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Equine Health Resource

Nitrate Toxicity

Consumption of nitrate fertilizers or an excess of nitrate-concentrating plant species can cause severe or fatal blood disease. It does this through eliminating the oxygen carrying capacity of the red blood cells.

Nitrate toxicity is much more common in ruminants (cows, sheep and goats) than it is in horses. In ruminants, the rumen bacteria efficiently convert less toxic nitrate to very toxic nitrite. Unlike ruminants, horses are hindgut fermenters and convert less of the nitrate to nitrite.

Nitrite is absorbed into the blood, where it converts hemoglobin (the oxygen carrying molecule in red blood cells) to a form that cannot carry oxygen (methemoglobin). This starves the tissues of oxygen, resulting in cell death.

Signs of nitrate toxicity include respiratory distress, weakness, colic, diarrhea, muscle tremors, wobbly gait, high heart rate, brownish discoloration of the gums, seizures and/or sudden death. The blood of animals with nitrate toxicity is brown instead of red.

Laboratories can test plants for nitrate levels, and body tissues and fluids for level of nitrite and methemoglobin.

Treatment requires methylene blue and supportive care. The goal is to revert methemoglobin back to hemoglobin.


Prevent your horses from ingesting nitrates through good stabling and grounds management. Keep fertilizers away from horses.

Ensure water sources are not contaminate by fertilizer runoff. Ensure animals are kept off recently fertilized pastures as instructed by the fertilizer manufacturer.

Learn about common nitrate accumulating plants that grow in your area. Do not feed moldy or wet hay.

Helpful Outside ResourcesCredible Equine Health Information on the Internet

Author: Doug Thal DVM Dipl. ABVP


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