Wait 5 Days to Graze after First Freeze to Avoid Prussic Acid Poisoning


By Brent Bean, Sorghum Checkoff Agronomist

The first freeze is nearing. With that comes the possibility of prussic acid accumulating in sorghum.

Cattle and horses are susceptible to prussic acid poisoning. However, if precautions are taken, prussic acid will disappear and cattle and horses can safely consume sorghum forage, either through grazing standing stalks or by consuming hay or silage.

What is Prussic Acid?

Production of prussic acid, also known as hydrogen cyanide, occurs naturally in all types of sorghum when the plant tissue is damaged from a freeze or drought. This damage causes dhurrin, a product found in sorghum, to come into contact with enzymes, producing prussic acid.

Although all sorghum types can produce prussic acid, sudan grasses produce the least. Grain sorghum and Johnson grass produce the most. Accumulating almost exclusively in the leaves, new leaf growth can be particularly high in prussic acid. Very little can be found in the stalk and none can be found in the grain.

How can I avoid it?

After the first hard freeze, it is suggested to wait at least five days before grazing. By that time, the prussic acid in the dead plant tissue will be released into the atmosphere making the forage safe to consume. Grazing sorghum following a light non-killing freeze poses the greatest risk, since any new growth can be particularly high in prussic acid.  If cattle or horses have been grazing sorghum prior to the non-killing freeze, they should be removed immediately and not allowed to graze the sorghum until five days after a killing freeze has occurred.

To reduce the risk of poisoning from prussic acid, feed cereal grain to animals prior to releasing them to graze. This will give the animals more time to adjust to the prussic acid levels and provide more time for the prussic acid to dissipate to safe level.

When consuming hay or silage, prussic acid is rarely a problem because the acid has had time to dissipate as a gas. However, if prussic acid levels were particularly high when the sorghum was harvested, the hay or silage should be tested prior to feeding.

When collecting samples for testing, it is best to collect several stalks with leaves from different areas of the field.  Samples should be kept cool and transported to the lab immediately.  Prussic acid is considered toxic if it contains more than 200 parts per million (ppm) on a wet weight basis.  Anything less than 100 ppm is considered safe.

Although  prussic acid is a common concern this time of year, it is seldom a problem if these precautions are taken.