Sudangrass, forage sorghums and sorghum-sudangrass crosses (all in the genus Sorghum) contain a non-poisonous chemical called dhurrin. If these plants are damaged by freezing, chewing or trampling), the dhurrin is converted into prussic acid (cyanide) which is potentially very dangerous for animals eating these crops.

Factors affecting prussic acid content in plants

Species
The vegetative portion of all sorghums contains prussic acid. Generally, however, prussic acid content in sudangrass is about 40 percent less than in most other sorghums. As a group, the sorghum-sudangrass hybrids have more prussic acid than sudangrass. Crosses have now been developed that contain extremely low quantities.

Plant parts
In the sorghums, young leaf blades normally contain higher prussic acid levels than old leaf blades or leaf sheaths or stems. The seedheads are low in prussic acid, and the seeds contain none.

Maturity
Highest prussic acid levels are reached before the boot stage. As plants mature, the stalks make up a greater proportion of the plant, causing prussic acid content in the total forage to decrease.

Drought
Severe drought is probably the most common cause of prussic acid poisoning. Drought-stricken plants are hazardous to feed because they are mostly leaves.

Freezing
Forage is usually considered safe to pasture or feed as green chop five to six days after a killing frost.

Fertilizer
If high nitrogen rates are applied to soils deficient in phosphorus and potassium, prussic acid levels usually increase.

Safe feeding of potentially hazardous forages

Pasture
Sorghum that has wilted and dried five to six days after being killed by frost is considered safe for grazing. The risk of prussic acid poisoning can be reduced by feeding ground cereal grains to the animals before turning them out to graze. The chance of problems on pasture can be further reduced by using heavy stocking rates (four to six head per acre) and rotational grazing to avoid cattle selectively grazing the leaves. If new shoots develop after a frost the crop should not be grazed until this new growth is 2 feet tall.

Green chop
Green chop forage is usually safer than the same material used for pasture because it is not selectively grazed. Whereas in the case of pasture only the leaves may be eaten, with green chop material the total plant is consumed. Stems act as safety devices `diluting’ the high prussic acid content of leaves.

Silage
Sorghum silage is generally safe for feeding. Although it could contain toxic levels of prussic acid while in storage, much of the poison escapes as a gas during fermentation and when being moved for feeding. However, as a precaution, do not feed new silage for at least three weeks after harvesting and storing.

Hay
The prussic acid content of sorghum hay decreases as much as 75 percent while curing and is rarely hazardous when fed to livestock.  PD

—From Penn State Field Crop News, Vol. 7, No. 33

Marvin Hall, Forage Specialist, Pennsylvania State University