation of both energy and NSC in their diets. That also makes warm season grasses useful in situations where forage bulk is required in the diet of horses without access to pasture.
These grasses are also commonly what is available and growing as pasture in warm climates. Therefore, they are the only forage a horse has access to.
But there are downsides (see below)!
Issues with Warm Season Grasses
Grasses that love to grow in warm climates go by a few different names. Those include warm season grasses, subtropical grasses, or C4 type grasses.
However, they all have one thing in common…a compound called ‘oxalate’.
Oxalate loves calcium! So much so that when oxalate is present, it will steal calcium off your horses.
That means horses grazing warm season grasses, or eating hay made from them, can be at risk of severe calcium deficiency.
The calcium deficiency is commonly so severe that bones get brittle and will eventually break. Or muscles stop working to such an extent that a horse can no longer stand up.
But, the onset of the deficiency, in many cases, is slow and insidious. And it creates non-descript and often varying symptoms that are frequently misdiagnosed. In fact, all too often the first a horse owner knows about high-oxalate grasses is when, heartbreakingly, he or she ends up with a horse with a broken leg!
Don’t let this be you or your senior horse!
Keep reading to learn some of the grasses that present the highest risk to your horse. Learn how to identify them, how the calcium deficiency occurs, and importantly, how to prevent it!
The good news is, it’s easy and cheap to prevent…you just have to know how!
Which Warm Season Grasses are Dangerous?
Nearly all warm season grasses contain some oxalate. But many of them, like paspalum, digit grass, and Australian rhodes grass, contain such small amounts that they don’t disrupt calcium absorption and metabolism enough to cause your senior horse any grief.
There are, however, a number of grasses commonly present in horse pastures and hay around the world that do present a significant risk to the health of your horse.
These are bermudagrass (also commonly called couch grass), teff, kikuyu, buffel grass, and the worst of them all—setaria!
It’s important that you be able to identify these grasses. That enables you know when your horse is at risk of calcium deficiency and when you need to take preventative action!
Let’s look at bermudagrass (couch) first.
Bermudagrass (Couch Grass)

Bermudagrass typically has a bright green to ‘blue-green’ leaf and will usually grow in a ground covering manner with long stolons, or runners (long stems that spread horizontally) growing across the ground.
Leaves are narrow (2 to 5 mm wide), might be sparsely hairy, and they alternate from side to side along the stem. The leaves often feel relatively coarse to the touch.
Leaves appear rolled where they emerge from the leaf sheath. The ligule, which is a little structure that wraps around the stem at the base of the leaf, is made up of fine little hairs.
Bermudagrass will frequently have seed heads that can be identified as finger-shaped spikelets that have a pink-purplish tinge, especially when flowering. Each spikelet will usually have 5 spikes, but it can vary from 3 up to 7 spikes. The spikes grow from a central point making them look a bit like an upside down umbrella.
Teff

Teff (Eragrostis tef) is a lime-green to green colour and grows in upright tufts, as opposed to running along the ground.
Leaves are narrow (2 to 4 mm wide) and the stems of teff are fine. Like bermudagrass, the ligule is made up of fine little hairs.
Teff is perhaps most easily identified by its seed head, which is a panicle. A panicle, in simple terms, is a branched seed head, that has a main stem, then branches that hold the seeds. Teff’s seed head is a loose, airy cluster of often weeping branches that each hold multiple tiny seeds.
Teff is most commonly used as hay for horses. Typically, good-quality teff hay is green to light green, fine stemmed, and leafy with the characteristic teff seedhead present.
Kikuyu

Kikuyu is usually a bright, almost lime green colour when it is young and growing. It is a grass that grows in long runners, so if you pull at it, you will often get what look and feel like leafy grass ropes.
Its leaf is a tiny bit hairy with a dominant vein running down the middle, which is especially visible and able to be felt from the back.
The stem is a bit flat, the leaves look folded (as opposed to rolled) where they are emerging from the leaf sheath. Typically kikuyu is soft to the touch, especially when young and green.
Kikuyu’s ligule is made up of really visible, fine little hairs. Something very unique to kikuyu is that it never has visible seed heads!
