Nitrate toxicity can be a serious problem in cattle and other ruminants. They are more prone to toxicity because the microbes in the rumen convert nitrate (NO3) to a more toxic nitrite (NO2) form. However, since horses are hindgut fermenters and do not have the extensive ability to ferment forages like cattle do, the risk is lower.
Plants such as sudan, johnsongrass, sorghum, corn and pigweed can be nitrate accumulators under the right set of environmental conditions. Additionally, excessive amounts of fertilization can increase nitrate concentrations in the plant. Normally, plants take up nitrate from the soil and convert it into protein in the leaves. When the plant becomes stressed by drought, cloudiness, cold weather or even too much moisture, the metabolic process slows down. However, the uptake of nitrates from the soil continues at a higher rate for a brief period of time, thus, building up excessive nitrates in the stem of the plant.
When nitrites are created from nitrates in the animal and then absorbed into the blood stream, they bind with hemoglobin to form methemoglobin, which cannot effectively carry oxygen. Nitrate poisoning may produce the following symptoms: difficulty breathing and suffocation, lack of coordination, muscle weakness or tremors, blue or pale mucous membranes, diarrhea, colic, and even death. Blood of affected animals has a chocolate brown instead of a deep red color and is more watery.
Forages can be tested for nitrate concentration while growing or in hay. Estimating growing forages is problematic because nitrate content can change by the hour or day. Once the forage has been cut for hay, the quantity of nitrate in the plant is locked in since metabolism will cease once the hay becomes cured. But even with hay, there can be nitrate concentration variation due to location in the field and sampling and testing error.
Table 1 lists general guidelines for feeding nitrate-containing forages to various classes of horses. One must also take into account the level of nitrates that may be present in the water and other feeds that will be offered to the horse on a daily basis.
If there is no other option but to feed a high nitrate-containing forage, then it should be diluted with other clean hay and feed to reduce the nitrate levels to acceptable limits. Table 2 (adapted from L.D. Lewis, 1995) demonstrates how to calculate the dilution of the nitrate-containing hay with feed alone. I would suggest further diluting the nitrate-containing hay by feeding at least one-half of the total amount of hay in the table as clean hay, which does not contain nitrates. This will further reduce the nitrate levels in the hay to be safe. Additionally, caution should be exercised when feeding amounts more than 50 percent of the horses' daily dry matter intake.
Horses do have a higher tolerance to nitrates than cattle, but extreme caution should be exercised when feeding it.