The caveat for sorghums is their chance to contain prussic acid (hydrogen cyanide) mostly when the first frost occurs. If the first frost is not a killing frost, more prussic acid will be produced on subsequent frosts until the sorghum plant is completely dead. Prussic acid toxicity occurs with all sorghum family plants, i.e., forage sorghum, hybrid sudangrass and sorghum-sudan. If you are using millet instead of sorghum, no prussic acid will be formed in frosted millet.
The best practice for dealing with frosted sorghum, is harvesting the sorghum and allowing it to ferment (10 days) before feeding it. This will dissipate all the prussic acid. If you are grazing, a more careful process should be followed. If the frost was a complete killing frost, allowing the cows or heifers back into the sorghum pasture after 7-10 days will be safe. If it was a light frost, more prussic acid will be formed at the next frost until the plant is no longer vegetative. One joke was to only send out one cow, the one who kicks to test the pasture. Then if she dies, you are not so concerned!
It is obviously wise to avoid night grazing during potential frost events. Almost all prussic acid poisonings happen when cattle are left out overnight and lower parts of a pasture froze without the grazier realizing it. However, prussic acid poisoning is rare since the causative factors have become well understood.
Is there an advantage to prussic acid, you bet! Even though rootworms have developed a resistance to GMO rootworm technologies, they will not survive prussic acid.
By Larry Hawkins, PAS