As a precaution, plant those hybrids known to be lower in prussic acid. Johnsongrass, shattercane and sorghum almum could contain dangerous levels of prussic acid, and may be hazardous in pure stands or as contaminants in sudangrass or sorghum-sudan cross pastures. Fence rows contaminated with these weedy species might also be hazardous. Pearl millet, another summer pasture crop, does not contain toxic levels of prussic acid. In the sorghums, leaf blades normally contain higher prussic acid levels than leaf sheaths or stems, the heads are low in prussic acid, and the seeds contain none.
Upper leaves have more prussic acid than older leaves.
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. However, the hazards associated with poisoning may decrease only slightly with age if animals selectively graze those plant parts that are high in prussic acid. Severe drought is probably the most common cause of prussic acid poisoning. Drought-stricken plants are hazardous to feed because they are mostly leaves.