Upper leaves contain more prussic acid than older leaves. Thus, prussic acid content of sudangrass, sorghum, and sorghum-sudangrass hybrids 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 only slightly with plant maturity if animals selectively graze the more tender new growth - those plant parts high in prussic acid. 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 reached 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 high in prussic acid than sudangrass. As a group, sorghum-sudangrass hybrids also contain more prussic acid than sudangrasses. Some varieties tend to be lower in prussic acid than others. Growers should select varieties with lower prussic acid potential. The free prussic acid in this forage does not begin to decline until thawing and wilting begin.