Prussic Acid in Summer Cover Crops this Fall

Frost damaged volunteer sorghum x sudangrass from a summer cover crop blend in a central Oklahoma crop field.
( Oklahoma State University )

Many of the wheat producers in the Southern Plains have begun growing summer cover crops that include mixtures of several plant species including sun hemp, cow peas, forage sorghums or sudangrass (or their hybrid sorghum x sudangrass). Several producers I have talked to are considering using these green cover crops as a fall stockpile forage for cows or weaned calves. Other producers that planted small grains directly into the standing cover crops have seen volunteer sorghum or sudan come up in the planted wheat.

With the average frost date for most of central and northern Oklahoma behind us, and reports of a light frost from producers in areas of north central parts of the state, prussic acid may be an issue in certain forages commonly found in fields and pastures.

Prussic acid is found when susceptible plants are under drought conditions or after a light frost. Prussic acid is a form of cyanide and will absorb through the rumen wall and reach the blood stream. When prussic acid combines with hemoglobin in the red blood cells, the cells will take up oxygen but will not be able to release it. This causes suffocation. The oxygen saturation in the red blood cells causes the blood to have a bright cherry red color. The symptoms of prussic acid poisoning are similar to the symptoms of nitrate poisoning, but in nitrate poisoning the red blood cells cannot take up oxygen, causing blood to be a dark chocolate brown color. The symptoms include: anxiety, weakness, labored breathing, and death. Many times the first symptom observed by producers is dead animals.

The plants that have the most issues with prussic acid poisoning are (in order of susceptibility) grain sorghum (most), johnsongrass, sorghum-sudan hybrids, and finally pure sudangrasses. Millets (such as pearl millet) do not produce prussic acid, but are nitrate accumulators.

Prussic acid is higher in younger plants than older plants, higher in leaves than stems, and higher in upper leaves than lower leaves. This means the volunteer in the newly planted small grain fields are likely to have the highest levels.

What to do…

  • Do not allow animals to graze fields until plants reach 18 to 24 inches.
  • Do not graze drought damaged plants until 4 days following a good rain.
  • Do not graze frost damaged plants during the first 7 days following a frost or until plants are completely dried out and brown in color.
  • Frost damaged plants that are not killed by the frost can be grazed within 4 days if they are greater than 30 inches tall.
  • Frost damaged plants that are not killed by the frost and are < 30 inches tall should have cattle kept off for 10 to 14 days.

The great weather for wheat pasture this fall will make it hard to keep cattle off until the volunteer sorghums and sudangrasses are completely dried out. Hopefully, the danger will be short lived and we can take advantage of our outstanding fall pasture.

Frost damaged johnsongrass in a waterway next to a wheat field south of Mulhall.

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