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Precautions When Utilizing Sorghum / Sudan Crops as Cattle Feed
Dr. Bob Larson

Situations when caution is warranted:

New growth - cyanide risk
New growth after a frost - cyanide risk
New growth after heavy fertilization - cyanide risk
Stressed or dead growth - cyanide risk or nitrate risk
Harvested during a drought - cyanide or nitrate risk
Rain following a drought - nitrate risk
Grazing during cool, cloudy days - nitrate risk

Cyanide / Prussic Acid Toxicosis

While growing, sudangrass and sorghum produce cyanogenic glucosides. When water is mixed with cyanogenic glucosides, cyanide is released from the chemical reaction and becomes hydrocyanic acid (prussic acid). While in the bonded form, cyanide and glucosides aren't poisonous, however, then the bond is broken due to the presence of certain enzymes, the highly toxic cyanide or prussic acid is formed. Those enzymes may be present in the plant itself, other feeds or the digestive juices of the animal. Stress that retards the plant growth (including drought or frost) causes an increase in the glucoside level within the plant. Frost or wilting of the plants followed by rapid regrowth will increase prussic acid within the plant and the transformation into the highly toxic cyanide.

Sorghum, Johnsongrass, and Shattercane are much higher in prussic acid than sudangrass. Prussic acid is released very rapidly from the glucoside form in frozen leaves. This is the reason that frosted sorghum is dangerous to feed until it dries out. There is little danger of prussic acid poisoning in grazing most varieties of sudangrass. Plants that have a height of about six feet tend to not affect animals as much as shorter, stunted, plants experiencing regrowth. Favorable weather for plant regrowth after clipping, drought, frost, or grazing will result in new leaves that are likely to be very toxic and cause prussic acid poisoning. Small shoots are high in cyanide content and are desirable to cattle.

  • Accumulate high levels of cyanide early in growth stage
      i.e. new growth after frost, after heavy nitrate fertilization
  • Accumulate high levels of cyanide during periods of stress
      i.e. frost, drought, herbicide kill
  • Cyanide is more concentrated in young leaves than in older leaves or stems. New sorghum growth following drought or frost is dangerously high in cyanide. Pure stands of Indiangrass that are grazed when plants are less than 8 inches tall can posses lethal concentrations of cyanide.
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.
  • Don't graze or green chop before plants are 18 inches tall.
  • After a killing frost, wait at least 4 to 6 days before grazing to allow the released HCN to dissipate.
  • Don't green chop for 2 days after a killing frost.

    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.

    Level of prussic acid in forage (DM basis) and effect on animals

    ppm HCN Effect on animals
    0-500 Generally safe; should not cause toxicity
    600-1,000 Potentially toxic; should not be the only source of feed
    1,000 and above Dangerous to cattle and usually will cause death

    Clinical signs: bright red blood, mucus membranes may be cyanotic, salivation, rapid respiration, difficult respiration, muscle fasciculations, nervous signs, convulsions

    Necropsy lesions: Venous blood may be bright red if necropsy occurs soon after death If necropsy is delayed, blood is dark and clots slowly Lung and liver are congested - hemorrhage on many serosal surfaces Rumen is distended with gas and a bitter almond odor may be detected when opened

    Treatment: (As long as a heart beat is present, intravenous therapy can result in recovery.)

    1. 10 ml Sodium nitrite (20% soln.) + 30 ml Sodium thiosulfate (20% soln.) This solution is given IV (40 ml mixture per 1000#)
    2. Methylene blue at 4 to 6 gm/1000# IV

    Nitrate Toxicosis

  • Accumulate nitrate due to excessive fertilization
  • Accumulate nitrate due to water stress: i.e. drought or rain after a drought
  • Nitrate accumulates at night, on cloudy days and when environmental temperatures are cool
  • Nitrate levels are highest in the roots and stems, lower in leaves, and almost no nitrate accumulates in the flower and seed. (Most of nitrate is concentrated in lower 1/3 of stalk)

    Clinical signs: Usually seen w/n 6 hr of ingestion but can be as long as a week

    • Signs are related to anoxia resulting from methemoglobinemia.
      • Depression
      • Cyanotic (blue) to brown cast of mucus membranes(mouth, vulva)
      • Rapid, weak pulse
      • Muscle tremors, ataxia, weakness
      • Exercise will may signs worse - remember the animals are suffocating
      • Abortion may result a few days after an episode, even in animals that did not appear to be affected

    Diagnosis: Elevated methemoglobin levels (1:20, blood:distilled water, refrigerated or frozen - then delivered to the laboratory)

      Chocolate brown colored blood is suggestive

    Necropsy lesions: Postmortem lesions are limited to the chocolate brown cast of the blood, mucus membranes, viscera and muscles

    Differential Diagnosis: Chlorates (desiccant herbicides) also produce methemoglobinemia - treatment is the same but may have relapses when chlorates are the cause.