Buffel Grass

Buffel grass has a darker green to almost blueish colour. Unlike bermudagrass and kikuyu which tend to grow along the ground, buffel grows in a tussocky bunch.
Its leaves are sparsely hairy, up to 1 cm wide and also have a prominent vein or mid-rib down the centre. The leaves feel rough to the touch.
The emerging leaves are rolled (as opposed to folded) and again, its ligules are hairy.
Buffel grass seed heads are like little bottle brushes that are up to 10 cm long and frequently purpleish in colour, aging to a straw colour as the plant matures.
Setaria

Setaria is grey-blue-green in colour, and like buffel, grows in tussocky bunches. With the entire plant reaching 1 to 2 meters tall at maturity!
Leaves are broad, flat and usually hairless. They are folded (as opposed to rolled) where they emerge from the leaf sheath. The base of young leaves are compressed into a fan shape and are often reddish around the stem.
Setaria’s ligules are also hairy.
Its seed heads are like a long bottle brush…8 to 25 cm long! They will vary in colour from purplish to brown.
How Much Oxalate Do these Grasses Contain?
Bermudagrass and teff contain—on average—around 10 g/kg dry matter of oxalate. Kikuyu and buffel contain—on average—around 15 g/kg dry matter of oxalate. These grasses pose significant risk to senior horse health if not managed properly. But their risk pales in comparison to that posed by Setaria.
Setaria can contain anywhere from 35 to nearly 80 grams of oxalate…twice to more than 5 times the oxalate in kikuyu and buffel grass. It has been known to cause fatal calcium deficiency in less than 2 months.
Oxalate levels do vary by season, plant maturity, and plant part. Oxalate levels are typically highest during the season when plants experience the most growth. Plants in the early vegetative stage of growth are higher in oxalate than more mature plants. Leaves are higher in oxalate than stems.
Be aware that these grasses are more dangerous when they are young and leafy. Mowing paddocks to keep these grasses short in an attempt to reduce intake is likely counterproductive and will potentially result in putting your horse at higher risk of oxalate-induced calcium deficiency.
It is possible to have your pastures laboratory tested for oxalate. However, practically it appears to work well to feed based on an average expected oxalate level for the grasses your horse might be grazing.
Calcium in Horses
To understand the potential impact of these grasses on your senior, let me first explain normal calcium metabolism.
Under normal conditions:
- Calcium’s major roles in the horse’s body are providing strength to bones and enabling normal muscle contraction and relaxation.
- Because the heart is a muscle that must consistently contract and relax to keep your horse alive, blood calcium levels must be maintained within a very tight range so that muscle contraction and relaxation—therefore heartbeat—can occur correctly.
- If blood calcium gets too low, muscle function is affected and the heart cannot beat properly, which will ultimately result in death.
- In order to ensure blood calcium levels never get too low, the body has developed a very clever system where there is a store of calcium in the bones. Should blood calcium start to drop, calcium is released from the bones to ‘top up’ the blood levels.
- Then when there is excess calcium from the diet, the bones top up their calcium stores.
So, calcium is constantly fluxing in and out of the bones. And dietary calcium is topping up blood calcium levels and bone calcium stores.
When Horses Eat These Grasses
When a horse grazes cool season or low oxalate warm season grasses that don’t have high levels of oxalate, normal calcium metabolism is occurring. They absorb calcium from the pasture as well as from other ingredients in their diet. That calcium is topping up both the blood calcium and bone calcium levels.
But, when your horse grazes the high oxalate, warm season grasses, the oxalate ‘steals’ calcium from your horse. That changes the way this finely balanced calcium metabolism works.
Remember, the number one priority for your senior horse is to maintain blood calcium levels in order to keep its heart beating.
When horses graze the high-oxalate grasses, these grasses don’t contribute to topping up blood calcium levels. So, to maintain blood calcium, the bones will release calcium and blood calcium levels are maintained.
This is the normal process…the smart safety catch our horses have built in to keep them alive.
Calcium Problems
The problem arises when this keeps happening, day after day, week after week, month after month. Calcium has to keep being released from the bones to top up blood calcium levels. That means bone calcium stores don’t get an opportunity to be topped up!
