Nitrate poisoning in cattle is caused by the consumption of an excessive amount of nitrate or nitrite from grazing crops, hay, silage, weeds, drinking water, lubricating oil, fertilizer, etc.  In addition, drought is also a major cause of nitrate poisoning.

All plants contain some nitrate, but excessive amounts are likely to occur in forages, which have been grown under conditions of excessive fertilization and/or stress.  The buildup of nitrates in soil brought on by excessive fertilization with poultry litter or animal manure is a common cause of nitrate accumulation in plants.  Commercial fertilizers aren’t likely to cause excessive nitrate accumulation in plants when recommended application rates and practices are followed.  However, plant species and adverse environmental conditions before harvest affect the concentration of nitrates even more than available nitrogen in the soil.  Direct ingestion of fertilizers that contain nitrates can be toxic to livestock.  Any stress condition which causes an abrupt decrease in plant growth may contribute to plant nitrate accumulation, even with a normal nitrogen supply.

Nitrate toxicity in cattle is primarily a problem associated with some annuals, certain perennial weeds and some cool-season grasses (fescue, bromegrass, orchardgrass, small grains and ryegrass). Warm-season grasses, generally, cause fewer problems. Corn and sorghum have occasionally caused problems. Sorghum-sudan hybrids have probably caused more problems than any other summer annual forage.

Accumulators of nitrates, ranked from highest to lowest, are as follows:

•Weeds
•Corn
•Sorghums
•Sudangrass
•Cereal grains
•Tame forage grasses

While these plants are the most common cause of nitrate poisoning, any grass or weed beyond the seedling stage with a high stem-to-leaf ratio is more likely to be a problem than a less stemmy forage.

Most forages contain some nitrate. Nitrate is not particularly toxic to cattle. When feeds containing nitrate are eaten by ruminants, the nitrate is normally broken down to ammonia and converted by bacteria into microbial protein. Nitrite, one of the intermediate products involved in this process, is the cause of “nitrate poisoning.” Some of the nitrite is absorbed into the animal’s bloodstream where the nitrite is capable of changing the normal oxygen-transporting substance, hemoglobin, into methemoglobin, a substance that cannot carry oxygen from the lungs to the tissues.

Nitrates also cause dilation of blood vessels, and further lack of oxygen is caused by peripheral circulatory failure. Lack of oxygen to the tissues may be the cause of abortions, which sometimes occur following nitrate poisoning. Abortions due to nitrate should be accompanied or preceded by some evidence of nitrate problems in the adult animal, including death or bluish discoloration of unpigmented areas of the skin or mucous membranes.

The most likely signs of nitrate poisoning are difficult and painful breathing, cyanotic membranes, rapid breathing, muscle tremors, weakness, low tolerance to exercise, incoordination, diarrhea, frequent urination, dark- to chocolate-colored blood and collapse. Milk production may also be reduced.

Nitrate poisoning may cause death within one-half hour to four hours after symptoms appear. Symp-toms usually appear when methemoglobin reaches 30 to 40 percent, and death occurs when methemoglobin reaches 80 to 90 percent.

Another condition can occur that complicates the diagnosis of either nitrate or urea poisoning. Excess ammonia in the rumen, which occurs in urea toxicity and may occur in nitrate toxicity, may prevent the absorption of magnesium and cause grass tetany. A complex chemical salt is known to form at about pH 6.2 to 6.4 when magnesium is present with ammonia and phosphorus. This insoluble salt apparently cannot pass through the rumen and intestinal walls into the bloodstream. When this condition occurs, the animal suffers from magnesium deficiency even though the ration contains an adequate level of magnesium.

Treatment and Prevention of Nitrate Toxicity

An injection of 1 percent solution of methylene blue (4 mg per pound of body weight) into the bloodstream is the preferred treatment to aid in the reduction of methemoglobin to hemoglobin. This dose may be repeated in 20 to 30 minutes if the initial response is not satisfactory.

Recommendations for preventing nitrate toxicity are as follows:

• Follow recommendations for nitrogen fertilization, and be careful not to exceed 4 tons of poultry litter yearly per acre on cool-season grasses. The risk will be minimized by spreading litter uniformly and limiting application to 2 tons per acre per application.
• When a crop is grown under conditions that cause nitrate accumulation, delay harvest of the crop until conditions improve to permit nitrate content to drop to a safe level.
• Consider making silage of drought-damaged forage. The ensiling process reduces the nitrate level 40 to 60 percent.
• If high levels of nitrate have accumulated in plants, raise the cutter bar and leave more stem, the portion of the plant with the highest concentration of nitrate, in the field.
• Have suspected forage tested before feeding to cattle.
• Dilute toxic forage by mixing it with nontoxic forages and/or energy feeds such as molasses or corn. Use forage nitrate analysis to determine dilution rates. Energy feeds, such as shelled corn, when fed daily at a minimum of 2 pounds per head, will offset production losses as long as the average forage NO3-N concentration does not exceed 1,500 ppm.
• Feed a nutritionally balanced ration. Iodized salt and vitamin A or green feed supplementation lessen the toxicity of nitrates.
• Adapt cattle slowly to elevated levels of nitrate. Don’t give hungry animals a full feed. Never exceed maximum recommended levels of nitrate intake.
• Feed suspect forage in small amounts several times a day rather than all at one feeding.
• If forages are high in nitrates, ask your county Extension agent about an analysis of the drinking water.
• Be aware that forage regrowth and volunteer plants are highly suspect following nitrate fertilization and drought.
• Observe animals closely for signs of toxicity, and call a veterinarian immediately if symptoms are observed.

For more information check out this great video from the University of Tennessee Agricultural Extension…

And these publications…

Nitrate Poisoning in Cattle, Dr. Shane Gadberry, Extension Livestock, March 20th, 2009

Nitrate Poisoning in Cattle, University of Arkansas Extension Publication, FSA 3024