Used to be that Heinz 57, JG and Coastal Bermuda were the only choices around here for hay, including horse hay. When I first moved out here 40 years ago, daughter had a horse and all it ate was JG hay. Horses roaming with cows eat what's available including JG and SS (Sorghum-Sudan). Never saw a horse down.
Put some NPK on it and it goes nuts. Never heard of a case PA poisioning with that nor SS which is what is planted now-a-days for volume hay production primarily along with the volunteer, even in stress times.
Nitrates in excess with nothing to consume it causing N poisoning doesn't happen around here either. Most hay patches are less than full fed on NPK. Only things bright green are corn fields.
Did have a friend that overdid the N and turned hungry, weined calves in on it with nothing else.....it was tall fescue! He lost a few.