Oxalates are naturally occurring compounds that bind calcium in the equine diet, preventing absorption of this mineral from the horse’s gut. When horses consume plants with high levels of these compounds, oxalate toxicity can occur.

Horses with oxalate toxicity show signs of calcium deficiency, including bone weakness, enlarged facial bones (big head disease), muscle tremors, and incoordination. Horses with higher calcium requirements, such as pregnant, lactating or growing horses, are more sensitive to oxalates in the diet.

Fortunately, most forages that are fed to horses in North America contain low levels of oxalates. However, tropical grasses such as pangola grass, seteria and buffel grass can accumulate hazardous levels of oxalates, putting equines grazing on these grasses at risk of toxicity.

If tropical hay or pasture makes up the bulk of your horse’s diet, submit a forage sample for analysis to measure the level of oxalates. Horses consuming high-oxalate forages require supplemental calcium to help prevent calcium deficiency.

Work with an equine nutritionist to ensure calcium supplementation is appropriately balanced with phosphorus to maintain healthy mineral ratios.

Oxalates in the Equine Diet

Oxalates are natural compounds found in some plants that support plant survival by forming protective structures like needles to deter predators. [17] Most plants can synthesize oxalates, but concentrations vary greatly between plant species.

When ingested by animals such as horses, oxalates from plants are released during digestion and bind to free minerals in the digestive tract, particularly calcium, magnesium, and potassium.

Once bound, these minerals are not available for uptake and utilization by the animal, leading to their excretion. If large quantities of oxalates are consumed, or if they are consumed consistently over a long period, the horse may develop nutritional deficiencies, particularly calcium deficiency.

In horses, calcium plays a crucial role in bone strength, muscle activity, and metabolic function. In severe cases, calcium deficiencies can have long term health effects on the horse.

Equine diets containing less than 0.5% oxalates are considered safe and unlikely to contribute to calcium deficiencies. [7] However, diets with more than 0.5% oxalates are at risk of blocking calcium absorption and causing a deficiency.

In one study, ponies receiving a diet of 1% oxalates had 60% less calcium available for absorption, and diets with 6% oxalates completely blocked calcium absorption. [18]

High-Oxalate Forages

Some forages grazed by horses, particularly from tropical plant species in South America, Africa, and Australia, can contain over 6% oxalates. Such high oxalate levels pose a risk to horses, especially if these forages form a significant part of their diet.

While North American grasses and legumes typically do not have high levels of oxalates, the following subtropical and tropical forages have moderate levels: [3]

  • Buffel grass (Cenchrus ciliaris)
  • Pangola grass (Digitaria eriantha)
  • Setaria (Setaria sphacelata)
  • Kikuyugrass (Pennisetum clandestinum)
  • Guinea Grass (Panicum Maximum, Green Panic Grass)
  • Para Grass (Urochloa mutica)
  • Signal Grass (Urochloa decumbens)
  • Napiergrass (Pennisetum purpureum)

These oxalate-accumulating plants have concentrations ranging between 1-5%. [3] Levels of oxalates vary within the plant anatomy, with leaves typically containing higher amounts than stems. [8][9]

Many other plants, including spinach, rhubarb, tea, and black pepper, are known to have high-oxalate concentrations (between 3 – 15%), but are not usually consumed by horses. [6][7] Several common pasture weeds, including lambsquarter, curled dock, and the redroot pigweed, are also high in oxalates. [3]

While Teff hay is sometimes cited as high in oxalates, there is no research showing that Teff grass cultivated in North America has high oxalate levels.

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Calcium Deficiency

Diets with a high proportion of oxalates are a concern for horses because they can contribute to a secondary calcium deficiency. A secondary nutritional deficiency occurs when an animal gets enough of a specific nutrient in their diet, but the absorption or availability of that nutrient is impaired, leading to signs of deficiency.

In horses with oxalate poisoning, there may be enough calcium in the horse’s diet, but oxalates block the absorption and use of that calcium. This can result in weakened bones, increased susceptibility to fractures, and other adverse effects.

Severe calcium deficiency associated with acute oxalate toxicity in horses can also result in permanent kidney damage, which can become life-threatening.

Types of Oxalates

To understand how oxalates in the equine diet can contribute to calcium deficiency, it’s important to differentiate between the types of oxalates found in plants and forage.

