Sandy Johnson, extension beef specialist, Colby and John Holman, cropping system agronomist, Garden City
Plants in the sorghum family are known to be a potential risk for prussic acid (also known as hydrogen cyanide, HCN) poisoning. Many plants contain a compound known as dhurrin that, when exposed to enzymes, is converted to hydrogen cyanide. The hydrogen cyanide interferes with oxygen transport and, if enough is consumed, can quickly result in shortness of breath, convulsions, and death.
The good news is that Dr. Mitch Tuinstra’s lab at Purdue University identified a dhurrin-free variety of forage sorghum. A limited amount of this genetic material was available for purchase this year, and availability should increase in the future. However, this technology is currently licensed to only one company, and that company is liquidating, so that could slow the availability of this technology to producers. Tests with the dhurrin-free variety have illustrated its high palatability. Both in tests with grazing sheep and cattle and the need to fence rabbits out of test plots. In fact, in KSU test plots rabbits have preferentially fed on this particular dhurrin free variety compared to other varieties. Part of the role of dhurrin in plants is as a deterrent to insect feeding by facilitating the release of HCN. The dhurrin is converted to HCN when cells are disrupted by chewing or freezing. So far in research plots an increase in insect feeding or damage was not observed.
A common misconception about prussic acid, or perhaps a weakness in educational efforts, is that when a plant dries down (after a hard frost or as hay dries), the HCN is formed as the cells rupture, but dissipates after a period of time. However, if levels are very high before the dry down, the prussic acid levels do not always decrease to a safe level.
Because tracing some of the literature on this issue has been hard to come by, we ended up with some good material to generate some of our own data. A field of forage sorghum was cut in early August, at 3 to 4 feet tall. A nice rain after that allowed for some regrowth, but conditions remained dry through the fall and winter.
Since regrowth is known for higher levels of prussic acid, and drought stress contributes, this field was suspected to contain very high levels. Samples of regrowth from across the field were collected, chopped and split into 18 subsamples for analysis at 6 different time points. One set of samples was shipped directly to the lab for analysis, and the remaining samples were dried and delivered to the lab over the next several months. Samples were stored at room temperature after drying and before being delivered to the lab.
Figure 1 below shows the prussic acid content of 3 samples at each time point. Samples collected in October still contained lethal levels in April.
Samples from the remaining standing material were collected in triplicate in November (1787 mg/kg), December (844 mg/kg) and January (1150 mg/kg). Every attempt was made to collect similar material to the composite sample initially collected, but the samples could not be as consistent as those that were dried and stored. Prussic acid levels decreased over time in the field more than the sample stored at room temperature, but samples in the field were still at a level to cause toxicity.
The important point to this data is that if prussic acid is potentially high when you harvest forage, 1) sample some regrowth or the smallest plants prior to harvest and 2) test the forage prior to feeding. If a field only had tillers or regrowth in a small area, testing a composite sample of 20 or more bale cores may miss it. However, if you look for tillers or regrowth that are of a height that would become part of a bale, testing that material directly could be very valuable. If regrowth is very limited the risk of prussic acid will be greater and may need to avoid grazing this regrowth even after a freeze event.
Look for more information on dhurrin-free varieties when you purchase seed. The dhurrin-free variety (SPDF708 PF) was part of the KSU 2024 & 2025 forage variety tests, and that information is available here (https://www.agronomy.k-state.edu/outreach-and-services/crop-performance-tests/forages/hay-and-silage/). If not growing a dhurrin-free variety, see Managing the Prussic Acid Hazard in Sorghum for more information on how to safely manage around this potential risk.