Eventually, so much calcium gets mobilised that the bones become weak and your horse will now have a disease known as “nutritional secondary hyperparathyroidism” (NSH). This is commonly known as ‘bighead disease.’
NSH or bighead disease is really just generalised osteoporosis, or weak bones. This puts your horse at high risk of bone fractures, with tragic leg and pelvic fractures the most common.
It is worth noting that NSH can also be cause by diets that are excessively high in phosphorus and/or too low in calcium. In fact the original name for the disease was bran or millers disease because it was frequently observed in horses working in flour milling facilities that were commonly fed the high-phosphorus wheat bran without adequate calcium supplementation.
What Are the Signs of NSH?
It is really important to be able to recognise NSH so you can quickly reverse the calcium deficiency should your horse ever be affected by this condition.
The most characteristic sign, as the common name for this disease suggests, is a big head. In young horses, so much calcium is leached from the jaws and skull that the bones become spongy and enlarge, creating the well-recognised big head appearance.
However, it is important to remember that the name ‘bighead disease’ is misleading. This is because in mature horses, the disease is rarely accompanied with the characteristic ‘big head’ that is seen in young horses.
Mature and senior horses can still get bony changes on their heads. A bump running between their eyes or on the bridge of their noses is relatively common.
Early Clinical Signs of NSH
What you are more likely to see in the early stages of the disease is generalised soreness and a shifting lameness that moves around all four legs . Horses will favour certain legs over others depending on which ones are most sore.
I almost always find these horses are also generally unhappy. They don’t like being touched and are sour and grumpy. When the calcium deficiency is severe, you might find they become reluctant to move. You also might notice that they start to move with an odd gait. Other signs could include rough coats, weight loss and doing ‘poorly’ even when being fed plenty of feed, and compromised hoof health. When the disease is well progressed, their teeth might fall out.
One common sign that is almost always misdiagnosed is some form of noisy breathing. The nasal passages become restricted. This creates odd breathing sounds, especially if your senior is still working.
In the final stages of disease, the bones become so weak that bone fracture is almost inevitable. Horses will most commonly break legs or fracture their pelvises when rolling.
If they are lucky to escape fractures, it is possible for the disease to deplete the bones of calcium so much that blood calcium levels can no longer be maintained. This results in horses that are unable to stand up and ultimately ends with heart failure and death.
All of it is heartbreaking!!
But the good news is, it is also all preventable! Here is how!
Preventing Severe Calcium Deficiency
First things first: If your horse is already symptomatic, seek help from an experienced nutritionist and your veterinarian with managing your horse’s recovery.
If your horse is grazing these pastures or eating hay made from these grasses—but is not symptomatic—take heart. Preventing severe calcium deficiency is easy and cheap!!
The main thing you have to do is simply add enough calcium to your senior’s diet to prevent calcium deficiency!
However, it needs to be done properly, with enough calcium added to prevent the disease. And enough magnesium and phosphorus must be provided to keep the ratios of these minerals correct in the diet.
Following are some options to help you do this.
Option 1: Specialised Balancer Pellets
There are some well-formulated balancer pellets available on the market that are high in calcium and balanced with magnesium and phosphorus. These will add enough calcium to your senior’s diet to prevent calcium deficiency when they are consuming a diet that is predominantly high-oxalate forages.
These balancer pellets give you a convenient and reliable option for preventing oxalate-induced calcium deficiencies! However, be mindful that these products usually don’t contain enough calcium to fully counteract the negative effects of setaria!
For horses grazing bermudagrass, teff, kikuyu, and buffel that also have some alfalfa/lucerne hay included in their diets, normal balancer pellets might suffice. They need around 3% or 30 grams/kg of calcium, which is likely sufficient to prevent calcium deficiency.
Have an experienced and qualified nutritionist check your senior’s diet. Or you can use an online nutrition calculator to ensure there is enough calcium if you plan to use your senior’s usual balancer pellet together with alfalfa/lucerne.
Option 2: Add Your Own Calcium
You can also add calcium to your senior’s diet to prevent calcium deficiency if they are eating high-oxalate grasses. The key to doing this well is making sure you are adding enough calcium and balancing it with magnesium and phosphorus.
