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Nitrate and Prussic Acid Poisoning in Cattle

Fast Facts

Prussic acid or nitrate toxicity can kill animals in a matter of hours, so it is important to understand the conditions that cause buildup. 
  • There are no visual signs of toxicity in forages.
  • 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.
  • Excessive fertilization with poultry litter or animal manure is the most common cause of nitrate buildup in plants.
  • Johnsongrass, pearl millet, and sorghum/sudangrass are the forages most often found with high nitrate levels, but others can accumulate high nitrate under stressful conditions.
  • Any stress on the plant which causes abrupt stoppage of growth can also contribute to nitrate buildup (drought, herbicide damage, even late afternoon wilting).
  • Nitrates will persist in the grass when hayed. Prussic acid does not. If considering cutting for hay, consider a nitrate test first, especially if field received heavy N fertilization.
  • Nitrates are higher in younger plants than older ones. Higher concentrations are in the stem and lower third of the plant.
  • Prussic acid is also higher in young plants than older ones, but it concentrates more in the leaves than the stems.
  • Regrowth contains the most nitrates and prussic acid.
  • If haying, raising the mower even a couple of inches could make a difference in hay nitrate content. Grazing brand new regrowth after bushogging or cutting isn't recommended, especially if the field received heavy N fertilization.
  • Prussic acid is also known as hydrocyanic acid.
  • Prussic acid dissipates after being hayed. For this reason, testing anywhere but directly in the field isn't very reliable.
  • Fields that received a lot of N, but are deficient in P and K are susceptible to toxic concentrations of nitrate.

What are the signs of nitrate poisoning in cattle?

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
  • collapse
  • milk production may also be reduced

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

Observe animals closely for signs of toxicity, and call a veterinarian immediately if symptoms occur.

How do you treat nitrate poisoning in cattle?

  • Stop feeding the hay that's being fed, or get them off the pasture they're grazing.
  • Contact your veterinarian immediately.
  • An injection of 1% 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.

How can I prevent nitrate poisoning in my cattle?

  1. Follow recommendations for nitrogen fertilization, and be careful not to exceed 2 tons of poultry litter yearly per acre on pastures, especially on johnsongrass or warm-season annual grasses. The risk will be minimized by spreading litter uniformly and limiting application to 2 tons per acre per application.
  2. 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.
  3. Consider making silage of drought­ damaged forage. The ensiling process reduces the nitrate level 40 to 60 percent.
  4. 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.
  5. Have suspected forage tested before harvesting for hay or feeding to cattle. Contact your county agent for help testing.
  6. 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.
  7. Feed a nutritionally balanced ration. Iodized salt and vitamin A or green feed supplementation lessen the toxicity of nitrates.
  8. Adapt cattle slowly to elevated levels of nitrate. Allow animals to graze only a couple hours per day for a few days on suspected fields. Never turn hungry animals in to a suspected field – allow them to graze other pasture first or feed hay before turn-in. Never exceed maximum recommended levels of nitrate intake.
  9. Feed suspect forage in small amounts several times a day rather than all at one feeding.
  10. If forages are high in nitrates, ask your county agent about an analysis of the drinking water.
  11. Be aware that forage regrowth and volunteer plants are highly suspect following nitrogen fertilization and drought.
  12. Observe animals closely for signs of toxicity, and call a veterinarian immediately if symptoms are observed.

What causes nitrate to accumulate in plants?

  • 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.
  • Excessive fertilization with poultry litter or animal manure is a common cause of nitrate accumulation in plants.
  • 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. Some of these conditions are:

  1. Lack of Sunlight – Shaded valleys, continued cloudy days and high plant populations may contribute to excess nitrates in plants.
  2. Detrimental Weather – Drought and high temperatures or low humidity, cold temperatures, hail damage and frost may slow or stop plant growth and cause nitrates to accumulate.
  3. Herbicides – Spraying with herbicides such as 2,4­-D may result in temporary high nitrate levels in some weeds. Herbicides disrupt the normal enzyme systems of plants, and this action interferes with the reduction of nitrates and their conversion into protein. Judicious, timely spraying of pastures to control young weeds will actually reduce the nitrate hazard because those weed species which are normally high in nitrates are killed early in the year. This reduces the overall threat of nitrate toxicity through more of the season.
  4. Diseases – Diseases may destroy photosynthetic and/or conductive tissue that could cause nitrates to accumulate in plants by interfering with nitrate reduction, protein synthesis or the manufacture and translocation of carbohydrates.
  5. Imbalance of Soil Nutrients – A balance of soil nutrients is important in preventing nitrate accumulations. A lack of trace minerals, such as molybdenum, copper, iron, magnesium, sulfur or manganese that are involved in the enzyme system for using nitrates or potassium, may cause nitrates to accumulate in plants.

Have your forages checked!

Nitrates will persist in the grass when hayed. Prussic acid does not. If considering cutting for hay, think about doing a nitrate test first, especially if field received heavy N fertilization. 

