There may not be any problems, but testing the hay for nitrates is always a good idea. Any prussic acid would be gone by now. Johnsongrass could accumulate some nitrates, but it is a vigorous grass (tough to stress).
So for a forage source for beef cows, sample the hay and test it for moisture, crude protein, total digestible nutrients (TDN) (energy), and nitrates. Remember, forage testing labs will only take a probe sample, so you may need to contact your local Cooperative Extension office to see if they have a forage probe that you can borrow.
