Upper leaves contain more prussic acid than older leaves. Thus, prussic acid content of sudangrass and sorghum is highest in the earlier stages of growth. As plants mature, the stalk content increases, causing the prussic acid content in the total forage to decrease. Sudangrass should not be grazed or green chopped until it has reached a height of at least 18 to 20 inches. Sorghum-sudangrass hybrids should not be grazed or green chopped until they have reach a height of at least 24 to 30 inches.
Sorghum is generally unsafe for pasture or green chopping until after plants reach maturity.
Species and Variety Sorghums are generally much higher in prussic acid than sudangrass. As a group, sorghum-sudangrass hybrids also contain more prussic acid than sudangrasses. Some varieties, such as Piper and Trudan sudangrasses, tend to be lower in prussic acid than others. Growers should select varieties that tend to be lower in prussic acid potential. Frost Prussic acid is released very quickly from the glucoside form in frozen leaves, and hence frosted sudangrass can be very dangerous until it has dried out.