From what I have been able to find out all the only time you need to be concerned about Nitrate poisioning is when the Sudan (Johnson Grass, Hay Grazer, etc) is under stress and has been highly fertilized.
I plan on turning my cattle in the hay field where I had Hay Grazer planted. The field was not heavly fertilized and it's certainly not stressed so I don't expect to have any problem.
Go out to one of the university web sites (like A&M in Texas) and do a search for Nitrate poisioning. I think you will find the information you're looking for.