Q: Is nitrate poisoning a concern with horses?
A: If nitrite is absorbed in blood in sufficient quantities it may convert hemoglobin to methemoglobin. Methemoglobin does not release oxygen to tissues, and can interfere with the animal's ability to use oxygen. Nitrates are normally found in forages, however, some forages can accumulate large concentrations of nitrates during dry or drought conditions. Ruminate animals like cattle and sheep are reported to be about 10 times more susceptible to nitrate poisoning than horses because their rumen converts nitrate to nitrite. The same reaction may take place in the cecum (hindgut) of horses to a lesser extent. Thus horses are generally tolerant of higher concentrations of nitrate in forage than cattle are.
The symptoms of nitrate poisoning in horses include: difficulty breathing, bluish-colored mucous membranes, weakness, tremors, and possibly death. Research has shown that feeding hay containing 1.5 to 2% nitrate to pregnant and non-pregnant mares resulted in clinically normal animals, even though higher than normal levels of nitrate were detected in blood samples.
As a general rule, most horses should not be fed hay containing more than 1.5% nitrate. DHIA (320-352-2028), Dairyland (320-240-1737) and the Minnesota Veterinary Diagnostic laboratory (612-625-8787) can test hay for nitrate concentrations. If forage contains between 1.5 to 2% nitrate, it should not be fed without diluting the forage with other feedstuffs (i.e., forages/grains lower in nitrate). If the forage is over 2% nitrate, it should not be fed at all. Nitrate exposure has also been associated with goiter (or hypothyroidism) because of the potential for nitrate to interfere with iodine. Offering iodized salt is the most practical prevention for goiter.
By Mike Murphy, DVM, University of Minnesota