Oxalates can exist in two forms within plants: soluble or insoluble. In grasses, oxalates are predominantly found in the soluble form. [3]

Soluble Oxalates

Soluble oxalates are compounds that dissolve in digestive fluid, making them easily absorbed by the horse’s body. Soluble oxalates include:

  • Sodium oxalates
  • Potassium oxalates
  • Ammonium oxalates

Soluble oxalates can bind calcium in the digestive tract or within the blood, forming complexes that are not easily utilized by the body. This reduces the amount of free calcium available for the horse. [1][3]

  • Binding Calcium in the Gut: Soluble oxalates from forage dissolve in the horse’s digestive fluid, binding to free calcium in the digestive tract. Once soluble oxalates bind calcium, they become insoluble and are no longer available for absorption by the digestive tract.
  • Binding Calcium in the Blood: If the horse has low levels of calcium in their diet, there will be limited calcium in the digestive tract for free oxalate to bind to. The unbound oxalate is absorbed into the horse’s bloodstream where it can bind to circulating calcium ions. This can lead to a rapid drop in blood calcium levels (hypocalcemia).

Insoluble Oxalates

Insoluble oxalates are compounds that do not dissolve easily, remaining undigested as they pass through the horse’s gastrointestinal tract. Insoluble oxalates include: [1][2]

  • Calcium oxalates
  • Magnesium oxalates
  • Iron oxalates

Recent research shown that microbes in the horse’s hindgut can break down insoluble oxalates, releasing bound calcium. However, since calcium is only absorbed in the horse’s small intestine, free calcium in the hindgut cannot be absorbed and remains unavailable to the horse. [2]

Symptoms of Oxalate Poisoning

Oxalate poisoning in horses can manifest in two forms: chronic and acute, each characterized by different onset times, symptoms, and levels of severity.

Chronic Toxicity

Chronic oxalate toxicity occurs after ongoing, consistent intake of low concentrations of oxalates from the diet. Symptoms of chronic oxalate poisoning may take months to appear. [10]

Chronic oxalate toxicity in horses is characterized by high levels of parathyroid hormone (PTH), which is involved in regulating calcium levels in in the blood and bones. In affected horses, PTH levels are persistently elevated as the body struggles to compensate for low calcium levels in the blood.

High levels of PTH triggers the release of calcium from bone tissue, which can lead to softening of the bones over time. Since a horse’s bones provide a large reserve of calcium, horses can generally consume a high-oxalate diet for months without showing clinical signs of calcium deficiency.

Acute Toxicity

Acute oxalate toxicity occurs rapidly when a horse suddenly ingests a large amount of soluble oxalates in a short period of time.

This leads to an immediate disruption in calcium absorption from the diet, causing a rapid drop in blood calcium levels. Consequently, horses develop signs of hypocalcemia (low blood calcium).

Symptoms of hypocalcemia in horses include:

  • Muscle twitching
  • Tremors
  • Staggering and collapsing
  • Labored breathing
  • Thumps” or synchronous diaphragmatic flutter
  • Neurological symptoms, such as anxiety and difficulty swallowing

If you notice any of these signs in your horse, contact your veterinarian immediately for diagnosis and treatment.

Kidney Failure

Both chronic and acute oxalate poisoning can also lead to renal (kidney) failure. Symptoms of kidney failure include:

  • Weight loss
  • Ventral edema, or fluid accumulation between the front legs and along the belly
  • Changes in drinking behavior, such as increased drinking
  • Changes in urination behavior, such as increased urination, decreased urination, or urinating small volumes

Ingestion of high levels of oxalates results in the formation of calcium oxalate crystals in the horse’s bloodstream, which are then excreted in the urine.

However, these crystals often build up in the kidneys, resulting in kidney stones and renal damage. When kidney function is compromised past a certain point, renal failure can occur. [18]

Nutritional Secondary Hyperparathyroidism

When horses experience high levels of parathyroid hormone due to low calcium absorption, they may be diagnosed with Nutritional Secondary Hyperparathyroidism (NSH), or Big Head Disease. This condition can develop in horses after consuming high-oxalate forages for two months or more. [10][11]

Big Head Disease is characterized by excessive loss of calcium from bone, resulting in fibrous tissue attaching to the bone instead. This process alters the structure of the facial bones, giving them the distinctively misshapen appearance typical of horses with NSH.

The primary signs of NSH in horses include: [11]

  • Spontaneous bone fractures
  • Lethargy
  • Stiffness and lameness
  • Diarrhea
  • Enlarged or misshapen facial bones and head
  • Teeth grinding and excess chewing
  • Laboured breathing

Diagnosis and Treatment

Diagnosis and treatment for oxalate poisoning in horses varies depending on whether the condition is chronic or acute.

Acute Oxalate Toxicity

Acute oxalate toxicity can be tentatively diagnosed from a combination of factors, such as the presence of high-oxalate forages in the diet, clinical signs of hypocalcemia (low calcium) and a positive response to treatment.

Definitive diagnosis is possible using a blood test to measure calcium ions. However, this laboratory test can take several days to return results. Instead of waiting for these results, horses are usually treated based on showing signs of hypocalcemia.

