Final answer:
The grazing of young sorghum-sudangrass hybrids can lead to prussic acid poisoning due to the higher concentrations of hydrogen cyanide in plants less than 18-24 inches tall.
Step-by-step explanation:
The starting height for grazing sorghum-sudangrass hybrids should not be less than 18-24 inches because of the risk of prussic acid poisoning. Prussic acid, also known as hydrogen cyanide, is found in higher concentrations in young or stressed plants. When these plants are too short, they can contain dangerous levels of prussic acid that, when ingested by animals, can be rapidly absorbed into the bloodstream and potentially lead to respiratory failure and death. Allowing the plants to reach the recommended height minimizes this risk.