It depends on the ranch, most ranchers poison the pests, but not every year. If it is a year following poisoning it will not be great shooting. But to answer your question, there is still good shooting in September if you know where to go.
I’ve known some deer hunters that shoot prairie dogs in the off time. I suppose with the larger guns one doesn’t need the action as when set up with a varmint gun. A relative went in April a few times, had ok shooting on adult dogs.