Oxalates are naturally occurring compounds in certain plants that bind tightly to calcium in the gut, forming insoluble calcium oxalate crystals that the body cannot absorb. For horses, this is a significant concern because some tropical and subtropical pasture grasses — particularly buffel grass, setaria, kikuyu, pangola, and signal grass — contain high oxalate levels. When a horse grazes primarily on high-oxalate pasture, the oxalates lock up dietary calcium, creating a functional calcium deficiency even when total calcium intake appears adequate on paper. Over time, the body compensates by pulling calcium from the skeleton, leading to nutritional secondary hyperparathyroidism, commonly known as “big head disease” because the facial bones become visibly swollen as calcium is resorbed and replaced with fibrous tissue.
If your horse grazes on tropical or subtropical grasses known to be high in oxalates, you must supplement calcium to compensate — typically an additional 40 to 80 grams of calcium carbonate (limestone) per day on top of normal intake, depending on the oxalate level of the pasture. Alternatively, limit time on high-oxalate pasture and provide oxalate-free hay. Temperate grasses (timothy, orchard, brome) are low in oxalates and do not pose this risk.
Oxalate is not a nutrient, so deficiency does not apply. The concern is excess oxalate exposure, which causes signs of calcium deficiency: shifting lameness, stiff gait, reluctance to move, swollen facial bones (big head), loose teeth, spontaneous fractures, and poor condition despite adequate feed.
Covered above — high dietary oxalate causes nutritional secondary hyperparathyroidism (big head disease), bone demineralization, fractures, lameness, and dental issues. Acute oxalate poisoning from plants like sorrel or rhubarb can cause kidney damage, but chronic pasture-related oxalate exposure is the more common equine concern.