Nitrate forage testing is a $6 service offered by the University of Arkansas Diagnostic Lab.

If you have questions or need a test contact your county extension agent.

*Note: Unfortunately there are no reliable tests for prussic acid in forages because the levels can change so rapidly after taking a sample.

Prussic Acid Poisoning in Cattle

What are the most common signs of prussic acid poisoning in cattle?

The symptoms of prussic acid poisoning are very similar to that of nitrate poisoning. Symptoms of prussic acid poisoning include anxiety, progressive weakness and labored breathing, and death may follow when lethal amounts of hydrocanic acid (HCN*) are consumed. However, the dead animals may be found without visible symptoms of poisoning. Animals may also show increased rate of respiration, increased pulse rate, gasping, muscular twitching and convulsions. Death from prussic acid poisoning often occurs more rapidly than nitrate poisoning in affected animals.

*Hydrocyanic acid (HCN) or prussic acid is generally found in stressed plants and is formed by enzymatic action on compounds called the cyanogenetic glucosides (dhurrin) when growth is adversely affected.

What is the treatment for prussic acid poisoning?

Get the animals off the pasture they're grazing. Contact your veterinarian immediately. Animals affected by prussic acid poisoning may be treated with a sodium nitrite-­sodium thiosulfate combination. It must be injected intravenously and very slowly. The dosage and method of administration are critical. Prussic acid poisoning from hay is very uncommon.

Consult a veterinarian immediately to correctly diagnose prussic acid poisoning and to determine the proper treatment.

What is the relationship between prussic acid and nitrate poisoning?

Prussic acid poisoning is most commonly associated with johnsongrass and sorghum-sudangrass. Pearl millet and corn do not produce prussic acid. Under certain stressful conditions (especially prolonged drought or cool, cloudy weather), these grasses may produce prussic acid or accumulate high levels of nitrates in their stems. There is little or no relationship between prussic acid and nitrate poisoning. However, HCN poisoning is often confused with nitrate poisoning since environmental conditions and animal symptoms of the two disorders are somewhat similar

How to Prevent Prussic Acid Poisoning

  1. Do not allow animals to graze fields with succulent, young, short growth. Graze only after plants reach a height of 18 to 24 inches.
  2. Do not graze drought damaged plants in any form, regardless of height, within four days following a good rain. It is during this period of rapid growth that accumulation of prussic acid in the young tissue and of nitrates in the stems is most likely to occur.
  3. Do not graze wilted plants or plants with young regrowth.
  4. Do not rely on drought damaged material as the only source of feed. Keep either dry forage or green chop from other crops available at all times. Uneven growth as a result of drought can best be utilized as silage or hay.
  5. Do not use frost damaged sorghum as pasture or green chop during the first seven days after the first killing frost.
  6. Delay pasturing for a least seven days or until the frosted material is completely dried out and brown colored. Do not rely on frosted material as the only source of feed. Do not graze at night when frost is likely.
  7. Do not turn hungry cattle onto a pasture of sorghum, sorghum sudan hybrid, or johnsongrass. Fill them up on hay or other forage first, and begin grazing in the late afternoon.
  8. An option for using potentially toxic forage is to harvest it as hay or silage. Prussic acid levels decline in stored forages. Well cured hay is safe to feed.

Susceptible Plants in Arkansas- Sorghum & Johnsongrass 

Johnsongrass in a field
Johnsongrass (pictured above) is a warm-season perennial grass found statewide in Arkansas. It is often a weedy invader in other forages. It can cause prussic acid poisoning when wilted from frost or drought. It can accumulate toxic levels of nitrate if over-fertilized with nitrogen or litter.

The amount of HCN found in plant tissue varies among species. Of all the plants grown in Arkansas, those belonging to the sorghum category are most likely to contain potentially toxic levels. Grain sorghum contains the most, followed by johnsongrass, sorghum-sudan hybrids and then sudangrass.

However, johnsongrass may be the plant of most concern since it grows wild throughout the state and infests many pastures. Johnsongrass commonly has problems with nitrate accumulation. Prussic acid can also build up in stressed johnsongrass, much like nitrates.

Close grazing for several years usually eliminates johnsongrass from pastures.

Millet is free of prussic acid. HCN may be produced by a few other plant species. Wild cherry trees can produce toxic levels, and HCN poisoning occurs most often when animals consume wilted leaves after trees have been damaged by storms or pruning.

Signs of toxicity in Johnsongrass

Unfortunately, there's no hard and fast rule on when to graze or not graze it. Despite common belief, there's no real visual symptoms of toxicity. That white powdery stuff that gets on the leaves is most likely the fungal disease, powdery mildew, which is unrelated to prussic acid or nitrates.

Read more about Johnsongrass problems

View our forages ID to search forage images

For more information about preventing nitrate and prussic acid poisoning

Contact your county extension agent

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