Avoid prussic acid poisoning

By Steven Huff

UT Cocke County Extension

Avoiding prussic acid poisoning should be a primary concern for cattle producers. Prussic acid is a compound that acts very similar to cyanide and its lethal affects happen very quickly after it is ingested. Livestock are can be exposed to prussic acid through freeze- or drought-stressed forages. The precursor compound is located in different cells than the enzyme that turns it into prussic acid. So, when something like a hard frost makes the cells rupture, they mix and prussic acid accumulates.

In Tennessee, prussic acid poisoning is most often a problem with sorghums (grain and forage), sudangrass, sorghum, sudangrass hybrids and johnsongrass. New-growth leaves produce much more prussic acid than mature leaves or stems. Aside from forages, trees that are commonly found around pasture perimeters also contain prussic acid. These include wild cherry, apple and peach. Make sure to remove broken or wilted limbs and downed trees.

Similar to nitrate poisoning, high application rates of nitrogen fertilizer immediately prior to drought or freeze can increase the risk of prussic acid accumulation. To avoid this risk, split nitrogen applications for high-risk forages. Selective herbicide (ex. 2,4-D) application can increase prussic acid accumulation for several weeks. Fortunately, prussic acid is different from nitrates in that it is a highly volatile compound that will dissipate over time.

According to Southern Forages (one of the best resources for forage management in the Southeast), “if forage having high prussic acid content is ensiled, it will usually be safe to feed within three weeks after silo fill (or wrapping). Hay that is dry enough to safely bale will not have toxic levels of prussic acid.” Waiting at least 7 days before grazing after a killing frost will allow prussic acid to dissipate from standing forages.

Prussic acid affects cattle by interfering with their ability to use oxygen. This happens very quickly in ruminants because the rumen microbes produce enzymes that make it enter the blood rapidly. Cattle that are affected by prussic acid first become excited. Muscle tremors, labored breathing, staggering and excessive salivation soon follow. These symptoms can be treated by a veterinarian but, due to the rapid onset of prussic acid poisoning, cattle that consume it are usually found dead.

The most effective control of prussic acid poisoning is accomplished by managing highrisk forage grazing very closely during late fall and immediately after a drought ending rain.

Going into November of this year, Johnsongrass would be the most likely problem, especially since most of the state has experienced drought conditions. If relatively warm conditions persist into November and rains stimulate some late season Johnsongrass growth (after being stunted by drought), prussic acid concentrations could reach extremely high concentrations after the first killing frost.

Following are some suggested management practices that should be of help to you in replying to questions you might receive from concerned producers:

  • Do not graze “hungry” cattle on sorghum or sorghum-sudangrass hybrids and Johnson Grass following a “killing frost” until the plant has dried which will take from 7 to 10 days.
  • Do not graze short regrowth forage following hay harvest or following a period of “close” grazing until it has an opportunity to dry.
  • Feeding some grass hay or other forage that would be free of prussic acid would aid in reducing the intake of the suspected forage.
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