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OSU Extension

College of Food, Agricultural, and Environmental Sciences

October 6, 2020 - 8:00am --

With the arrival of fall and cooler temperatures, I would like to remind farmers who have grazing livestock or who will be taking a final cutting in the next month or so to be aware of some potential health concerns related to frost-damaged plants.

Below is a summary based on an article by Mark Sulc, OSU Extension Forage Specialist, where he addresses nitrate toxicity and prussic acid toxicity.

Nitrate toxicity becomes more of an issue in drought conditions and also when the forage is in an active growing state when a heavy frost arrives. This is common in grasses like fall planted oats and other small grains, millet and sudangrass. The risks are lessened when grazing livestock compared to greenchop or hay that is cut immediately after a frost. Nitrate levels can be tested at most labs. A forage sample analysis is worth the vet bills or potential fatality of livestock.

Prussic acid toxicity is a potential in several forage species and some weeds. These plants convert naturally occurring compounds into hydrogen cyanide (prussic acid) when freeze damage occurs. Young plants that are frosted will have the highest proportionate levels. Risk is typically highest right after the first hard frost.

Sulc lists several forage species that have potential to be toxic, starting with the most likely. Grain sorghum, Indiangrass, Sorghum-sudangrass, Sudangrass hybrids, Piper sudangrass, and least likely, Peral millet and foxtail millet.

A few plant species not intentionally grown for livestock consumption that may contain toxic levels of prussic acid include Johnsongrass, Shattercane, Chokecherry, Black cherry, and Elderberry.

Soils with high fertility levels are more likely to increase the level of toxicity in these plants.

Forages made into hay or silage will have a very low risk for prussic acid poisoning as the hydrogen cyanide dissipates during dry-down.

Symptoms in livestock include “excess salivation, difficult breathing, staggering, convulsions, and collapse.” (Sulc) When lethal doses are consumed, livestock are likely to perish within 1-2 hours.

 Lab analyses that report prussic acid levels between 600-1000 ppm are potentially toxic and should be avoided or mixed with other feed sources to dilute the concentration below 500ppm. Levels over 1000 will result in cattle fatality. Prussic acid poisoning is a greater concern in our ruminant species (sheep, goats, cattle) and less of a concern with pigs or horses.

Additional precautions taken from Sulc’s article are below.

  • “Making hay does not reduce nitrate levels in the forage, but the hay can be tested and diluted sufficiently with other feeds to make it safe for animals.
  • Ensiling forage converts nitrates to volatile nitrous oxides, or ‘silo gases’. These gases are highly toxic to humans. Safety practices include removing tarps from a portion of the silo a day or two before removing the silage from the bunker.
  • Do not graze on nights when frost is likely. High levels of toxic prussic acid are produced within hours after a frost, even if it was a light frost.
  • Do not graze after a killing frost until plants are dry, which usually takes 5 to 7 days.
  • After a non-killing frost, do not allow animals to graze for two weeks because the plants usually contain high concentrations of prussic acid.  
  • New growth may appear at the base of the plant after a non-killing frost. If this occurs, wait for a killing freeze, then wait another 10 to 14 days before grazing the new growth.
  • Do not allow hungry or stressed animals to graze young growth of species with prussic acid potential. To reduce the risk, feed ground cereal grains to animals before turning them out to graze.
  • Use heavy stocking rates (4-6 head of cattle/acre) and rotational grazing to reduce the risk of animals selectively grazing leaves that can contain high levels of prussic acid.
  • Never graze immature growth or short regrowth following a harvest or grazing (at any time of the year). Graze or greenchop sudangrass only after it is 15 to 18 inches tall. Sorghum-sudangrass should be 24 to 30 inches tall before grazing.
  • Do not graze wilted plants or plants with young tillers.”

While horses are at lower risk for prussic acid poisoning, they can develop founder or colic as plant carbohydrates are converted into sugars after a frost. The suggestion is to wait one week after a killing frost before returning horses to pasture. Additionally, Bruce Anderson (University of Nebraska) reports that between 1.5-3 pounds of fallen maple leaves can be harmful to horses.


Matthew Nussbaum is an OSU Extension Agriculture and Natural Resources Program Assistant and may be reached at 330-264-8722.

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