Corn for Silage
Technical
Insight 502

NITRATE AND PRUSSIC ACID TOXICITY IN FORAGE

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Nitrate is a natural product formed from the oxidation of organic compounds. Most of the soil nitrogen absorbed by plant roots is in nitrate form. Normally, nitrate in a plant is rapidly converted to amino-acids by the enzyme nitrate reductase. This reduction requires energy from sunlight, adequate water, nutrients, and favorable temperature. When plants are stressed, the nitrate-to-protein conversion is disrupted and nitrates begin to accumulate.

Why Nitrates are toxic?

After a plant is eaten, rumen bacteria rapidly reduce nitrates in the forage to nitrites. Normally, the nitrites are converted to ammonia and used by rumen microorganisms as a nitrogen source. If nitrate intake is faster than its breakdown to ammonia, however, nitrites will begin to accumulate in the rumen. Nitrite is rapidly absorbed into the blood system where it oxidizes hemoglobin to methemoglobin. Red blood cells containing methemoglobin cannot transport oxygen, and the animal dies from asphyxiation.

Animals under psychological stress (sick, hungry, lactating 0r pregnant) are more susceptible to nitrate toxicity. Toxicity is related to the total amount of forage consumed and how quickly it is eaten however if forages contain more then 6,000 ppm nitrate, they should be considered potentially toxic. Cattle and horses are generally affected by nitrate toxicity more then most, although all animals are susceptible.

Symptoms to look for

  • Reduced appetite
  • Weight loss
  • Diarrhea
  • Runny eyes

Acute toxicity is usually not apparent until methemoglobin approaches lethal concentration. Symptoms of this include:

  • Cyanosis (Bluish color of mucus membranes)
  • Labored breathing
  • Muscular tremors
  • Eventual collapse
  • Coma and death (usually follow these symptoms within 2-3 hours)

Diagnosis and treatment of nitrate toxicity should be performed by a veterinarian. However, in acute cases where time is limited, an antidote of methylene blue can be injected to convert the methemoglobin back to hemoglobin.

Plant factors

Plant species Nearly all plants contain nitrate, but some species are more prone to accumulate nitrate than others. Crops such as forage sorghum, grain sorghum, sudangrass, sudan-sorghum hybrids and pearl millet are notorious nitrate accumulators.

Stages of Growth Nitrate content is nearly always highest in young plant growth and decreases with maturity. Sorghum’s and sundan grasses however are exceptions because concentrations usually remain high in mature plants. If plants are stressed at any stage of growth they accumulate nitrate.

Plant parts Nitrates normally accumulate in stems and conductive tissues. The highest nitrate levels occur in the lower one-third of the plant stalk. Concentrations tend to be low in leaves because nitrate reductase enzyme levels are high there.

Environmental factors

Drought Roots continually absorb nitrate, but high day time temperatures inhibit it’s conversion to amino acids. During severe drought, lack of moisture prevents nitrate absorption by plant roots. Following rain however the roots rapidly absorb nitrate and accumulate high levels. After a drought-ending rain, it requires 7-14 days before the nitrates will be metabolized to low levels, provided environmental conditions are optimum.

Sunlight Plants growing in field corners may be shaded and are frequently high in nitrates. Extended periods of cloudy weather increase nitrate content. Dangerously high levels can occur when wet, overcast days follow a severe drought.

Frost, Hail, or Disease. Conditions such as hail, light frost, or plant disease can damage plant leaf area and reduce photosynthetic activity. With less available energy, nitrate reduction is inhibited and nitrates accumulate in the plant.

Temperature. Low temperatures retard photosynthesis of warm-season plants and favor nitrate accumulation. Extremely high temperatures also increase nitrate concentrations by reducing nitrate reductase enzyme activity.

Management Factors

Fertilization Nitrogen fertilization increases soil nitrate levels and the subsequent uptake by plant roots. Nitrogen from decomposing organic matter also can contribute to nitrate accumulation. Applying high amounts of manure or other fertilizer, particularly in the late season, increases concentrations. Split nitrogen applications provide better nutrient distribution and reduce the potential for toxicity. In addition to excess nitrogen, an imbalance of other soil nutrients can affect forage nitrate levels. Plants growing in soils deficient in phosphorus, potassium, and some trace elements have high nitrate concentrations.

Herbicides Selectively spraying weeds routinely high in nitrates can reduce the potential hazard to livestock. Weeds damaged but not killed by a herbicide will have high nitrate levels because of depressed enzyme activity and reduced leaf area.

Harvest Technique When roughage are made into silage, fermentation normally reduces nitrate levels by 40-60%. Forages with extremely high nitrate levels at harvest may still be dangerous after ensiling and should be analysed before feeding. If forages are harvested as hay, nitrate concentrations remain virtually unchanged over time.  High nitrate forages may be grazed, but a dry roughage should be fed first to limit intake. Stocking rate should not be too high because overgrazing forces cattle to eat the stems, which contain the highest nitrate levels. Cattle should be removed from potentially susceptible forage for 7 to 14 days after a drought ending rain. Lush regrowth of heavily fertilized grasses contains high nitrate levels and should not be grazed. If plants are fed as green chop, the harvested forage should be fed immediately after cutting, not allowed to heat up.

