Nitrates in Livestock Feeding

Rick Rasby and Rick Stock, Extension Beef Specialists
Bruce Anderson, Extension Forage Specialist
Norm Schneider, Veterinary Toxicologist

Nitrate poisoning in cattle occurred long before the use of nitrogen fertilizers. In the late 1800s there were reports of cornstalk poisoning in Nebraska, and nitrate poisoning from oat hay in North and South Dakota and from weeds in the high-organic matter soils in Florida and Wisconsin.

Nitrate concentrations in feeds for livestock depends more on plant species and environmental conditions prior to harvest than on the amount of available nitrogen in the soil.

What is Nitrate Toxicity?

Nitrite is one of the intermediate products in the breakdown of nitrate and is the cause of nitrate poisoning. Some nitrite is absorbed into the bloodstream. Nitrite in the bloodstream changes hemoglobin to methemoglobin.

Hemoglobin carries oxygen from the lungs to other tissues but methemoglobin is incapable of carrying oxygen. Thus, nitrates become a problem when enough methemoglobin is produced that the oxygen carrying capacity of blood is reduced to a critical level. If enough methemoglobin is produced (more than 80 percent of the total hemoglobin) the animal will die.

The toxic level depends both on the amount of nitrate in the feed and how fast the feed that contains nitrate is consumed. For example, it takes about twice as much nitrate to kill a ruminant when the nitrate comes from forages that are eaten over a long period of time compared to a supplement or a drench that is consumed quickly.

Symptoms of Nitrate Toxicity

Nitrates in blood also cause blood vessels to dilate and are responsible for peripheral circulatory failure. Lack of oxygen to the fetus probably causes abortions that sometimes occur following nitrate poisoning. Abortion due to nitrate is accompanied or preceded by some evidence of nitrate problems in the adult animal, including chocolate-colored blood and bluish discoloration of unpigmented areas of the skin or mucous membranes.

Reporting Nitrate levels

Table 1. Methods of Reporting Feed Nitrates Concentrations (Dry Basis).

Formulas for Converting Methods of Reporting.

Both nitrate nitrogen (NO 3 N) and nitrate ion (NO 3 - ) will be used to evaluate and discuss nitrate concentrations in feed and water in this NebGuide.

Deaths have been reported when nitrate content in the overall diet contained as little as .21 percent NO 3 N (.93 percent NO 3 - ). Rations containing substantially more than .21 percent NO 3 N have been fed without harm. One such instance was reported from Missouri where researchers fed rations containing .28 to .33 percent NO 3 N from sudangrass hay. Steers were slowly adapted to the high nitrate feed and performed satisfactorily.

Recent research indicates that beef heifers can safely graze sudangrass that contains high levels (.21 percent NO 3 N ) of nitrates. Sudangrass can contain up to .65 percent NO 3 N (28,690 ppm NO 3 - ), but cattle grazing these pastures tend to selectively graze the leaf portion of the plant that is not high in nitrate, and slowly adapt themselves to feeds that may be high in nitrate. If stocking rates are heavy for a pasture suspected of high nitrate concentrations it may cause cattle to consume plant parts such as stem bases that are high in nitrate, increasing the possibility of nitrate poisoning.

Causes of High Nitrates in Forage

All plants contain some nitrate, but excessively high amounts are likely to occur in forages having been grown under stress conditions such as:

1. The amount of nitrate in plant tissues also will depend on:

2. plant species;
3. stage of maturity;
4. part of the plant;
5. nitrogen fertilization.

With normal growing conditions, nitrate content will be slightly higher in the morning than in mid-afternoon, and may be higher on cloudy days than on sunny days. For example, oat hay harvested after cloudy weather is likely to be much higher in nitrate content than oat hay harvested during sunny periods.

Weather. Not all drought conditions cause high nitrate concentrations in plants. Some moisture must be present in the soil with nitrate for absorption and accumulation. If the major supply of nitrates for the plant is in the dry surface soil, very little nitrate will be absorbed by plant roots. In plants that survive through drought, nitrates often are high for several days following the first rain.

