From what I've researched if it has all been baled from the same field at around the same time you should be ok and I would be perfectly comfortable with just testing one bale. I know some IR horse owners who will send multiple samples of the same load to see how consistent it is, but when I was testing I didn't have the money for that, especially when I bought hay in smaller quantities.

Riding a horse is not a gentle hobby, to be picked up and laid down like a game of solitaire. It is a grand passion. It seizes a person whole and once it has done so, he will have to accept that his life will be radically changed. Ralph Waldo Emerson

Tack for Sale! http://fhotd64476.yuku.com/topic/67755/Saddle-girth-pad-and-bridle