Nitrates in Dairy Forages

Early frosts raise the concern of nitrate accumulation in forages. High nitrate levels are particularly toxic to cattle, less so for horses.

What Are Nitrates?

Nitrates are one of the raw materials taken up by the plant to be used for protein synthesis. The nitrates are converted to nitrites and then to ammonia which is the actual end product used by the plant. Under certain conditions (eg. frost, drought, hail) plant metabolism does not proceed normally and nitrates accumulate in the leaves and stem of the plant. The plants most at risk are annuals such as barley, oats and millet, grasses and weeds. Legumes are not known to accumulate significant levels of nitrate.

Why Are Nitrates Harmful to Cattle?

Under normal circumstances, nitrates from the feed are converted, in the rumen, to nitrites and then to ammonia. The ammonia is either excreted or used for other purposes in the body. When feeds high in nitrates are consumed by cattle, these normal metabolic pathways are overwhelmed and the very toxic nitrites accumulate in the bloodstream. Nitrites bind the hemoglobin and reduce the blood's oxygen carrying capacity.

What Are the Symptoms of Nitrate Poisoning?

Clinical signs of nitrate poisoning are related to lack of oxygen in the blood. Acute poisoning usually occurs within as little as four hours after consuming toxic levels of nitrate. Gasping, rapid respiration is the predominant sign, along with muscle tremors, rapid weak pulse, weakness, with the affected animals going down and into terminal convulsions. Pregnant females which survive nitrate poisoning may abort a few days after exposure due to lack of oxygen to the fetus. Post-mortem examinations reveals dark, chocolate-colored blood. Low daily intake of nitrate apparently causes no evidence of disease. There is usually little or no opportunity for treatment of severely affected animals. However, if the affected animals are found, methylene blue is an effective treatment.

Why Do Nitrates Accumulate in Forages?

Forage Type

Nitrates accumulate in some forages and weeds more than they do in others. Cereal crops, grasses and weeds are more likely to accumulate nitrates than legumes.

Level of Fertilization

High levels of nitrates in the soil can occur as a result of heavy nitrogen fertilization and manure application.

Stage of Forage Development

Early cut forages commonly contain higher nitrate levels than the same forage when almost mature.

Light Intensity and Temperature

Poor light intensity (shade) and high temperatures lead to high nitrate levels in plants. Nitrates accumulate during the night and dissipate rapidly on bright sunny days with moderate temperatures.

Frost Damage

Nitrate levels are highest just before sunrise which is when frosts are most likely to occur. Any surviving leaves would be shaded by the frost damaged leaves above limiting the potential for nitrate reduction.

Hail Damage

Immediately after a hailstorm, nitrate content can be expected to be quite low, because hailstorms usually occur after a hot, bright, sunny day. Since the leaves are usually the first to be destroyed, nitrates may accumulate over time.

Will Nitrates Decrease After Harvesting?

Nitrates are soluble in water. Considerable amounts of rain, while the hay is in the windrow, will leach out nitrates along with important nutrients. Hay dried quickly will lose very little nitrate. Ensiling will reduce the amount of nitrates to varying degrees.

How Should High-Nitrate Forages Be Fed?

Animals should not be allowed to consume feed containing more than 0.8% potassium nitrate (100% dry matter basis). Feeds containing more than this amount should be physically mixed with "nitrate-free" feeds to ensure the total nitrate level is below 0.8%. Samples may be submitted to feed testing laboratories for a nitrate analysis. These results must be viewed with caution,however, as there can be tremendous variation in nitrate content among plants taken from the same field. Feeding adequate levels of vitamin A and energy will help to reduce the potential for nitrate toxicity.

For further information contact:

Karen Dupchak
Farm Production Extension, Animal Nutritionist
Manitoba Agriculture Food, and Rural Initiatives
204-545 University Crescent
Winnipeg, MB R3T 5S6
Phone: 204-945-7668
Fax: 204-945-4327