Is there cyanide in my pasture? Absolutely.

Each year I get many calls and questions about just how safe or dangerous is it to have Johnsongrass in a pasture when a frost occurs.

The short answer is it can be very dangerous for your livestock, potentially even deadly. The longer answer is the rest of this article.

Johnsongrass facts

• It is a member of the sorghum family.

• It is a perineal plant.

• It is a warm season plant, which means it loves summer and dies back or goes dormant during cold temperatures.

• Johnsongrass is very drought-tolerant.

• It can grow back and grow new plants from being cut or eaten or froze back from its roots.

• It also can and does spread by seeds.

• Cattle love to eat it. The best way to get it out of a field is to let cattle graze that field. They will eat it until it cannot sprout back.

• Johnsongrass is not native to the United States; it was brought here as a fast-growing, nutritious forage plant and to help stop erosion by its fast growth and reproduction abilities.

• Johnsongrass produces prussic acid, or cyanide, when it is stressed.

Wait a minute; what? Johnsongrass makes cyanide, and cattle love to eat it? Then why are all the cattle that eat it not dead? It has to do with when they eat it.

Typically, the grass is perfectly safe for animals to eat, no prussic acid present. But we can potentially see the worst effect when the plants get frosted upon. This stresses the plant and that triggers the production of prussic acid. Now the big question is: How long does the cyanide stay in the plant?

That is both a simple question with a simple answer, and a much more complex one.


First, the easy, short answer for making dry hay. If there is a killing frost, and the Johnsongrass turns completely brown, it is dead. If this is the case, you can cut it at any time you want. It just needs to lay in the field for two or three days before baling as dry hay. You do not have to wait any amount of time to cut it, just let it cure before baling.

This is not different than you would normally treat most hay this time of year, since with the cooler temperatures and reduced sunlight, it will take that long for those thick stems to cure anyway.

The important thing to remember is it does not make any difference how the Johnsongrass is killed, killing frost or mowing, it is still 72 hours after death that it is safe for the animals to eat again.

If you are making haylage that will contain Johnsongrass, treat it as you normally would for making any grass into haylage. Again, given the temperatures and sunlight this time of year, it will dry slower than in the heat of summer.


Again, if the plants are killed by frost, the animals must not eat it for a minimum of 72 hours to give the cyanide time to dissipate. If there is a lot of Johnsongrass in the pasture, and not a lot of other desirable forages for them to graze, you may need to take them out of that pasture for three or four days.

Now this is where the answer can get a bit more complex. If the frost is not a killing frost, but instead of what I call a burn back frost, the wait is potentially much longer. If we get a burn back frost, it severely stresses the plant, which causes it to make cyanide, as does a killing frost. However, if the plant is not killed, the toxin can stay in the plant for as long as 10 days to two weeks.

So, with a burn back frost, we need to keep the animals from eating the stressed plant for at least 10 days and preferable two weeks. This may mean moving them to a Johnsongrass free pasture, if you can find one. Or put them in a lot and feed some hay for two weeks. Either way is preferable to having one or more of the animals die.

I would think that so far things are still pretty simple. Now, let’s complicate them. Say we have a burn back frost, and we move the cattle out of the dangerous pasture. Six days later we have a killing frost, do we still need to keep the animals out of that pasture for another eight days?

The answer is no. When the plant is killed, no matter what happened before, the clock starts over, and it is for the 72 hours. This is the same as if we cut it after a burn back frost. After we cut it, the clock starts the 72-hour countdown, and we can forget about the remainder of the 10 to14 days.

When it is damaged but not killed, wait 10 to 14 days to graze it. When it is killed, by frost, freeze or cutting it, 72 hours if the magic number.

This also applies to sorghum-Sudan grass. Since they are in the same family, they both create cyanide when stressed. So the same time frames apply when using it.


Stressed Johnsongrass affects other ruminants such as sheep and goats, but also horses, donkeys and others of the equine ilk.

The danger to ruminants is cyanide and nitrate poisoning. To horses, the dangers are much different.

For horses, the danger actually lies more when we are in drought situations and Johnsongrass is the predominant grass in the field as others have died or gone dormant, and lots of hay is not offered for them to eat.

The issues this causes in horses are, predominantly, neuropathy and a thing called teratogenesis (damaging effects to a fetus). Cyanide and nitrate poisoning are very, very rare in horses.

The effects of horses eating stressed Johnsongrass occur after a few weeks to months of continuously eating Johnsongrass or other types of sorghum, at any stage of growth. So because of this accumulated affect, hay that contains Johnsongrass, or sorghum/Sudan grass, is not recommended to be fed to horses.

For more information, call your local extension agent. In Boyd County, call (606) 739-5184.

LYNDALL HARNED is the Boyd County Extension Agent.

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