Sure not an expert on this subject but everything I've read indicates that the Sudan's retention of Nitrates in the leaves is a result of stress on the plant. I think BHB eluded to this earlier. If your field where the Johnson grass, Haygrazer, (Sudan) is growing has been fertilized and it has or is in a drought condition you need to worry about the poisoning. Otherwise it should not be a problem to graze or bale the grass. My cattle love the stuff but then I don't cultivate it and I darn sure don't waste fertilizer on it.
Sorry to hear about your loss.