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Prussic Acid Poisoning

As the first frost date approaches, producers often have concerns about the risk of prussic acid poisoning in livestock. Certain forage plants, especially sorghums and related species are associated with an increased risk of death loss because of prussic acid poisoning. Understanding how poisoning occurs and what factors are involved in contributing to those conditions will help producers take management steps to minimize their risk.

What is Prussic Acid and How Does Poisoning Occur?

Prussic acid, also known as hydrocyanic acid or cyanide, is a rapidly acting, lethal toxin. Prussic acid inhibits oxygen utilization by the animal at the cellular level resulting in suffocation. Ruminants are more susceptible because the rumen microbes have enzymes that release the prussic acid in the digestive tract. Death often occurs within minutes of exposure.

Some plants, particularly sorghums and sudangrass, accumulate cyanogenic (prussic acid producing) glucosides in the outer tissue layers of the plant. The enzymes that would trigger the prussic acid production and located in other plant tissues, specifically the leaf.

Under normal conditions these is no contact between these compounds and therefore no risk of poisoning. However, any factor that causes the plant cells to rupture and these compounds to combine can lead to prussic acid release. The damage could be caused by frost and freezing, or anything else that leads to cell rupture such as crushing, trampling, chewing, or chopping.

What Factors Increase Risk?

Cyanogenic glucosides that can lead to prussic acid formation are found in the greatest concentration in the leaf portion of young, rapidly growing plants. New regrowth following drought, grazing, or any other form of environmental stress is often dangerously high in prussic acid. Plants grown in soils with high levels of nitrogen, but low levels of phosphorus and potassium are also a greater risk.

Cyanide is a gas and will gradually dissipate as frosted tissues dry. Waiting seven days or more for gases to completely leave plant tissues greatly reduces the risk of loss under grazing conditions. Prussic acid content also decreases greatly during hay curing or the ensiling process. Most losses in grazing conditions occur when hungry or stress animals consume young plants or regrowth.

What Levels Are Safe to Feed?

It is difficult to give precise answers as to what levels might cause problems due to variation between areas of a field, from one sample or plant to another, and because of differences in amount (and speed) of forage intake. These values should be interpreted as general guidelines.

Table 1. Level of prussic acid in forage (dry matter basis) and potential impact on livestock.
HCN, ppm (dry matter basis) Effect on Livestock
0 - 500 Generally safe
600 – 1000 Potentially toxic, should not be the sole source of feed
Greater than 1000 Dangerous to cattle, do not feed

How Can I Reduce the Risk of Loss?

Here are some suggested management steps to reduce the risk surrounding grazing or feeding forages with the potential for prussic acid production.

  • Do not graze sudangrass, sudangrass hybrids, or sorghum until the plants are at least 18 to 24 inches tall.
  • Be especially cautious grazing short regrowth that occurs after grazing, harvesting, or a light frost.
  • Never turn out cattle that are hungry.
  • Do not graze susceptible forage crops following a series of light frosts. Wait 7 to 10 days (or longer) before grazing under those conditions.
  • Defer grazing after a killing frost until the plant has dried, usually about 7 days.
  • Harvesting as hay or silage usually results in lower concentrations of prussic acid compared to fresh samples. Test any suspect hay or silage samples before feeding.