Nitrates and Feeding Problems for Horses

By Dr. Matry Adams, PAS, Equine Nutritionist for Southern States

Plants accumulate nitrates as a natural process. Nitrogen is absorbed through the roots of the plant and then stored as nitrate in the stems. This nitrate is later converted into protein in the plant’s leaves. Excessive amounts of nitrate can be accumulated in grass pasture or hay as a result of excessive fertilizer or manure application. High nitrate levels can also be caused by prolonged drought or cloudy weather conditions. Nitrate and nitrite can also be present in water from wells and ponds. The only sure way to determine toxic nitrate or nitrite levels is to send in a sample of hay or water and have a chemical analysis performed. The upper safe limit in water for all livestock is 100 mg/liter for nitrate nitrogen and 10 mg/liter for nitrite-nitrogen.

Nitrates interfere with an animal’s ability to carry oxygen in the blood. Nitrates are converted to nitrites in the animal’s body and forms methemoglobin, reducing the amount of oxygen available for the animal. A high level of nitrate in a forage can also cause reduced feed consumption, decreased growth rates, lowered milk production, and abortion. In severe cases, death can occur. 

Nitrate poisoning may produce the following symptoms: suffocation, labored breathing, incoordination, and blue or pale mucous membranes. The most spectacular symptom of nitrate toxicity is chocolate brown coloration of the blood. Other symptoms include diarrhea, frequent urination and frothing at the mouth. Horses can tolerate higher levels of nitrates in forages than ruminants (cattle, goats and sheep) because research has shown that horses absorb nitrates in the small intestine before they would travel into the large intestine to be converted into nitrites, which are responsible for toxicity symptoms.  Ruminants convert nitrates into nitrities in the rumen, which are absorbed into the bloodstream and can quickly cause symptoms of nitrate poisoning. A research study showed that non-pregnant mares can tolerate almost 2.0% nitrate in the total dry matter of their diets safely.