Forage Facts
The potential for high nitrate concentrations in crops such as corn, sorghum, canola, cereal grains, and some grasses occurs after exposure to drought, hail, frost, cloudy weather, or soil fertility imbalance. Nitrates accumulate in the lower portion of these plants when stresses reduce crop yields to less than those expected, based on the supplied nitrogen fertility level. Feeding harvested forages or grazing plants that are high in nitrates can be toxic to livestock because the metabolism products from nitrates interfere with the ability of blood to carry oxygen causing asphyxiation.
Why Nitrates are Toxic
Nitrate toxicity is a misnomer because nitrite (NO2), not nitrate (NO3) is poisonous to animals. After a plant is eaten, rumen bacteria rapidly reduce nitrates in the forage to nitrites. Normally, the nitrites are converted to ammonia and used by rumen micro-organisms as a nitrogen source. But, if nitrite intake is faster than its breakdown to ammonia, nitrites will begin to accumulate in the rumen. Nitrite is rapidly absorbed into the blood system where it converts hemoglobin to methemoglobin. Red blood cells containing methemoglobin cannot transport oxygen, and the animal dies fro asphyxiation.
Animals under physiological stress (sick, hungry, lactating or pregnant) are more susceptible to nitrate toxicity than healthy animals. Toxicity is related to the total amount of forage consumed and how quickly it is eaten, but generally if forages contain more than 6,000 ppm nitrate, they should be considered potentially toxic.
Level of forage nitrate (dry matter basis) and potential effect on animals.
| ppm Nitrate (NO3) | Effect on Animals |
| 0-3,000 | Virtually safe. |
| 3,000-6,000 | Moderately safe in most situations; limit use for stressed animals to 50% of the total ration. |
| 6,000-9,000 | Potentially toxic to cattle depending on the situation; should not be the only source of feed. |
| 9,000 & above | Dangerous to cattle and often will cause death. |
Symptoms
Symptoms of nitrate toxicity may appear within a few hours after eating or not for several days. Signs a few hours after eating or not for several days. Signs of toxicity include reduced appetite, weight loss, diarrhea, and runny eyes. However, these are nonspecific symptoms of numerous disorders and are not a reliable diagnosis of nitrate poisoning. Lower nitrate levels can cause abortion without any other noticeable symptoms.
Acute toxicity usually is not apparent until methemoglobin approaches lethal concentrations. Symptoms include cyanosis (bluish color of mucus membranes), labored breathing, muscular tremors, and eventual collapse. Coma and death usually follow within two to three hours. Postmortem confirmation of nitrate toxicity is chocolate-colored blood; however, the color will change to dark red within a few hours after death.
A veterinarian should perform the diagnosis and treatment of nitrate toxicity. In acute cases where time is limited, an antidote of methylene blue can be injected to convert the methemoglobin back to hemoglobin.
Plant Factors
Nearly all plants contain nitrate, but some species are more prone to accumulate nitrate than others. Crops such as forage and grain sorghum, sudangrass, hybrid sorghum-sudan, and pearl millet are notorious nitrate accumulators. Canola and other brassica species (e.g. radishes, turnips) take up nitrogen effectively and can accumulate dangerous concentrations of nitrates. Weed species such as kochia, lambs-quarters, sunflower, pigweed, and Johnsongrass also are often high in nitrate. Under certain environmental and managerial conditions, corn and cereal grains like wheat and oats, and other plants can accumulate potentially toxic levels of nitrate. Under extreme stress, legumes such as alfalfa and soybeans also can accumulate nitrate.
Nitrate content generally is highest in young plant growth and decreases with maturity. Sorghums and sudangrasses, however, are exceptions because concentrations can remain high in mature plants. If plants are stressed at any stage of growth, they can accumulate nitrate.
Highest nitrate levels occur in the lower one-third of the plant stalk. Nitrate concentrations tend to be low in leaves because nitrate reductase enzyme levels are high there. Grain does not contain appreciable amounts of nitrate. In canola, nitrate concentrations can be 10 times greater in petioles than in leaves.
Guidelines to Reduce Nitrate Toxicity
- Pay close attention to potentially troublesome plant, such as sorghum, sudangrass, other summer annuals, and brassica species, which often have high nitrate levels.
- Avoid excessive application of manure or nitrogen fertilizer.
- When harvesting high-nitrate forages, raise the cutter bar 6 to 12 inches to exclude basal stalks. This will minimize harvesting many weed species that have accumulated nitrate from shading.
- Delay harvesting any stressed forages after conditions improve. Two weeks of favorable weather generally are required for plants to reduce accumulated nitrate.
- Never feed green chop that has heated after cutting or been held overnight.
- Harvest plants containing high levels of nitrate as silage rather than as hay.
- Have representative samples of suspect forage analyzed before feeding.
Suspected Forage
Forage suspected to contain high nitrate levels should be tested by a laboratory before feeding to livestock. Unfortunately, different laboratories may report nitrate level as nitrate (NO3), nitrate-nitrogen (NO3-N), or potassium nitrate (KNO3). Potassium nitrate, nitrate nitrogen, or percent nitrate can be converted to ppm nitrate using the following conversion factors:
Potassium Nitrate x 0.61 = Nitrate (ppm NO3)
Nitrate-Nitrogen x 4.42 = Nitrate (ppm NO3)
% Nitrate x 10,000 = Nitrate (ppm NO3)
Feeding High-Nitrate Forages
Before feeding or grazing potentially troublesome plants such as sorghum, sudangrass, or canola, have the forage analyzed for nitrates. Environmental conditions in Kansas create high nitrate concentrations in some forages virtually every year. In situations where this is a possibility, nitrate analysis is necessary to determine if the feed is potentially toxic. High-nitrate forages can be fed to animals if proper precautions are taken.
Publication
Nitrate Toxicity