Even in cases of acute oxalate toxicity, a horse’s calcium levels might appear normal on a blood test if there’s adequate mobilization of calcium from the bones to sustain normal ranges. Despite this, bloodwork can be an important step to rule out other disorders and guide treatment. [1]

Treatment

Treatment for acute oxalate toxicity is focused on resolving hypocalcemia by restoring blood calcium levels to ideal range. The treatment for low blood calcium levels is an intravenous calcium infusion delivered directly into the horse’s bloodstream. [18]

In some horses, intravenous administration of calcium stops muscle tremors and twitches immediately. Others may require this infusion along with oral calcium given directly into the stomach through a nasogastric tube. [18] 

Note: nasogastric intubation should only be performed by a veterinarian. There is no safe way to intubate a horse at home.

Chronic Toxicity

Chronic oxalate toxicity, characterized by NSH, is diagnosed with a combination of physical examination, dietary analysis and observation of symptoms. [12] Bloodwork measuring parathyroid hormone levels provides a definitive diagnosis. [13]

Treatment

Treatment for NSH linked to chronic oxalate toxicity includes the removal of any high-oxalate forages, as well as ensuring there are no high-oxalate plants in the horse’s pasture.

Calcium supplementation is required to help restore levels of this mineral in bone. Supplemental magnesium along with vitamin D can also be beneficial for supporting normal calcium metabolism. [12]

Feeding Guidelines

The only way to determine oxalate levels in your horse’s hay or pasture is with forage testing from a laboratory. If your horse consumes tropical grasses that can accumulate high oxalate levels, conduct a forage analysis and work with an equine nutritionist to make sure your horse’s diet is safe.

Follow these guidelines to keep your horses safe from oxalate toxicity: [14]

  • Soluble Oxalates: Ensure that the total diet comprises less than 0.5% soluble oxalates, calculated on a dry-matter basis
  • Calcium-to-Oxalate Ratio: Plants with high oxalate content can be included in the diet as long as they maintain a minimum calcium-to-oxalate ratio of 0.5:1

If your horses graze high-oxalate forages, work with an equine nutritionist to formulate a balanced diet, especially if you have pregnant, lactating or growing horses, who are more sensitive to oxalates in the diet.

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Balancing a High-Oxalate Diet

High-oxalate forages should not be the primary forage source for a horse. However, this is sometimes unavoidable if there are limited forage options available.

When feeding high-oxalate forages is unavoidable, dietary strategies can help mitigate their effects. Strategies include diluting with other forages or feeding your horse supplemental calcium.

Dilution

To maintain the overall oxalate content of your horse’s diet below 0.5%, dilute your horse’s forage with a lower-oxalate hay. Alfalfa is commonly used to balance diets high in oxalates because it contains high calcium and low oxalate levels.

For example, feeding your horse solely Seteria grass with a 1.0% oxalate concentration risks oxalate toxicity. However, if this grass constitutes only 30% of your horse’s total dry matter intake and you provide the remaining 70% as a low-oxalate forage, the overall diet will remain within the safe limit of 0.5% oxalates.

Calcium and Phosphorous

Another strategy to offset high-oxalate consumption is increasing the amount of calcium fed to your horse. Common calcium supplements used in equine diets include limestone and dicalcium phosphate.

In a study of horses with chronic oxalate poisoning, normal blood calcium levels were successfully restored following supplementation. [15] Supplemental calcium has also been shown to prevent oxalate poisoning in horses introduced to high-oxalate grass. [15]

To balance a diet with high oxalate levels, increase the rate of calcium supplementation to achieve a ratio of at least 0.5 grams of calcium per gram of oxalates. For example, a 500 kg (1100 lb) horse eating forage with a 1% oxalate concentration will consuming roughly 10 grams of oxalates per day. To balance this amount, the horse should be fed 5 grams of supplemental calcium per day.

Oxalates can also contribute to phosphorous deficiency in horses, so supplemental phosphorous should be provided alongside calcium. [1] Ensure calcium and phosphorous are provided in balanced amounts to prevent side effects, including NSH. A calcium to phosphorus ratio between 1.5:1 to 4:1 is considered ideal.

Work with an equine nutritionist to determine the correct amount of calcium and phosphorus to feed to your horse. A 500 kg (1100 lb) horse can consume up to 100 grams of calcium carbonate (limestone) or 200 grams of dicalcium phosphate per day.

To maximize the absorption of calcium and phosphorus from the diet, it is more effective to increase the frequency of supplementation than to increase the dose. [12]

Mad Barn’s Omneity is a comprehensive vitamin and mineral supplement that provides 11 grams of calcium and 6 grams of phosphorus per serving. One serving of Omneity can balance forages with up to 2% oxalates for a typical horse.

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Soak Forages

Soaking forages and feed prior to feeding your horse may also help reduce oxalate levels. When feeds are soaked, soluble oxalates dissolve in the water and are removed from the feed. [16]

For example, soaking raw soybeans, peas, and other beans has been shown to decrease soluble oxalate content by up to 56%. [16] While soaking forages can decrease their oxalate content, this process may also deplete other minerals, potentially leading to nutritional imbalances that need to be corrected with supplementation.

