Danger in the Grass
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Author: Dr Nerida McGilchrist | PhD Equine Nutritionist and Founder of My Happy Horse
Horses are natural grazers and there are few scenes more peaceful than watching horses with their heads down munching away at pasture. But sometimes what you may be unaware of is that some grasses will do real damage to your horse’s bones. And sadly, in far too many cases, the first a horse’s owner is aware of this danger lurking in their grass, is that their horse will suffer a broken leg, or pelvis and the only option is euthanasia. Heartbreaking.
These calcium sucking, bone destroying grasses come from the ‘warm season’ (also commonly known as the sub-tropical) group of grasses. The dangerous grasses contain large amounts of a compound called oxalate which loves calcium and will steal it from your horse, weakening their bones in the process.
To protect your horse, it is important you can recognise the dangerous grasses and correctly supplement your horse with additional calcium. This article aims to help you understand how to do both of these things so you can keep your horse safe!
How does grass destroy bone?
Let’s look first at how grasses can literally destroy bones. As I mentioned, the dangerous grasses contain a compound called oxalate. Because of oxalate’s structure, it loves to grab hold of calcium which then makes it unavailable for absorption, creating a severe calcium deficiency in the process. At least some of the oxalate is also absorbed into the body which then allows it to grab hold of calcium from the blood and drag it out of the body via the kidneys. The resulting calcium deficiency is the first part of the process.
The second part of the process to understand is that your horse MUST maintain blood calcium levels within a very strict range. Calcium is crucial for muscle contraction and relaxation and if your horse allows its blood calcium level to drop, their heart would literally stop beating… disaster!
To prevent that from happening, whenever your horse needs to top up its blood calcium levels, they will pull calcium from their bones. This is a very clever, 100% normal physiological process that horses and a myriad of other species have that allows them to keep blood calcium at the correct level.
Under normal circumstances, on diets that meet calcium requirement, it’s a constant process of borrowing a little bit of calcium from the bone here, but paying it back there, and their bones maintain a healthy ‘bank account’ of calcium.
When your horse is on these calcium sucking grasses however, your horse borrows calcium and then borrows again and again and again until eventually their ‘bank account’ of calcium is depleted, their bones are weakened and they are at extreme risk of bone fracture.
This condition is a disease officially known as Nutritional Secondary Hyperparathyroidism (NSP), more commonly known as ‘bighead disease’.
Hence it is SUPER important that you learn to recognise these grasses so that you can adjust your horse’s diet accordingly to prevent the bone calcium depletion from happening!
Which grasses are dangerous?
There are many high oxalate grasses found in our horse’s pastures, but the most common ones you should be aware of are kikuyu, couch or bermudagrass (same grass, different names), buffel grass and setaria.
Here is how to spot them…
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. And something very unique to kikuyu is that it never has visible seed heads!
Couch grass / Bermudagrass
Couch (also called bermudagrass) is usually a medium green colour and forms a dense, springy “lawn-like” mat when it’s growing well. It spreads in long runners along the ground — and if you tug at it, you’ll often pull up wiry, creeping stems that root at the joints like little pegs. And something very distinctive about couch is its seed head: when it does go to seed, it often looks like a little star or windmill, with 3–7 thin “fingers” radiating out from one point.
Buffel grass
Buffel grass has a darker green to almost blueish colour. Unlike kikuyu which tends to grow along the ground, buffel grows in a tussocky bunch. And its 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! The base of young leaves are compressed into a fan shape and are often reddish around the stem. And its seed heads are like a LONG bottle brush… 8 to 25 cm long! And they will vary in colour from purplish to brown.
Beware setaria
Setaria can contain anywhere from 35 to nearly 80 grams of oxalate per kg of dry matter… twice to more than 5 times the oxalate in kikuyu and buffel grass, which both contain around 15 g/kg of dry matter. Setaria has been known to cause fatal calcium deficiency in as little as 6 weeks in lactating broodmares.
So if you have this grass, you NEED to know you have it so you can feed in a way to prevent rapid and severe calcium deficiency occurring.
Early symptoms
The name Bighead disease is misleading as typically only younger horses will get the classic ‘bighead’ appearance. In mature horses, what you are more likely to see is a shifting, hard to pin down, chronic lameness that will move around their legs as they favour one or more over another. Over the years I have also seen a pattern of horses being sore in the muscles and unhappy or ‘sour and grumpy’, preferring not to be touched!
You may also notice an unusual sound as they breath, particularly when working, an odd gait, bony changes to the front of the skull, like a ridge between their eyes, a rough coat, dental issues, weight loss and poor hoof health.
Prevention is better than cure!
If you have read this article and believe your horse already has bighead disease, please seek out an experienced nutritionist to help you correct your horse’s diet and feed for recovery! In the short term, remove them from the high oxalate pasture and provide them a diet that introduces plenty of lucerne hay.
If instead you recognise you have one or more of the dangerous grasses in your horse’s pasture, it is really important you check your horse’s diet to make sure that the diet contains enough calcium to counteract the effect of the oxalate in the pasture.
To prevent bighead disease, you must be feeding calcium in a ratio of 0.5 parts calcium to 1 part oxalate while also keeping the calcium in the correct ratios with magnesium and phosphorus.
You can do this by feeding a specialised balancer pellet that has been correctly formulated specifically for horses on high oxalate pastures. Or by adding a calcium supplement formulated for this situation (being wary of marketing claims that only ‘organic calcium’ is suitable, these claims are not substantiated in any way!).
Or you can add additional calcium, magnesium and phosphorus to your horse’s diet. How much you need to add depends on what else your horse is being fed!
For horses just on grass pasture, you will need to add approximately 80 grams per day of limestone, 50 grams per day of dicalcium phosphate and 20 grams per day of high quality magnesium oxide (Causmag). Because it is so crucial you get these mineral levels right, I strongly recommend you use a diet formulation tool or an experienced nutritionist to calculate your horse’s diet. The stakes are too high to guess!
If your horse is on setaria pasture, at a minimum you will need to feed approximately 3 times these levels of mineral and at least 2 kg/day of lucerne hay. And you really MUST have your horse’s diet properly calculated... Setaria is so dangerous it should be listed as toxic for horses!
Hay can also cause disease
I want to just quickly note here that high oxalate hays like teff hay and bermudagrass hay also contain enough oxalate to cause issues for horses if fed with a diet that doesn’t contain enough calcium. If you are feeding these hays, you also need to take care to balance the diet correctly.
Awareness is the first step!
Most disastrous outcomes for horses on these high oxalate pastures occur simply because their owners are totally unaware of the danger lurking in their grass. To be aware of whether or not your pastures contain any of these dangerous grasses is the first step toward preventing serious problems for your horse. So go check out your paddocks, and if you aren’t sure what your grasses are, ask for help in identifying them, because once you know whether or not your horse is at risk, you can start the process of balancing their diet to keep them safe!
About the Author
Dr Nerida McGilchrist
Dr Nerida McGilchrist is an Australian equine nutritionist with a PhD and over two decades of experience. As the founder of Equilize Horse Nutrition, and advisor to some of the world's largest nutrition companies, she’s built an international reputation for blending science with practical solutions. Now, she’s bringing her expertise to My Happy Horse to make advanced nutrition accessible to all.