University of Missouri
Reviewed April 26, 2006
Precautions When Utilizing
Sorghum / Sudan Crops as Cattle Feed
Situations when caution is warranted:
Cyanide / Prussic Acid Toxicosis
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.
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
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.)
Clinical signs: Usually seen w/n 6 hr of ingestion but can be as long as a week
Diagnosis: Elevated methemoglobin levels (1:20, blood:distilled water, refrigerated or frozen - then delivered to the laboratory)
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 **
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.
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.
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.
Bob Larson, LarsonR@missouri.edu 573-882-7848
College of Agriculture Food and Natural Resources
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