Feeding high nitrate forage

  •  Before feeding potentially troublesome plants such as sorghum and sudangrass, analyze the forage for nitrates. High nitrate forages still can be fed to animals if proper precautions are taken.
  • Adapt cattle to high nitrate feeds gradually
  • Dilute with other feeds – after three – four weeks the animal normally becomes adjusted to nitrates and the proportion of high nitrate forage can be increased
  • Supplement grain – Grain dilutes the amount of nitrate in the total ration and provides the energy necessary for bacteria to quickly convert nitrite to ammonia. Molasses also can provide needed energy for nitrite reduction but may be cost prohibitive.
  • Feed a balanced ration – Formulate rations to ensure adequate protein, vitamin A and other nutrients.
  • Do not feed to stressed livestock
  • Provide clean drinking water

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Prussic acid poisoning

Prussic acid poisoning is caused by cyanide production in several types of plants under certain growing conditions. Sorghums and closely related species are the plants most commonly associated with prussic acid poisoning. These plants possess a cyanogenic molecule called dhurrin in their epidermal cells. Inhealthy, intact leaf tissue dhurrin is nontoxic. However, mesophyll cells located beneath the epidermis have an enzyme that removes HCN from dhurrin. If the leaves become damaged, dhurrin and its hydrolyzing enzyme will intermix and release cyanide.

Why prussic acid is toxic

Once eaten, cyanide is absorbed directly into the bloodstream and binds to enzymes in the cell. This cyanide complex prevents hemoglobin from transferring oxygen to individual cells and the animal dies from asphyxiation.

Prussic acid acts rapidly, frequently killing the animal within minutes.

Symptoms include:

  • Excess salivation,
  • Difficult breathing,
  • Staggering, convulsions, and collapse.

Death from respiratory paralysis follows shortly. The clinical signs of prussic acid poisoning are similar to nitrate toxicity, but animals with cyanide poisoning have bright red blood that clots slowly, whereas animals poisoned with nitrate have dark, chocolate- colored blood. The smell of bitter almonds is often detected in animals poisoned with cyanide. If there is no doubt about the diagnosis, and a veterinarian  is not present, the animal can be treated with simultaneous injections of sodium nitrate and sodium thiosulfate. Sodium nitrate releases the cyanide from the cell, which then binds with the sodium thiosulfate to form a nontoxic complex that is excreted. Animals alive one to two hours after the onset of visible signs usually recover.

[quote]Animals alive one to two hours after the onset of visible signs usually recover.[/quote]

Prussic acid concentration factors

Plant Species. Crop species most commonly involved with prussic acid poisoning are sorghums, Johnsongrass, and sudangrass. Potential cyanide production among varieties and hybrids of most summer annual forages varies widely. Grain sorghums are potentially more toxic than forage sorghums or sudangrass, whereas hybrid pearl millet and foxtail millet generally have very low cyanide levels.

Plant age and condition. Young, rapidly growing plants are likely to contain high levels of prussic acid. New sorghum growth following drought or frost is dangerously high in cyanide. Pure stands of Indiangrass that are grazed when the plants are less than 8 inches tall can possess lethal concentrations of cyanide.  Generally, any stress condition that retards normal plant growth may increase prussic acid content. Hydrogen cyanide is released when plant leaves are physically damaged by trampling, cutting, crushing, chewing, or wilting.

Drought and Frost. Drought-stunted plants accumulate cyanide and can possess toxic levels at maturity. Freezing ruptures the plant cells and releases  cyanide. After a killing frost, wait at least four days before grazing to allow the released HCN to dissipate.  Prussic acid poisoning is most commonly associated with regrowth following a drought-ending rain or the first autumn frost. New growth from frosted or drought-stressed plants is palatable but dangerously high in cyanide.

Soil Fertility. Plants growing in soil that are high in nitrogen and low in phosphorus and potassium tend to have high cyanide concentrations. Split applications of nitrogen decrease the risk of prussic acid toxicity.

Animals. Most losses occur when hungry or stressed animals graze young sorghum growth.

Harvest Technique. Prussic acid concentrations are higher in fresh forage than in silage or hay because HCN is volatile and dissipates as the forage dries. However, if the forage had an extremely high cyanide content before cutting, or if the hay was not properly cured, hazardous concentrations of prussic acid could remain. Hay or silage that likely contained high cyanide concentrations at harvest should be analyzed before it is fed.

 

Summary Guidelines To Avoid Prussic Acid Poisoning

  • Do not allow hungry cattle to graze where prussic acid may be a problem.
  • Do not allow animals to graze potentially troublesome plants after a light frost or after rain has ended a summer drought.
  • Chop or ensile plants high in cyanide to reduce toxin levels.
  • Have representative samples of any suspect forage analyzed before feeding.

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Revised: June 2015
Expires: June 2018