I thought I remembered 2 shots a month apart, then yearly after. I don't believe that rolling the bale out makes any difference. I might not be remembering right but if I am it has to do with how much air is getting into the bale as it cures, and round bales don't get enough because of their size. I remember recently a woman was running a "rescue" locally and 2 horses died. It was supposed to be from botulism from a round bale but I'm not sure it that is true or not. Because horses died the another "rescue" came in and seized all her animals and said that it wasn't botulism, they starved to death. There was a picture of a couple of healthy enough looking horses munching on a round bale, but it was considered abuse because they were standing in mud so I tend to believe it was botulism.