Frost, hail and low temperatures all interfere with normal plant growth and can cause nitrates to accumulate in the plant. Frost and hail may damage, reduce or completely destroy the leaf area of the plant. A decrease of leaf area limits the photosynthetic activity of the plant so nitrates absorbed by the roots are not converted to plant proteins and instead are accumulated in the stem or stalk.

Most plants require temperatures above 55°F for active growth and photosynthesis. Nitrates can be absorbed quickly by plants when temperatures are low, but conversion to amino acids and protein occurs very slowly in plants during times of low temperature.

Herbicides. Herbicides, such as 2, 4-D, tend to disrupt normal plant processes and can result in temporary high nitrate content in plants. However, spraying pastures and silage crops to control weeds actually may reduce the nitrate hazard of these feeds, especially when weeds high in nitrates are killed.

Disease. Plant diseases interfere with normal growth and development. This can cause nitrate to accumulate by interfering with nitrate reduction, protein synthesis or manufacture and translocation of carbohydrates.

Plant Species. Plants vary in the amount of nitrate that accumulates in various tissues. Certain weeds, such as pigweed, kochia, puncture vine and lambsquarter, often are high in nitrate. Oats and millet cut for hay at an immature stage often have high nitrate concentrations. Sorghum and sudangrasses often store high amounts of nitrates. Brome and orchard grass store very little nitrate under normal growing conditions. Legumes generally do not contain high nitrate concentrations. The nitrate content of some representative forages sent to the University of Nebraska during the early 1970s for nitrate analyses is shown in Table II.

Stage of Growth. Stage of growth markedly changes the nitrate content of forages. Nitrate concentrations usually are higher in young plants and decrease as the plant matures. However, plants grown in soils with excessive nitrates or grown under stress may be high in nitrate content at maturity.

Plant Parts. Plant parts closest to the ground contain the most nitrate. Leaves contain less nitrate than stalks, and the seed (grain) and flower usually contain little or no nitrate. Most of the plant nitrate is in the bottom third of the stalk.

Nitrogen Fertilization. Nitrates in the soil are the source of nitrate in plants. While a positive relationship exists between soil nitrates and nitrate in the plant, the effect of nitrogen fertilization appears to be less important than the conditions listed previously in causing high nitrate content in forages.

Table II. NO 3 N Content of Feedstuffs, Dry Basis

Harvest Methods Affect Nitrate Levels

Harvest forages suitable for silage at the stage of maturity where forage quality and quantity are optimal. Excessive nitrate in forages will not always be reduced to safe values during ensiling. If silage is suspected to be high in nitrate, analyze the feed for nitrates before feeding. The analysis will help in designing rations to prevent livestock losses from nitrate poisoning.

Harvest Near Maturity. Crops normally have lower nitrate levels at maturity, so crops such as corn or sorghums silage should be harvested as near maturity as possible. If the corn or sorghum field being harvested as silage has been identified as or is suspected of having high nitrate concentrations, raise the cutter head to selectively avoid stalk bases that have the highest nitrate concentration.

In crops such as oats for hay, it may be wise to harvest the crop at a more mature stage than desirable for maximum yield of digestible nutrients.

Harvest as Green Chop. Some forages are harvested in an immature stage and fed to cattle in the form of green chop. Green chop, high nitrate feeds that are piled in mounds and allowed to heat before feeding, are especially dangerous to feed to livestock. Heating causes the conversion of nitrate to nitrite, making the feed about 10 times as deadly. An analysis of nitrates will help in designing feeding programs to prevent livestock losses from nitrate poisoning.

Managing High Nitrate Feed

Frequent intake of small amounts of a high nitrate feed increases the total amount of nitrate that can be consumed daily by livestock without adverse effects, and helps livestock adjust to high nitrate feeds.

Feed long stem forages such as wheat, oat and cane hay that contain high amounts of nitrate in limited amounts several times daily rather than feeding large amounts once or twice daily. In addition, long stem hays suspected of nitrates can be fed in combination with hay low in nitrate with little risk of nitrate problems.

