Rug, a stack fire would be the worse case scenario.
Generally speaking you run into mold and or 'stack burnt' hay.
Stack burnt hay is when the hay cooks itself inside the bale(no fire, just high moisture/high heat curing). Alfalfa will turn a brown color, or if it's really bad, black.
The cows actually love to eat hay that is a little 'burnt'(it has an almost sweet, tobacco smell) but loses feed value and gives the cows the chits.
