There are two times to be worried about prussic acid (hydrogen cyanide) in Johnsongrass...
Live standing johnsongrass that's been frosted-- when it turns dark after a frost, give it about a week to make sure the prussic acid comes out of it before you turn the cows in on it.
The other time is when johnsongrass has been pretty badly drought stressed and then gets a good rain and greens up REAL quick and takes off growing again-- it can get prussic acid then.
When making hay prussic acid isn't a problem. When the grass is cut, it continues to metabolize until it wilts flat, and any prussic acid will come out of the stems (outgas) as it wilts down and dries down enough to make hay. Unless you're doing baleage or silage, prussic acid is eliminated by the natural curing of the hay.
Nitrates in johnsongrass (or just about any other grass for that matter, sorghums, sudax, ect) are usually only a problem in heavily fertilized patches that suffer drought stress severe enough to stunt the plants-- the grasses store the nitrates in the plant tissue in a concentrated form awaiting sufficient water to put on a growth spurt. If that water never comes, the concentrated nitrate just sits in the plants.
Nitrates DO NOT outgas from the stems and plant tissue during dry-down when making hay. If anything, they concentrate as the water evaporates out of the cut plants.
Hope this helps... OL JR