Livestock should have access to clean water at all times. Follow sound management practices conducive to a successful feeding program when high nitrate feeds are fed.

A balanced ration tends to reduce problems from nitrates in the ration. The adverse effects of feeding livestock feeds that have high nitrate concentrations appear to be greater if the ration is not properly balanced. When feeding feeds that are high or suspected to be high in nitrate, make sure the ration is balanced nutritionally for vitamins (A and E), macro minerals and trace minerals.

Allowing livestock to graze pastures suspected of having high nitrate levels is not without risk. Implementing one or more of the following management practices will reduce the risk of livestock losses to nitrate toxicity.

Do not feed hay, straw or fodder suspected of being high in nitrate when it is damp. Damp feed seems to be more toxic because some of the nitrate already has been converted to the more toxic nitrite before being consumed.

Using feeds that contain high nitrate concentrations is not without risk, but feeds high in nitrate can be fed successfully. Use the management practices mentioned above to reduce the chance of animal loss. Extensive losses can occur when non-acclimated, hungry livestock are permitted unlimited access to high nitrate feeds.

Nitrate in Water

Livestock drinking well water are not likely to experience nitrate toxicity.

Nitrate toxicity from water is more likely to occur when livestock drink water from ponds, road ditches, or other surface impressions that collect drainage from feedlots, heavily fertilized fields, silos, septic tanks or manure disposal lagoons.

As with feed, frequent intake of water containing nitrates appears to increase the total amount of nitrate that can be consumed daily without harmful effects. However, when evaluating possible toxic situations, the nitrate in both the feed and water must be considered because they are additive.

Nitrate toxicity is not likely to occur from water containing less than 100 ppm NO 3 N, provided animals are fed a balanced ration that is not high in nitrate, and sound feeding, watering and management practices are followed. When elusive nutrition or disease problems appear, a hasty implication of nitrate in water should not be made, even though water may contain more than 100 ppm NO 3 N.

Treatment

Administer a four percent (4%) methylene blue solution intravenously at a dosage rate of 4.55 mg per lb of body weight to treat cattle with nitrate poisoning. Treatment may have to be repeated every six to eight hours because the rumen may be full of forage or feed that contains nitrate, and nitrites will continue to move from the rumen into the blood stream. Mineral oil may be given orally to protect the irritated mucous membranes.

When many animals are affected, additional methylene blue is available and can be obtained from the three University of Nebraska veterinary diagnostic laboratories in Lincoln, North Platte and Scottsbluff.

Sampling and Testing Feeds for Nitrates

For suspected forages being put into an upright silo, take grab-samples for three successive days, then sub-sample and transfer to a plastic bag as mentioned above.

Long hay should be sampled using a hay probe. Sample those bales or stacks that represent the suspected hay. For baled hay, probe about 20 different bales and for hay stacks, sample each stack in six different areas to obtain a representative sample. Transfer the sample to a plastic bag, and compress the air out and seal before sending the sample to the laboratory.

Refer to NebGuide G77-331 Sampling Feeds For Analysis for more details on sampling.

It is difficult to obtain a representative sample from pastures suspected of nitrates that cattle are grazing. Cattle are selective in the plants and plant parts they consume, and a clipped sample will not represent what is actually being consumed. It is recommended you not test grazed forages for nitrate, but manage the grazing of such forages to reduce the problems due to nitrates, as mentioned above.

Most commercial feed laboratories will analyze feeds for nitrates. Contact your local extension agent to obtain information regarding laboratories in your area that test feeds for nitrates.

File G170 under: BEEF
A-13, Feeding & Nutrition
Revised July 1988; 12,000 printed.

Electronic version issued February 1996

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Elbert C. Dickey, Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources.

University of Nebraska Cooperative Extension educational programs abide with the non-discrimination policies of the University of Nebraska-Lincoln and the United States Department of Agriculture.