    ** Important to test forages and water for nitrate levels even if you are certain of diagnosis **

      Also can confuse cyanide toxicosis with nitrite toxicosis

    Treatment: Methylene blue 4.4 mg/kg IV in a 2-4% solution

    Mineral oil PO to lessen time that high nitrate material is in contact with the GIT

    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.

    Nitrate toxicity is a misnomer because nitrite (NO2), not nitrate (NO3), is poisonous to animals. After a plant is eaten, 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 oxidized hemoglobin to methemoglobin. RBCs containing methemoglobin cannot transport oxygen, and the animal dies from asphyxiation.

    Toxicity is related to the total amount of forage consumed and how quickly it is eaten, but, generally, if forages contain more than 6,000 ppm nitrate, they should be considered potentially toxic.

    ppm Nitrate Effect on animals
    0-3,000 Virtually safe
    3,000-6,000 Moderately safe in most situations; limit use for stressed animals to 50% of the total ration
    6,000-9,000 Potentially toxic to cattle depending on the situation; should not be the only source of feed
    9,000 and above Dangerous to cattle and often will cause death

    Because different laboratories report nitrate levels as either nitrate (NO3), nitrate-nitrogen (NO3-N), or potassium nitrate (KNO3), one should convert these values to ppm nitrate by using the following conversions.

    Potassium Nitrate X 0.61 = Nitrate (ppm)
    Nitrate-Nitrogen X 4.42 = Nitrate (ppm)
    % Nitrate X 10,000 = Nitrate (ppm)

    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. Weed species such as kochia, lambsquarters, sunflower, and pigweed also are routinely high in nitrate. Under certain environmental and managerial conditions, wheat, corn, alfalfa, soybeans, oats, Johnsongrass, and other plants can accumulate potentially toxic levels of nitrate.

    Nitrate content generally is highest in young plant growth and decreases with maturity. Sorghums and sudangrass are exceptions because concentrations usually remain high in mature plants. If plants are stressed at any stage of growth, they can accumulate nitrate.

    Nitrates normally accumulate in stems and conductive tissues. 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. Grain does not contain appreciable amounts of nitrate.

    Drought Nitrates accumulate in plants during periods of moderate drought because the roots continually absorb nitrate, but high daytime temperatures inhibit its conversion to amino acids. During a sever drought, lack of moisture prevents nitrate absorption by plant roots. Following a rain, however, the roots rapidly absorb nitrate and accumulate high levels. After a drought-ending rain, it requires 7 to 14 days before the nitrates will be metabolized to low levels, provided environmental conditions are optimum.

    Sunlight Nitrate reduction occurs in young leaves and requires light as an energy source. Shaded plants lack sufficient energy to convert nitrate to amino acids. 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 (less than 55 F) in the spring or fall retard photosyntheses of warm-season plants and favor nitrate accumulation. Extremely high temperatures also increase nitrate concentrations by reducing nitrate reductase enzyme activity.

    Applying high amounts of manure or other fertilizer, particularly in the late season, increases concentrations of soil nitrates and subsequent uptake by plant roots.

    Weeds damaged but not killed by a herbicide will have high nitrate levels because of depressed enzyme activity and reduced leaf area.

    When roughages are made into silage, fermentation normally reduces nitrate levels by 40 to 60 percent. Forages with extremely high nitrate levels at harvest may still be dangerous after ensiling and should be analyzed before feeding. If forages are harvested as hay, nitrate concentrations remain virtually unchanged over time.

    Guidelines to Reduce Nitrate Toxicity

    Pay close attention to potentially troublesome plants, such as sorghum and sudangrass, which often have high nitrate levels.

    Avoid excessive application of manure or nitrogen fertilizer.

    Raise cutter bar 6 to 12 inches to exclude basal stalks. This also will minimize harvesting many weeds species that have accumulated nitrate from shading.

    Delay harvesting any stressed forages. A week of favorable weather generally is required for plants to reduce accumulated nitrate.

    Back to MU Drought site

  • Univeristy of Missouri Extension Bob Larson, 573-882-7848
    College of Agriculture Food and Natural Resources

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