While most of the state will continue to have moderate fall temperatures this week, there are a few regions with frost risk on Thursday morning. This means it’s time for our annual sorghum species prussic acid reminders.
What is prussic acid?
Prussic acid is a cyanide-containing compound that forms from a compound called dhurrin, which is part of the defense mechanisms found in sorghum species. When cells are damaged from frost (or occasionally other stressors like sudden high insect feeding), chemical reactions in the plant cells cause dhurrin to convert into prussic acid. When mammals consume prussic acid, it can disrupt oxygen transfer and may be toxic even at relatively low levels (500-750 ppm cyanide in feed on a dry matter basis).
Which plants have a prussic acid formation risk?
Plants in the sorghum family have the risk of forming prussic acid after frosts. This includes forage species like sorghum and sorghum-sudangrass and also weeds like johnsongrass and shattercane.
How can you reduce risk of prussic acid poisoning?
Prussic acid is highest in the week following a frost, and concentrations are typically higher closer to the base of the plant. Reduce risk by not allowing animals to graze sorghum species in the week following a frost or when sorghum species plants are shorter than 18”.
Stored forages have a lower prussic acid risk than grazing. Ensiling can reduce dhurrin by over 50% and is generally the best option for reducing prussic acid poisoning risk in sorghum plants with suspected high dhurrin concentration. Historically it was thought that sun exposure during drying eliminated prussic acid risk in hay, but recent research from elsewhere in the Midwest indicates that dhurrin content is stable for at least 2 months in dry samples. While some dhurrin will break down over time in storage, remaining dhurrin can be remobilized from dry hay and converted into cyanide containing compounds in rumen fluid. Learn more about management options for reducing prussic acid risk from this factsheet from Iowa State University Extension (also linked in the Further Reading section).
What symptoms should you watch for?
If you have cattle grazing on sorghum species during the fall, watch for symptoms such as shortness of breath, staggering, or other indications of poor oxygen transfer. In pastures with unknown dhurrin levels, consider turning out only one animal at first to limit risk of losses.
Are sorghum species safe for ruminants?
Yes, despite the prussic acid risk, sorghum species are a safe, reliable forage option when best practices are followed. Sorghum and sorghum-sudangrass establish well and are very water-efficient in Ohio’s hot summer weather, which can greatly benefit many producers during the summer slump for cool-season grasses. In order to limit risks, only allow animals to graze sorghum species that are taller than 18” and remove animals from sorghum-based pastures when frost occurs.
Further Reading:
Gruss, Shelby M., Keith D. Johnson, Manoj Ghaste, Joshua R. Widhalm, Sandy K. Johnson, Johnathon D. Holman, Augustine Obour, Robert M. Aiken, and Mitchell R. Tuinstra. 2023. Dhurrin Stability and Hydrogen Cyanide Release in Dried Sorghum Samples. Field Crops Research 291 (February): 108764. https://doi.org/10.1016/j.fcr.2022.108764
Management Strategies of Prussic Acid Toxicity in Sorghum. 2025. https://shop.iastate.edu/cropr3199.html
Ohio Agronomy Guide, 16th Edition. 2024. https://extensionpubs.osu.edu/ohio-agronomy-guide-16th-edition/