Beware of prussic acid poisoning
The sorghum family of summer annual forage and grain crops, including sorghum, sudangrass, sorghum/sudan hybrids, and Johnson grass, produce abundant forage for use during summer and fall. A potential problem using this family of crops as forage is the potential for poisoning ruminant livestock with prussic acid.
These plants contain non-toxic compounds called cyanogenic glycosides. The problem is that these compounds can be degraded under certain conditions to hydrocyanic acid (HCN), or prussic acid, which is cyanide gas. The primary times of concern are when the plants are either immature or have been stressed or damaged by conditions such as drought or frost. The HCN potential differs among members of the sorghum family. In general, both grain and forage sorghums have the highest levels, while sudangrass has the lowest levels, and sorghum by sudan hybrids are intermediate. Additionally, there is variation among varieties and hybrids within each class.
The glycosides are most concentrated in immature plant material, including young leaves and the growing points of the plant. As the plant grows and matures, the level of glycosides is diluted in the older plant material. However, various stressors can cause plants to have elevated levels of toxic potential. Drought is one stress that can cause elevated cyanogenic glycosides. This is primarily because drought limits growth and prevents the plant from maturing to the point of diluting the glycosides.
Another important stressor that can elevate the toxicity of sorghum forages is frost. First, frost causes the rupture of cell membranes within the plant, allowing the cyanogenic glycosides to mix with enzymes that break them down to toxic HCN. Thus, it is important to allow several days (a week or more) for the cyanide to dissipate before allowing animals to graze after a frost. Second, sorghum plants often initiate regrowth after a light frost. This regrowth is new, immature leafy forage that has concentrated glycosides just like immature original growth.
There is much greater potential for poisoning from grazed than harvested forages. Harvesting, both for hay and silage, causes rupturing of plant tissue. This allows formation of HCN, but the time from harvest to feeding allows the HCN to dissipate so toxic levels are dramatically reduced. However, with grazed forage, the rupture of plant tissue and release of the HCN happens in the mouth and stomach of the animal, where it is absorbed and leads to rapid signs of toxicity. Because small amounts of HCN are highly toxic and act quickly, animals often die soon after being introduced to sorghum crops and before symptoms are observed.
In general, a variety of practices can help to reduce the risk of HCN poisoning when grazing sorghum crops:
1. Choose sudangrass or sorghum by sudan hybrids that have been bred for reduced levels of cyanogenic glycosides.
2. Allow sudangrass to grow to a minimum of 18 inches height and all other sorghums to at least 24 inches to allow adequate plant growth and maturity to dilute the cyanogenic glycosides.
3. High levels of nitrogen and low levels of phosphorous in the soil contribute to elevated HCN potential. Fertilization should be based on a soil test. Even them, use nitrogen fertilizer conservatively.
4. Feed cattle so their stomachs are full before introducing them to a new pasture of sorghum forage to limit their initial intake. Observe them for signs of HCN poisoning, which include excitability, agitation, elevated heart and respiration rates, gasping, tremors, foaming at the mouth, and convulsions, which are quickly followed by death. Be prepared to quickly remove them if symptoms present. Some people have suggested initially turning in a few test animals rather than your entire herd.
5. Because cattle are selective grazers and will tend to eat the youngest leaves and therefore most toxic parts of the plant, using rotational grazing with relatively high stock densities to force them to consume the less toxic parts of the plant with the most desirable parts.
6. Do not graze sorghum forages when we enter the time that frost is possible. Grazing can resume once hard freezes have killed all regrowth and a week or more have passed so the HCN has dissipated.
The sorghum family of summer annual crops can produce large quantities of palatable and nutritious forage. With careful management, the potential toxicity from prussic acid can be managed so these crops can be safely utilized to support livestock.
Ken Olson is an SDSU Extension Beef Specialist.