One of the biggest management challenges facing horse owners in Australia and many tropical regions is high oxalate pastures. These grasses—while often lush and palatable—can have a huge effect on your horse’s health and long-term wellbeing.
Oxalates are compounds that bind to calcium and other essential minerals, locking them up in forms that horses cannot absorb. Because calcium is critical for so many vital body functions - bone strength, muscle contraction, nerve signalling, cardiovascular function - this binding process can quickly lead to deficiency. When dietary calcium is not available, the horse’s body pulls calcium from its own skeleton, leading to weakness, pain, and in severe cases, the bone disease known as Bighead.
What Are Oxalates and Why Are They a Problem?
Oxalates are naturally occurring compounds found in many plants, especially tropical grasses (such as buffel, setaria, green panic, kikuyu and guinea grass), as well as some vegetables like rhubarb leaves.
They occur in two main forms:
Soluble oxalates – these pass into the bloodstream and are excreted by the kidneys.
Insoluble oxalates – when oxalates bind to calcium, they form insoluble crystals. These cannot be digested or absorbed, and not only deplete calcium but also cause inflammation and irritation to the gut lining.
Horses with continual exposure to high oxalate pastures often develop compromised gut integrity, inflammation, and mineral imbalances. Unlike cattle, who have rumen microbes capable of breaking down some oxalates before absorption, horses are hindgut fermenters. By the time oxalates reach the hindgut, it’s too late—the calcium has already been lost to absorption.
Why Calcium Matters
Calcium is a macronutrient, meaning horses need it in large amounts every single day. It plays roles in:
- Bone and skeletal strength
- Muscle contraction and relaxation
- Nerve conduction and signalling
- Nutrient transport and enzyme activity
Because calcium is so vital, the horse’s body will do everything possible to keep blood calcium levels stable. If dietary calcium is locked up by oxalates, the body will pull it from bones, leading to weakness, lameness, and in time, Bighead disease.
Symptoms of Oxalate Problems in Horses
Most horse owners associate oxalate issues with Bighead—the swelling and deformity of facial bones due to chronic calcium deficiency. But this is often the final stage, appearing only after weeks to months of damage. Earlier signs can be subtle:
- Behavioural changes (irritability, sudden reactivity, pain responses)
- Shifting lameness or shortened stride
- Muscle weakness, twitching, or spasms
- Poor performance or reluctance to work
- Digestive problems (colic, loose manure, poor gut health)
- Slowed recovery from injuries
- Cardiovascular stress
Signs of calcium mobilisation can occur within days of grazing high oxalate pastures, with Bighead often developing within 12 weeks if the issue is not addressed.
Which Horses Are Most at Risk?
While all horses can be affected, the consequences are more severe in horses with higher calcium needs:
- Young, growing horses (bone development)
- Pregnant and lactating mares (calcium for milk production)
- Performance horses (muscle function, skeletal stress)
- Horses recovering from injury (healing and tissue repair)
What Can Horse Owners Do?
1. Reduce Exposure Where Possible
Ideally, horses should be removed from high oxalate pastures, at least for part of the day. Even if grasses are mixed, horses often selectively graze oxalate-rich species because they are highly palatable.
Rotational grazing, supplementary hay, or overnight removal with ad lib roughage can help reduce intake.
2. Supplement Calcium and Minerals
If removal from oxalate pastures isn’t possible, calcium supplementation is essential. The goal is to provide enough calcium to:
Bind the oxalates present, and
Leave sufficient calcium free for absorption in the small intestine.
3. Support the Gut and Nervous System
Oxalates damage the intestinal lining and can cause chronic inflammation. Using herbal and nutritional support to protect, soothe, and restore gut integrity is critical. Nervine herbs can also help with behavioural changes and pain responses linked to calcium deficiency.
McDowells Herbal Recommendation
At McDowells, we recommend a holistic approach:
- Mineral balancing: Providing highly bioavailable forms of calcium and trace minerals to compensate for pasture deficiencies.
- Digestive support: Herbs such as Slippery Elm, Marshmallow, and Licorice to soothe and protect the gut lining.
- Herbal nervines: Herbs like Skullcap, Valerian, and Chamomile can help calm behavioural issues linked to pain and stress.
- Bone and tissue tonics: Herbs such as Comfrey and Yarrow to support connective tissue and recovery.
Our tailored herbal programs are designed to work alongside practical management strategies such as pasture rotation, supplementary roughage, and correct calcium supplementation.
Final Thoughts
Oxalate pastures are a widespread challenge for horse owners, but with the right management, nutrition, and natural support, it is possible to protect your horse from calcium deficiency, Bighead, and other oxalate-related problems.
If you are concerned about your horse’s exposure to oxalate-rich grasses, or you’ve noticed subtle behavioural or physical changes, we encourage you to reach out for a free consultation. Together, we can design a program to safeguard your horse’s bones, digestion, and long-term health.
