All About Grazing: Be aware of prussic acid and potential damage this fall


After our county fair in August, Athens County hosted the Ohio Forage and Grasslands Council Beef Grazing Tour.

One of the discussions during the tour centered on some patches of johnsongrass in a stockpiled field of fescue.


The host farmer mentioned that he was considering using the field for grazing in the mid- to late October period. There were questions about the safety of livestock grazing the johnsongrass at that time of year.

Johnsongrass is a perennial weed and it is also a warm season grass. It can be a good feed source for grazing animals.

However, this plant is a member of the sorghum family. All plants in the sorghum family have the potential for prussic acid poisoning of livestock. Other familiar forage plants in this family are forage sorghum, sorghum x sudangrass hybrids and sudangrass.


The risk of prussic acid poisoning begins with the fact that plants in the sorghum family contain varying concentrations of cyanogenic glucosides. Cyanogenic glucosides are compounds composed of a carbohydrate or sugar molecule chemically bonded to a cyanide molecule.

Johnsongrass has high to very high amounts of cyanogenic glucosides, forage sorghum and sorghum x sudangrass hybrids have intermediate to high levels and sudangrass has low to intermediate levels.


Cyanogenic glucoside concentration is just a measure of the potential toxicity of the plant. By itself, the cyanogenic glucoside compound is non-poisonous. Toxicity only results when the cyanogenic glucoside compound is broken apart and the cyanide molecule is separated from the sugar molecule.

Prussic acid or hydrocyanic acid (HCN), also known as hydrogen cyanide, is formed when the cyanogenic glucoside compound is broken apart.


An important question is: How do these cyanogenic glucosides get broken apart and form prussic acid? Plants in the sorghum family also contain an enzyme called emulsion. This enzyme can break apart the cyanogenic glucosides and cause the formation of prussic acid.

This enzyme and the cyanogenic glucosides are contained in separate plant tissues. Anything that causes plant cells to rupture can bring these two components together and result in prussic acid formation.

Plant cells can be ruptured by cutting, wilting, freezing, drought, crushing, trampling, chewing, or chopping.


Unfortunately, ruminant animals tend to be more susceptible to poisoning from cyanogenic glucosides than non-ruminant animals. When non-ruminant animals ingest forage with cyanogenic glucosides the forage goes directly into the stomach. The stomach is a strongly acidic environment that reacts with any free prussic acid to form low toxicity substances.

In contrast, when the ruminant animal ingests forage with cyanogenic glucosides, the forage passes into the rumen. The rumen in a forage-based diet is a mildly acidic to mildly alkaline environment.

Prussic acid

In addition, there are rumen micro-organisms and enzymes that contribute to the formation of prussic acid. Prussic acid is very toxic and is rapidly absorbed into the blood. It combines with hemoglobin in the blood to form cyanoglobin, which does not carry oxygen.

Prussic acid poisoning symptoms include an increased rate of respiration, increased pulse rate, gasping, muscular twitching or nervousness, trembling, foaming at the mouth, spasms or convulsions. Death occurs from respiratory paralysis and can happen quickly, within a 15- to 20-minute time span.

Prussic acid accumulation is not equal throughout the plant. It tends to accumulate in leaves as compared to stems and is found in higher concentrations in the upper or younger leaves of the plant as compared to the lower or older leaves.

Poisoning threat

In general, young, immature plants pose the greatest prussic acid poisoning threat to livestock.

It should also be noted that the young growth that follows clipping, drought, frost, or grazing can contain higher concentrations of prussic acid.

Of concern to graziers as we get into the fall of the year is that frost damage to plants in the sorghum family can lead to prussic acid formation. Livestock should not be allowed to graze any plants in the sorghum family immediately following a frost event.

However, because prussic acid is actually a gas, it will dissipate from the plant as the plant dries out and with the passage of time.

Grazing plant recommendations

The general recommendations regarding grazing plants in the sorghum family in the fall of the year are:

• Do not graze on nights when frost is likely. High levels of prussic acid are produced within hours after a frost.

• Do not graze after a killing frost until plants are dry, which is usually five to seven days.

• After a non-killing frost, do not allow animals to graze the frosted plants that are shorter than 30 inches in height for 10 to 14 days. Plants above that height can be grazed after a 4-5 day period.

New growth

New growth may appear at the base of the plant after a non-killing frost. This growth will contain high levels of prussic acid. Do not allow livestock to graze this growth. Wait for a killing frost and then give those plants another two weeks before livestock are allowed to graze.

Stockpiled field

The group on the beef tour at the Athens County farm that I mentioned at the beginning of this article came up with an additional option for the grazier other than to delay his planned grazing.

Since the johnsongrass in the stockpiled field was concentrated along the edge of the field bordering a road, the suggestion was made to just fence the johnsongrass out and continue with the plan to graze in mid- to late October.

This has been a good year for johnsongrass growth and while this weed can be grazed during the summer months, graziers need to beware of the potential of prussic acid poisoning when it is grazed during the fall.

(Rory Lewandowski is an OSU Extension educator in Athens County.)


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