Are you worried about high oxalate grasses in your horse paddocks? Don’t panic! 
Having a high oxalate horse pasture isn’t the end of the world – with a little understanding and some expert help, it is easy to supplement horses grazing high oxalate grasses to provide a healthy ration and avoid ‘bighead disease.’
Marketing spin and urban myths about pasture oxalates abound in the Australian horse community, striking terror into responsible horse owners. Tired of unsubstantiated claims and feeling pressured to buy expensive supplements? Read on to access scientific facts and veterinary recommendations to answer commonly asked questions, explain the risks and how to safely manage them.
What are high oxalate pastures?
Oxalates are a naturally occurring plant biomineral thought to help plants with calcium regulation and improve light dispersion to improve the efficiency of photosynthesis and growth.
Figure 1. The large black shape represents the oxalate molecule and the green triangles represent the inorganic calcium molecules which can be bound in the plant or in the gut.
Many plants contain oxalates but only some contain the very high levels that can induce bighead in horses.
Some weeds such as pigweed, soursob and dock are extremely high in oxalates.
What makes oxalates dangerous to horses?
Plants contain oxalates in either soluble or insoluble form. Insoluble oxalates are bound to calcium and magnesium inside plant leaves and stems. All the calcium and magnesium within a high oxalate plant can be oxalate-bound, making it unavailable horses grazing those plants.
Ingested soluble oxalates can be broken down in the stomach into oxalic acid. This free oxalate binds calcium inside the horse’s stomach and small intestine to form an insoluble calcium oxalate crystal which cannot cross the gut wall for absorption into the bloodstream. The calcium that is bound to the oxalate molecule continues on through the digestive tract to be passed in manure. Fortunately, oxalates are not like little pacmen with insatiable appetites! Each oxalic acid molecule can only bind a single calcium molecule.
Blood calcium levels are strictly regulated by hormones because calcium plays many important roles in bodily communications (through nerve function, muscle contraction and hormone secretion) and body stability (including skeletal strength, blood clotting, membrane stability and cell division).
When inadequate calcium is absorbed from the gut, the kidneys excrete less calcium and bones release calcium from storage to replenish blood calcium levels. If this continues over time and bones lose too much calcium, they become weak, fibrous and misshapen. The resulting weakened bone condition is known as equine nutritional secondary hyperparathyroidism (NSH) and is also called bighead, bran disease and fibrous osteodystrophy (FOD).
Figure 2. Horses can have Nutritional Secondary Hyperparathyroidism without developing a ‘big head.
Figure 3. A pony displaying the typical bony facial growths of classic ‘big head.’
Do horses get oxalate poisoning?
It is extremely rare for horses to experience oxalate poisoning. Oxalate toxicosis occurs when unbound oxalic acid is absorbed into the bloodstream and binds ionised blood calcium. Acutely low blood calcium and magnesium levels can cause hypocalcaemia (very low blood calcium)/grass tetany. Affected horses may have muscle tremors, staggering gait, loss of appetite, lethargy, and can die without prompt treatment.
Oxalate toxicosis more commonly occurs in ruminants due to microbial breakdown of insoluble oxalates in the rumen. High levels of free oxalic acid reach the small intestine from where it can be absorbed into the bloodstream. By contrast, horses are hindgut fermenters, so any fermentation of calcium oxalate occurs in the caecum, which is not a site for mineral and oxalic acid absorbtion.
How long does bighead take to develop?
The length of time it takes grazing horses to develop clinical signs of NSH associated with bone demineralisation depends on:
- the calcium levels that were present in the bone to begin with;
- the amount of oxalate consumed; and
- the amount of calcium consumed.
Most recorded cases have taken at least a few months to develop but it is like an overdrawn bank balance. Think of calcium as money, and bones as the bank. If you start taking out more than you put in, you’ll move into deficit. The time it takes to get into trouble depends on how much you had in the bank to start with, and how much you spend each day.
When is the risk of bighead highest?
The higher the oxalate level of the grass, and the more grass the horse eats, the more soluble oxalates will be present in the gut, increasing the risk of calcium deficiency. Broodmares and growing horses are most susceptible due to their higher calcium requirements.
The amount of oxalate that plants contain varies according to stage of growth and environmental conditions.
Find out how to safely supplement horses grazing high oxalate pastures here.