Although, most areas of the states have had significant frosts, we are still getting questions on Johnsongrass and cyanide (prussic acid) poisoning. Let’s overview the main points.
Do not graze Johnsongrass when frost is likely (including at night). Frost allows conversion to hydrogen cyanide within the plant. Do not graze after a killing frost until plant material is completely dry and brown (the toxin is usually dissipated within 72 hours).
If cut for hay, allow to dry completely so the cyanide will volatilize before baling. Allow slow and thorough drying because toxicity can be retained in cool or moist weather. Delay feeding silage 6 to 8 weeks following ensiling. The full publication can be found here.