Suitable ingredients to use include limestone/calcium carbonate for calcium, a high-quality magnesium oxide for magnesium, and a calcium phosphate (DCP, MCP, or MDCP) for phosphorus that comes with additional calcium.
The amount of calcium you need to add will depend on how much calcium is already being fed as hard feed or from other forages (such as alfalfa or lucerne hay). If you are feeding a complete/fortified feed at the manufacturer’s recommended feeding rate plus alfalfa/lucerne hay, you might not need to add any additional calcium to the diet.
Again, you should work with an experienced nutritionist or use an online nutrition calculator to correctly calculate the amount of calcium, magnesium, and phosphorus you need to add to your senior’s diet to prevent NSH.
Option 3: Use a Specially Formulated Calcium Supplement
There are some high-quality and well-balanced calcium supplements on the market that are specially formulated for horses grazing warm season, high oxalate pastures, or eating warm-season, high oxalate hay. These supplements make adding calcium, magnesium, and phosphorus to the diet simple.
If you choose to use one of these products, follow the manufacturers recommendations. If your horse is grazing setaria, seek advice from an experienced nutritionist. Or you can use a nutrition calculator to determine correct dose rates.
Beware of Mistruths
There are some products on the market that contain organic calcium. If their marketing claims were to be believed, only their very expensive organic calcium is capable of preventing NSH disease.
This is simply is not true, nor does it have any reliable scientific evidence to support it.
Fact is, you can use common limestone as a calcium source. It is one of the cheapest feed ingredients available and also is highly effective for preventing oxalate-induced calcium deficiency.
Summary
To recap keeping your senior safe on high oxalate warm season grasses, remember:
- Warm season grasses contain a compound called oxalate that will steal calcium from your horse.
- Bermudagrass (couch), teff, kikuyu, buffel grass, and setaria are five of the most common, high-risk grasses.
- Horses need to keep their blood calcium levels tightly controlled to make sure their muscles can function and their hearts can keep beating normally.
- If a horse’s blood calcium levels drop, the animal simply takes a little bit of calcium from its bones to top the level up in its blood.
- When horses consume ‘normal’ low oxalate forages, they absorb calcium from the forage and this, together with any calcium from the rest of their diet, tops up their blood calcium levels and replenishes any calcium they might have taken from their bones.
- When they consume high-oxalate forages, these grasses steal calcium from the horses. The forages don’t provide enough calcium to top up blood calcium levels, so horses have to take calcium from their bones to keep blood levels topped up. Unfortunately, bone calcium stores don’t get replenished.
- When this happens constantly over a period of time, the bones become depleted of calcium and become weak. This is a disease known as nutritional secondary hyperparathyroidism (NSH).
- The main symptoms of NSH are a shifting lameness, general soreness, horses that don’t want to be touched, and horses that are generally sore and grumpy.
You also might notice a rough coat, weight loss, poor hoof health, and noisy breathing when horses are working. - Preventing NSH is as simple as increasing the amount of calcium in your senior’s diet. You must keep calcium in the right ratio with magnesium and phosphorus.
Final Words
High oxalate grasses are the cause of far too many sad and untimely deaths in our horse population. The wonderful thing is that prevention is simple and cheap!
With the information provided here, you can make sure you are keeping your seniors safe, healthy, and happy, even when they are eating high oxalate, warm season forages!
Further Reading
- Nutritional and Non-nutritional Aspects of Forage
- A study of oxalate concentration in five grasses in the wet tropics of Queensland –
- Factors affecting the nutritive value of kikuyu grass (Pennisetum clandestinum) – a review
- Oxalate accumulation in fodder crops and impact on grazing animals – a review
- 10 Tips for Feeding Forage to Horses. Dr. Patricia Harris. MySeniorHorse.com
- Learn more about Dr. Nerida McGilchrist here. MySeniorHorse.com
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Nerida McGilchrist, PhD, has more than 20 years experience as an equine nutrition specialist. She has worked with horses ranging from top-level competitors to backyard senior ponies.View all posts