Nitrate Poisoning of Livestock

V-839 (Revised), September 1998

Dr. Charlie Stoltenow, Extension Veterinarian
Dr. Greg Lardy, Extension Beef Specialist

Nitrate poisoning can occur commonly in cattle raised in North Dakota and other areas of the western Great Plains.

Poisoning is usually associated with animals ingesting forage or feed with a high nitrate content.

Sheep and cattle are more susceptible to poisoning than non-ruminant species because microbes in their digestive tract favor the conversion of nitrate to nitrite.

Nitrate Poisoning

Nitrate in itself is not toxic to animals, but at elevated levels it causes a disease called nitrate poisoning. Nitrates are normally found in forages are converted by the digestion process to nitrite, and in turn the nitrite is converted to ammonia. The ammonia is then converted to protein by bacteria in the rumen. If cattle rapidly ingest large quantities of plants that contain high levels of nitrate, nitrite will accumulate in the rumen. Nitrite is ten times (10 X) as toxic to cattle as nitrate.

Nitrite is absorbed into red blood cells and combines with hemoglobin (oxygen carrying molecule) to form methemoglobin. Methemoglobin cannot transport oxygen as efficiently as hemoglobin, so the animal's heart rate and respiration increases, the blood and tissues of the animal take on a blue to chocolate brown tinge, muscle tremors can develop, staggering occurs, and the animal eventually suffocates.

Plants Factors Favoring Nitrate Poisoning

Plant Species and Agronomic Factors

The majority of nitrate poisoning cases in North Dakota occur with drought-stressed oats, corn, and barley. However, a number of other plants can also accumulate nitrate, including sudangrass, sorghum-sudan hybrids, and pearl millet. Table 1 lists common plants known to accumulate nitrate if conditions are favorable. Plants which have been fertilized have higher nitrate levels than non-fertilized plants. The abnormal accumulation of nitrate in plants is influenced by various factors such as moisture conditions, soil conditions, and type of plant. Plant stresses such as drought are associated with increased levels of nitrate in plants. Soils high in nitrogen readily supply nitrate to plants. Acidity, sulfur or phosphorus deficiencies, low molybdenum, and low temperatures are known to increase nitrate uptake by plants.

Table 1. Common plants known to accumulate nitrate.

       Crops    Weeds
--------------------------------
      Barley    Canada Thistle 
        Corn    Dock           
        Flax    Jimsonweed     
      Millet    Johnson Grass  
        Oats    Kochia         
        Rape    Lambsquarter   
         Rye    Nightshade     
     Soybean    Pigweed        
     Sorghum    Russian Thistle
  Sudangrass    Smartweed      
 Sugar beets    Wild Sunflower 
 Sweetclover
       Wheat
--------------------------------

Plant Parts

Plant parts closest to the ground (stalks) contain the highest concentrations of nitrates. Leaves contain less than stalks or stems, 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. Research from Oklahoma has shown that the lower 6 inches of the stem in pearl millet contains three times more nitrate than the top part of the plant. While difficult to do with drought-stressed forages, raising the cutter bar above 6 inches can reduce nitrate content of forages.

Stage of Plant Growth

Nitrate decreases as plants mature. Young plants have higher nitrate concentrations than mature plants. However, mature plants can still have excessive nitrate concentrations if environmental and soil conditions are favorable.

Weather Conditions Which Favor Nitrate Accumulation by Plants

Not all drought conditions cause high nitrate levels in plants. Some moisture must be present in the soil for the plant to absorb and accumulate nitrate. If the major supply of nitrates for the plant is in the dry surface soil, very little nitrate will be absorbed by the roots. In plants which survive drought conditions, nitrates are often high for several days following the first rain (as the plant regrows following drought).

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 in leaf area limits the photosynthetic activity of the plant, so nitrates absorbed by the roots are not converted to plant proteins but are accumulated in the stem or stalk instead.

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 periods of cool weather. This allows nitrate to accumulate in the plant.

Water

Water may be a source of toxic levels of nitrate for livestock. Water may become contaminated by fertilizer, animal wastes, or decaying organic matter. Shallow wells with poor casings are susceptible to contamination. Marginally toxic levels of nitrate in water and feed may together cause nitrate toxicity in animals. Remember to consider both sources of nitrate.

Fertilizer

Acute nitrate poisoning may occur if livestock consume nitrate fertilizer. Avoid grazing immediately after spreading fertilizer. Areas where the fertilizer spreader turns or areas where filling (and consequently spilling) take place may have excessive quantities of nitrate freely available to animals.

Clinical Signs

Clinical signs of nitrate poisoning are related to the lack of oxygen in the blood. Acute poisoning usually occurs between a half hour to four hours after consuming toxic levels of nitrate. Onset of symptoms are rapid and include:

• bluish/chocolate brown mucous membranes
• rapid/difficult breathing
• noisy breathing
• rapid pulse (150+/min)
• salivation, bloat, tremors, staggering
• weakness, coma, death
• dark "chocolate-colored" blood

Pregnant females that survive nitrate poisoning may abort due to lack of oxygen to the fetus. Abortions generally occur approximately 10-14 days following exposure to nitrates.

