I have about 2 acres of johnsongrass in my hay meadow that is mostly bermudagrass. Would I be safe to bale the hay and feed it to horses?

Cyanide concentration drops to safe levels in johnsongrass when cured for hay, and it is safe to feed.
The leaves and stems of johnsongrass contain a cyanide compound, which when metabolized, inhibits the body’s ability to absorb oxygen, in effect suffocating the animal; young shoots of johnsongrass contain the highest concentration of the toxin. Because horses do not metabolize the cyanide compound as efficiently as ruminant animals do, grazing healthy adult plants is unlikely to harm them, but circumstances that injure the plant—wilting, trampling, frost—can chemically liberate the cyanide within the leaves, rendering them dangerous to all species.
Horses rarely have problems eating johnsongrass, but when they do, it can be very serious. Clinical signs often come on quickly, and frequently the horse is simply found dead in the pasture. The grass has to be stressed, as can happen after it has been damaged (trampled, frost). When stressed, the grass develops cyanide, and horses that eat johnsongrass can encounter breathing problems, anxiety, staggering, convulsions, coma, and death. Horses that live one to two hours after the onset of these signs will usually recover.
If you have a horse that is exhibiting any of the signs listed above, call the vet IMMEDIATELY. The longer you wait to get treatment, the lower the chance that the vet can save your horse.