Prussic Acid Risk in Forages

As livestock producers continue to manage through drought, decreasing acres of native pastures, and rising costs, they are continually looking for additional forage options. Forage sorghums, sudangrass, and sorghum-sudan crosses are often used as summer pasture and sometimes fed as silage and hay as an alternative forage source. These crops are great forage options as they offer excellent forage yield coupled with the ability to use them as part of a crop rotation.

One risk of using sorghum crops for livestock feed is under certain conditions, they can present the risk of prussic acid poisoning. This risk can be managed with proper awareness and understanding.

What is Prussic Acid and Why is it a Concern?

Prussic acid is a rapidly acting lethal toxin. It inhibits oxygen utilization by the animal at the cellular level resulting in suffocation.¹ In general, this lethal toxin is enhanced via a reaction through some sort of physical damage such as freezing, chewing, or trampling.

Certain crops contain a higher level of prussic acid in their vegetative tissue making them more prone to cause issues. All sorghum crops contain prussic acid although the prussic acid content of sudangrass is about 40 percent less than other sorghums.² Sorghum-sudan hybrids generally have less prussic acid than straight sorghums, but typically more than straight sudangrass. Pearl millet, which is not a sorghum but is a summer pasture crop, does not contain toxic levels of prussic acid.

Symptoms of poisoning occur rapidly if prussic acid levels are toxic. Symptoms can include labored breathing, staggering, and other struggling for oxygen symptoms.

Factors Affecting Prussic Acid Content

The toxin concentration is most prevalent in newly growing, vegetative tissue (leaves, tillers, suckers). Because of this, the concentrations are higher on plants that haven’t reached maturity. Other factors leading to higher concentrations include:

• Drought – Drought stricken plants are typically mostly leaves and vegetative tissue.
• Young regrowth after grazing or cutting – New shoots and leaves have a higher concentration.
• Frost – Frost may kill a portion of the plant, but new growth may occur in the lower portion leading to higher toxin concentrations.
• Fertility – If high nitrogen rates are applied to soils deficient in potassium or phosphorous, prussic acid levels usually increase.²

Managing and Safe Feeding

Risks of poisoning can be reduced through some simple management steps:

• Do not graze sudangrass, sorghums, or sorghum-sudan hybrids until they are 24 inches tall.
• Never turn animals out hungry.
• Defer grazing after a frost for 6 to 8 days.
• Harvest as hay or silage and wait at least 3 weeks.
• Be cautious of grazing short regrowth and drought-stricken crops.
• Test and send samples to a forage lab to determine prussic acid level.

Conclusion

With careful consideration livestock producers and forage growers can us sorghum crops to take advantage of their high yield potential while still minimizing risks of prussic acid poisoning.

As with any forage program local experts should be consulted to determine the best practice for any given area.

Channel Agronomist
Pat Koenig

Sources:

¹Rusche, W. 2021. Prussic acid poisoning. South Dakota State University Extension. https://extension.sdstate.edu/prussic-acid-poisoning/.

²Rhykerd, C.L. and Johnson, K.D. Minimizing the prussic acid poisoning hazard in forages (Purdue University). Forage Information System. Oregon State University. https://forages.oregonstate.edu/fi/topics/pasturesandgrazing/grazingsystemdesign/prussicacidpoisening

Websites verified 4/20/23. 1221_444551