Grazing Livestock

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.


Nitrate accumulates in plants when poor growth conditions prevent the nitrate from being assimilated in the plant, including long periods of cloudy weather, cold temperatures, drought and mechanical damage. Nitrate accumulates in plant stalks, especially the lower portion. Nitrate levels do not dissipate in hay and will at least partially dissipate in silage (50%). Nitrate poisoning occurs when cattle consume more nitrate than they can assimilate.


Conditions Nitrate Prussic Acid
Plant parts affected Lower Stalk Young growth, new growth
Types of plants All plants, especially sorghum and pigweed Sorghum
Grazing problems Only when animals eat lower stalk Occur early in grazing period or anytime new growth is present
Death occurs 4 hours of consumption Within minutes
Affect of haying None, concentration stays the same Dissipates when properly cured

Prussic Acid After A Freeze

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.

Prussic Acid After Drought

Do not graze any sorghums that have been subject to drought or injury unless they are tested for hydrocyanic acid. After plants have grown rapidly, such as shortly after a rain or irrigation on previously drought-stressed fields or warm weather after a cool period, wait at least two weeks after the plants begin to grow before grazing.

Prevent Prussic Acid Poisoning

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 a 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.

Managing Forage With Nitrate Poisoning

Avoid grazing or harvesting lower stalks on risky fields. Monitor nitrate levels in lower stalks. Generally, nitrate levels will drop significantly 3-5 days after a good rain. It is best to harvest in the afternoon rather than in the morning.

Lab 1, PPM
Nitrate (N03)
Lab 2, PPM
Nitrate Nitrogen (N03-N)
0 – 3,000 0 – 690 Generally safe for all cattle
3,000 – 5,000 690 – 1,150 Generally safe for non-pregnant cattle. Low risk of early term abortions. Dairycattle ration should be less than 2500 ppm.
5,000 – 10,000 1,150 – 2,300 Some risk to all cattle. May cause late term abortions and weak newborn calves. May decrease milk production.
> 10,000 > 2,300 Potentially toxic to all cattle.

Prevent Nitrate Poisoning

To avoid nitrate poisoning, producers should never graze animals on possibly high-nitrate forages. A combination of poor body conditions and high nitrate levels can be deadly. Hay should be tested before feeding if high nitrate levels are in question. If the hay tests high in nitrate, feed together with an energy supplement or with a low protein forage. After proper fermentation, nitrate levels in hay are reduced by 40-60%. However, high-nitrate forages can produce nitrogen dioxide, so proper precautions must be taken when handled by humans.