Avoid Grazing Johnson Grass During the Fall Frost Season

— Written By Glenn Detweiler and last updated by Judy Moore
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Horse owners, producers of cattle, sheep, and goats, and all showmen need to avoid grazing Johnson grass, sorghum, Sudan grass, and Sorghum Sudan crosses during first frosts of the fall. When these plants are both stressed and the cell wall ruptures, two compounds mix causing a reaction which produces prussic acid or hydrocyanic acid (HCN), which is a gas. Once ingested by a ruminant animal, the HCN is released in the stomach and readily absorbed into the bloodstream. HCN does not affect oxygen from being carried by the red blood cells but it is believed to lock the doors of the cell wall at the muscle preventing the cell from getting the needed oxygen to function. This causes the animal to die from asphyxiation at the cellular level. Ruminant animals affected by prussic acid poisoning will exhibit a characteristic bright red blood just prior to and during death. Since horses are not ruminants but grazing animals, they have other symptoms for a long time before death will occur. Equine cystitis, muscle weakness, urinary tract failure, and neural degeneration can be caused by hydrocyanic acid (HCN).

Since these forages are very valuable other times of the year, we can make some observations about the plants and, with this gathered information, develop some management guide lines to utilize these forages without harming our animals. Things to remember include:

  • Any stress condition, such as drought or freeze damage, can increase HCN amounts.
  • There is more HCN in the leaves than stems. The top-most (younger) leaves contain more HCN than lower leaves.
  • HCN tends to become diluted in older plants, but top-most leaves may still contain dangerous amounts.
  • Sun-curing of hay will reduce HCN, especially if hay is crimped. HCN evaporates in a gaseous form.
  • Dryland sorghums tends to be higher in HCN potential than irrigated sorghum due to stress of the plant.
  • High nitrogen rates, regardless of phosphorus level, will increase HCN potential.
  • Imbalance of nitrogen and phosphorus in the soil increases HCN potential.
  • HCN potential in sorghum plants is genetically controlled and varies among types.

With this information let us make some logical management guidelines to allow a proactive approach when grazing prussic acid producing plants.

  • Do not turn hungry cattle onto the pasture. Feed some hay before turning in cattle late in the afternoon.
  • When turning cattle into a field with possible problems, ask your vet stop by incase poisoning occurs. Try turning in only a few animals to see how they do.
  • Prevent selective grazing of lush young regrowth by using rotation grazing.
  • Allow plants to reach at least 18 to 24 inches in height before grazing is allowed.
    (This management practice permits some dilution of the HCN.)
  • Do not graze after frost until all HCN producing plants are field cured (completely dead). It is important to make sure you have had a killing frost because the sorghum plants will initiate tillering after a light frost. These shoots will have very high levels of HCN. It is recommended to not graze for 7-14 days after a killing frost.
  • If HCN is a big problem, check with your seed dealer for varieties or hybrids that have a low HCN accumulation potential.
  • Soil sampling is essential in managing for HCN. Take a soil sample from the area to be planted. Do not apply excessive nitrogen fertilizer. Keep soil phosphorus and pH at an appropriate level for producing sorghum.

Some of the information and bullet points where provided by Mr. Gary Strickland of Cooperative Extension. If you have questions regarding prussic acid in forage plants do not hesitate to call your local N.C. Cooperative Extension Livestock Agent, Glenn Detweiler, at the Extension Office 704-736-8461 or text him at 405-219-1902.