Oxalates in Equine Forages

The importance of calcium in the diet of horses and ponies is crucial. When coupled with phosphorus, the two minerals compose up to 70% of the total mineral content in the body. Calcium is necessary for skeletal, cardiac, and smooth muscle function, nerve conduction, and a host of other metabolic reactions. Ensuring adequate calcium intake is imperative, but horsemen should also be aware of substances that can impede proper absorption of calcium, such as oxalates.

Oxalates bind calcium in the gastrointestinal tract of the horse, thereby prohibiting its uptake into the bloodstream and its subsequent use throughout the body. As calcium levels in the blood drop, hormones initiate resorption of calcium from the skeleton. This reallocation of calcium allows the muscular and nervous systems to function unhampered but only at the considerable expense of the skeletal system.

Ingestion of these plants is typically not an issue unless they represent a large fraction of available forage or if oxalate-laden plants are more palatable than other species in the pasture landscape. The concentration of oxalates in plants often rests on the climatic conditions and soil chemistry. Oxalates become more toxic as plants mature. In summer pastures, oxalate concentration may rise as calcium levels subside.

Oxalate poisoning can occur acutely, such as with the rapid introduction to and ingestion of oxalate-ridden plants, or slowly, such as with the continual grazing of oxalate-accumulating roughages.

Depending on whether ingestion is acute or cumulative, clinical signs may appear immediately or two to eight months following the onset of grazing pasture grasses high in oxalates. Degree of poisoning also depends on other variables such as the nutritional status of the horse and the amount of calcium in the diet. Typical signs of oxalate poisoning are labored breathing, staggering, recumbency, depression, gastroenteritis, and diarrhea. Severe colic may occur in cases of halogeton toxicity. Postmortem examination usually reveals calcium oxalate crystals in the kidneys and numerous other tissues and organs.

The safety of a diet known to contain oxalates depends on the calcium to oxalate ratio. A diet that has a ratio of 0.5:1 or greater is generally regarded as safe. Kikuyu, buffel, pangola, and green panic grasses, for example, have been reported to have calcium to oxalate ratios of 0.23:1, 0.22:1. 0.37:1, and 0.32:1, respectively, indicating all are potentially dangerous. A second way of determining safety of certain grasses is by calculating percentage of oxalate in the dry matter of a diet. Total oxalate intake should not exceed 0.5% of dry matter. On a dry matter basis, for instance, halogeton can contain 34% soluble oxalates, and greasewood can be composed of 10-20% soluble oxalates, both of which far exceed recommended levels.

Oxalates are most detrimental to weanlings, yearlings, and lactating mares because of the substantial calcium requirements of these horses. Furthermore, these horses usually need considerably more feed than their adult sedentary counterparts, so the quantity of oxalates ingested will likely be greater. Proper skeletal growth of young horses is impossible in the face of calcium deficiency. Similarly, lactating mares fed oxalate-rich forages will produce calcium-deficient milk for their foals, which may contribute to developmental orthopedic problems.

Fortification of feeds is of little use if horses are concomitantly consuming substances that inhibit absorption of vitamins and minerals. Therefore, every attempt should be made to reduce oxalate-containing plants in pastureland. A reputable agronomist should be called to identify pasture grasses, and a ration evaluation by a qualified equine veterinarian or nutritionist should be conducted if consumption of plants containing oxalates cannot be avoided.