Forage sorghum, sorghum-sudangrass, and sudangrass are important forage crops throughout the United States. While sorghum is a valuable forage crop, sorghum species can produce prussic acid, which can be toxic to livestock. Prussic acid, also known as hydrogen cyanide (HCN), can cause acute toxicity and death. Potential of HCN toxicity (HCN potential) is directly related to dhurrin content, which is the precursor to HCN. Dhurrin is broken down through cell disruption, such as chewing or freezing, which causes rapid HCN release. This week, we will look at the characteristics and causes of HCN. Next week, we will look at feeding and testing options.
Characteristics affecting dhurrin content and hcn potential
Species/Varieties - Dhurrin content in sudangrass is about 40 percent less than in most other sorghums. As a group, the sorghum-sudangrass hybrids have more HCN potential than sudangrass and forage sorghum has more HCN potential than sorghum-sudangrass and sudangrass. Johnsongrass, shattercane and sorghum have high HCN potential, and may be hazardous when found in pastures and fence rows.
Plant Age - Sorghum age has a significant effect on dhurrin content. Plants have larger amounts of dhurrin in the early growth stages or early regrowth. Older plants, however, can increase HCN potential as a result of environmental factors, as described below. Delay grazing until the plants have reached a height of 18 to 24 inches to avoid HCN toxicity under good growing conditions.
Tissue Type - The vegetative portion of all sorghums can contain dhurrin. HCN potential varies within a single sorghum plant depending on the plant tissue. Tillers, commonly called shoots, can contain a large amount of dhurrin, too. Examine fields for the presence of young tillers on more mature sorghum plants.
Drought - In the western and southern United States, ingestion of sorghum species during drought can cause cyanide toxicity in livestock. Nitrate levels also can be high in drought-stressed plants.
Frost - Sorghum can become stressed in frost conditions leading to an increase in dhurrin. It is important to remove livestock from sorghum when a frost is predicted. After a frost, keep livestock off frosted sorghum plants for at least one week. This recommendation is a rolling suggestion, meaning that every time another frost occurs, livestock should be removed from the frosted plants for another week, until the plants have been completely killed with a hard frost. After a non-killing frost, sorghum should be scouted for regrowth because it can contain a large amount of HCN. If regrowth is apparent, you should wait 10 to 14 days or until the regrowth has reached 24 inches tall before grazing or chopping.
Nitrogen Application - Overfertilization with nitrogen can cause the crop to be toxic by two different mechanisms, HCN potential and nitrate concentration.
Alicia Boor is the Agriculture and Natural Resources agent with K-State Research and Extension – Cottonwood District. Contact her by email at aboor@ksu.eduor call 620-793-1910.
