Growing Beef Newsletter
July 2025, Volume 16, Issue 1
Prussic acid management
Shelby Gruss, ISU extension forage specialist, and Scott Radke, DVM, MS, DABVT Clinical Associate Professor
“As we reach halfway through summer, many producers may be preparing to harvest or graze summer annuals. These annuals are valuable tools for mitigating the "Summer Slump" commonly experienced in cool-season pastures. Summer annuals, including forage sorghum, sorghum × sudangrass, and sudangrass, offer excellent forage options due to their drought tolerance, high yield potential, and nutritional value when managed correctly. However, proper management is essential to minimize the risks associated with prussic acid.
Prussic acid, also known as hydrogen cyanide, is released when plant cells are damaged through chewing or mechanical actions, leading to the breakdown of the compound dhurrin into prussic acid. While other species, such as cherry trees, can also produce prussic acid, dhurrin is specific to sorghum species. Additionally, wild sorghum weeds, such as johnsongrass and shattercane, can present significant toxicity risks. Understanding factors influencing prussic acid potential helps ensure safe forage use.
Prussic acid potential typically varies by species, with forage sorghum having the highest potential, followed by sorghum × sudangrass, and sudangrass with the lowest. Weed species like johnsongrass and shattercane also pose high risks. Varieties and hybrids within species can differ significantly; consult your seed dealer about specific varieties to manage these risks effectively.
Plant maturity and specific plant tissues also influence prussic acid levels. Younger plants (less than 18 inches tall) have elevated prussic acid potential, which decreases as the plants mature. Leaves contain higher levels of prussic acid compared to stems, with young leaves at the top of the plant having the highest concentration.
Environmental conditions further affect prussic acid potential. Frost events pose significant risks for producers, particularly in the Upper Midwest, by elevating prussic acid levels temporarily after a frost. Additionally, environmental stresses such as drought and high nitrogen fertilization can elevate prussic acid risks, although these conditions typically result in increased nitrate levels rather than prussic acid alone.
Proper management strategies can mitigate prussic acid risks, ensuring safe utilization of sorghum species as forage. Here are a few key guidelines:
- Do not graze plants until they are taller than 18 inches.
- Following a frost, remove animals and wait at least one week before grazing again. Each subsequent frost resets this waiting period until a killing frost occurs. After a killing frost, wait another week before grazing.
- Grazing poses the greatest risk since animals selectively graze young leaves. Thus, avoid grazing plants that are immature (<18 inches tall) or frost-stressed.
- Ensiling is considered one of the safest methods for using sorghum species, as chopping and ensiling processes can reduce prussic acid potential by 50% or more.
- While hay was historically believed to minimize prussic acid risk, recent insights suggest this may not be reliable. Therefore, ensure plants are taller than 18 inches and avoid harvesting immediately after a frost. Additionally, drying hay adequately in Iowa's humid conditions is challenging, making other methods like ensiling preferable.
Following ingestion of forages that contain high concentrations of cyanogenic glycosides (CG), the onset of clinical complications is fairly rapid and is likely to occur within minutes to hours. Severe respiratory distress characterized by heavy, strained breathing may be observed. Anoxia within tissues as a result of the body’s inability to utilize oxygen may lead to ataxia and tremors with possible progression to seizures.
Provided these clinical signs, cyanide should be considered as a potential differential in cases involving neurologic signs and acute death. Although such signs may be observed, cattle are usually found dead due to the rapid nature of cyanide. The rapidity of onset is exemplified by cases in which cattle observed to graze such suspect forages have been reported to collapse and die after several steps.
Photo: Heart from cow following suspected cyanide poisoning after consuming young johnson grass after a rain.
Due to the acute nature of intoxication, lesions are not commonly observed. Hemorrhage around the heart is non-specific but has been noted in cases of cyanide intoxication. Bright cherry red blood and an almond odor emanating from the rumen are two gross observations commonly reported to be associated with poisonings, according to the literature. However, a diagnosis should not be solely based on these observations, or lack thereof. Determining if the blood is “cherry” red is entirely subjective in nature, and not all practitioners may be able to smell the odor in question.
In suspected cases of cyanide poisoning, blood, sealed frozen rumen content, and liver should be submitted for analysis. Laboratories should be contacted to determine whether or not testing is offered on these samples. Suspected forage should also be submitted.
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