Diagnosis

Diagnosis of nitrate intoxication is based on observed clinical signs and the possibility of exposure to toxic plants or water. A veterinarian should be consulted for a definitive diagnosis. Laboratory analysis can be performed on suspected plants, water, stomach contents, blood, urine, and aqueous humor of the eye of dead cattle to confirm the diagnosis. Postmortem specimens of rumen contents are of little value for nitrate determination because most nitrate in the rumen is reduced by anaerobic fermentation to ammonia.

Samples from fresh grass or dry forages need to be representative of the field or bales in question. These samples should be packaged in a clean plastic bag and shipped to the laboratory for analysis. Water samples must be collected in a sterile bottle. When collecting from a water system, let the water flow for a couple of minutes before collecting the sample.

Results of chemical analysis are interpreted according to guidelines in Table 2. These guidelines apply to livestock only.

Table 2. Interpretation of laboratory results.

-------------------------------------------------------------------------------
                                                        Recommendations
      KNO3               NO3-N               NO3        for use in livestock
-------------------------------------------------------------------------------
  - - - - - - - - - Level of Nitrate - - - - - - - - -   
Forage
                            
  0-10,000 ppm        0-1,500 ppm       0-6,500 ppm     Generally considered 
                                                        safe for livestock.
10,000-30,000 ppm   1,500-4,500 ppm   6,500-20,000 ppm  CAUTION. Problems 
                                                        have occurred at this 
                                                        level. Mix, dilute, 
                                                        limit feed forages in 
                                                        this level.
   >30,000 ppm        >4,500 ppm        >20,000 ppm     DANGER -- DO NOT FEED.
                                                        Potentially toxic.
                                             
Water                                         
                                               
    0-720 ppm         0-100 ppm          0-400 ppm      Generally safe for 
                                                        livestock.
  720-2,100 ppm      100-300 ppm       400-1,300 ppm    CAUTION. Possible 
                                                        problems. Consider 
                                                        additive effect with
                                                        nitrate in feed.
   >2,100 ppm         >300 ppm          >1,300 ppm      DANGER. Could cause 
                                                        typical signs of 
                                                        nitrate poisoning.
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------

Treatment

Animals can be treated by intravenous injections of methylene blue. Commercial preparations intended for treatment of prussic acid poisoning only should not be used to treat nitrate poisoning. It should be noted that methylene blue is not approved by the FDA for use in food-producing animals. You must consult your veterinarian before using this treatment.

Prevention

Prevention of nitrate poisoning is best achieved by controlling type and quantity of forage offered to livestock. Avoid forages with potentially toxic levels of nitrate or at least dilute them with feeds low in nitrate. When in doubt, have feeds and forages analyzed for nitrate before grazing or feeding them. Forages with sub-lethal nitrate levels can be fed to livestock with appropriate precautions. No single level of nitrate is toxic under all conditions. When grazing, feed a dry roughage first to reduce the amount of affected plants ingested by hungry animals. Harvested forages that are high in nitrate can often be safely fed by mixing with other feeds to reduce the total dietary intake of nitrate. Contact your veterinarian or extension personnel if you need assistance in determining the correct ratios of high and low nitrate forages to blend to develop a ration for a particular class of livestock.

Management Guidelines

• Drought-stressed small grain forages and other forages suspected of being high in nitrates should be tested before feeding.
• Dilute high nitrate forages with other forages or feedstuffs which are low in nitrates. This can bring the nitrate level of the diet down low enough where it is safe to feed.
• Frequent intake of small amounts of high nitrate feed helps adjust livestock to high nitrate feeds and increases the total amount of nitrate which can be consumed daily by livestock without adverse effects.
• Allow cattle time to adapt to increased nitrate in the diet. If nitrate levels are not excessively high (9000 ppm nitrate) the animals can adapt to increasing amounts in the feed.
• Allow livestock access to fresh, nitrate-free water at all times.
• Be sure you don't overstock pastures when grazing high nitrate forages. Overstocking increases the amount of high nitrate plant parts (stems and stalks) that are consumed by livestock.
• Do not strip graze high nitrate forages. Strip grazing also increases the amount of stem and stalk material consumed by livestock.
• Do not allow hungry cattle access to high nitrate forages or pastures. Feed cattle hays or forages low in nitrates before turning them onto high nitrate pastures.
• Supplement cattle grazing high nitrate forages with other low-nitrate feedstuffs such as low nitrate forages, feed grains, or byproducts.
• If possible, graze cattle on high nitrate pastures during the day and remove them at night for the first week of grazing. This reduces the amount of high nitrate forage consumed and helps acclimate cattle to the high nitrate levels.
• If possible, don't graze high nitrate pastures until one week after a killing frost.
• Observe cattle frequently when you turn into a suspected field or pasture in order to detect any signs of toxicity.
• Cattle in poor health and condition, especially cattle suffering from respiratory disease, are more susceptible to nitrate poisoning.
• Consider harvesting and feeding high nitrate forages as silages. Nitrate levels are reduced by the fermentation process that occurs when feeds are ensiled.
• Do not allow cattle access to areas where fertilizers are stored.
• Do not feed green chop which has heated after cutting or which has been held overnight. Heating favors the formation of nitrite which is more toxic than nitrate.

If you have questions concerning submitting samples to a laboratory for analysis, you can contact the North Dakota State University Veterinary Diagnostic Laboratory at 701-231-8307.

V-839 (Revised), September 1998