Pasture Management

Research shows that proportions of soluble and insoluble oxalates in plants is influenced by levels of other nutrients. Plants with high potassium levels tend to have more soluble oxalates while plants with high calcium tend to have more insoluble oxalates. [4]

It’s difficult to modify the levels of insoluble oxalates in plants, but amounts of soluble oxalates can be modified through pasture management practices such as fertilization. Studies indicate that using too much potassium fertilizer together with nitrogen fertilizer can increase oxalate levels in forage. [5]

While more research is required, reducing the use of potassium fertilizers may help limit soluble oxalate concentration in forages.

Summary

Oxalates are naturally occurring compounds found in some types of plants that can reduce the available calcium in a horse’s diet.

  • Oxalates can contribute to calcium deficiency in horses, resulting in serious health conditions such as nutritional secondary parathyroidism (Big Head Disease) or acute hypocalcemia
  • High oxalate plants are rare in North America; however, they are more common in tropical areas
  • Forages that are known to be high in oxalates, like buffel grass, should be tested before they are fed to horses
  • Equine diets should not contain more than 0.5% oxalates and should not exceed a ratio of 0.5:1 oxalates to calcium; this can be achieved by diluting high-oxalate forages or adding supplemental calcium to the diet

Always consult with an equine nutritionist or veterinarian to ensure the best care for your horse. You can submit your horse’s diet online for a free consultation with our qualified nutritionists.

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References

  1. McKenzie. R. A. et al., The Effect of Dietary Oxalate on Calcium, Phosphorus and Magnesium Balances in Horses. J Agric Sci. 1981.
  2. Allison. M. J. and Cook. H. M., Oxalate Degradation by Microbes of the Large Bowel of Herbivores: The Effect of Dietary Oxalate. Science. 1981.
  3. Rahman. M. M. and Kawamura. O., Oxalate Accumulation in Forage Plants: Some Agronomic, Climatic and Genetic Aspects. Asian Australas J Anim Sci. 2011.
  4. Rahman. M. M. et al., Effects of Levels of Nitrogen Fertilizer on Oxalate and Some Mineral Contents in Napiergrass (Pennisetum Purpureum Schumach). Grassl Science. 2008.
  5. Jones. R. and Ford. C., Some Factors Affecting the Oxalate Content of the Tropical Grass Setaria Sphacelata. Aust J Exp Agric. 1972.
  6. Mou. B., Evaluation of Oxalate Concentration in the U.S. Spinach Germplasm Collection. HortScience. 2008.
  7. Goyal. M., Oxalate Accumulating in Fodder Crops and Impact on Grazing Animals – A Review. Forage Res. 2018.
  8. Rahman. M. M. et al., Short Communication – Oxalate and Silica Contents of Seven Varieties of Napier Grass (Pennisetum Purpureum). S Afr J Anim. 2020.
  9. Rahman. M. M. et al., Changes in Oxalate and Some Mineral Concentrations of Setaria Sphacelata under Cutting and Uncutting Conditions. Pak J Biol Sci. 2014.
  10. Walthall. J. C. and McKenzie. R. A., Osteodystrophia Fibrosa in Horses at Pasture in Queensland: Field and Laboratory Observations. Aust Vet J. 1976. View Summary
  11. Hebert. E. W. and Dittmer. K. E., Acute and Chronic Oxalate Toxicity in Miniature Horses Associated with Soursob (Oxalis Pes‐Caprae) Ingestion. Equine Vet Educ. 2016.
  12. Stewart. J. et al., Bighead in Horses – Not an Ancient Disease. Aust Equine Vet. 2010.
  13. Lacitignola. L. et al., Nutritional Secondary Hyperparathyroidism in Two Ponies. Open Vet J. 2018.
  14. McKenzie. R. A. and Schultz. K., Confirmation of the Presence of Calcium Oxalate Crystals in Some Tropical Grasses. J Agric Sci. 1983.
  15. McKenzie. R. A. et al., Control of Nutritional Secondary Hyperparathyroidism in Grazing Horses with Calcium Plus Phosphorus Supplementation. Aust Vet J. 1981. View Summary
  16. Shi. L. et al., Changes in Levels of Phytic Acid, Lectins and Oxalates during Soaking and Cooking of Canadian Pulses. Food Res Int. 2018.
  17. Franceschi, V.R. and Nakata, P.A. Calcium Oxalate in Plants: Formation and Function. Annu Rev Plant Biol. 2005
  18. Herbert, E.W. and Dittmer, K.E. Acute and chronic oxalate toxicity in Miniature Horses associated with soursob (Oxalis pes-caprae) ingestion. Equine Vet Ed. 2016.