A question that arises every year after first frost is the safety of grazing sorghums.  All of the sorghums, including johnsongrass, can contain toxic concentrations of cyanide after a frost.  The cyanide is commonly referred to as prussic acid, thus prussic acid poisoning.  The plants contain cyanide in a bound form at all times; however, it takes some sort of plant damage to result in free cyanide which is toxic.  The cyanide is bound in the plant vacuoles and the damage from the frost allows the bound cyanide to come in contact with an enzyme in the plant cytosol thus releasing the free and toxic cyanide (prussic acid).  An average figure is that the sorghums contain approximately 250 mg of cyanide/kg of plant tissue and a lethal dose is between .5 and 3 mg/kg of body weight.  Thus a 1,200-pound cow would only need to consume between 2.5 and 14 pounds of sorghum to be toxic.  Considering that her daily intake is about 30 pounds this is not much!  Making the sorghum into hay will result in the prussic acid disappearing.  Also, after 3 to 5 days the prussic acid is generally gone.