Like MikeC said, there are alot of factors. You cant do anything about the weather, but you can control where you store them. Hard sloping ground for good drainage and no mud will save you alot of waste. Also, if you do not use a tarp, do not stack the bales because it only traps the water. For years we used the old New Holland round baler and it made a terrible, loose bale. I really believe there were times we lost 1/3 of the hay before we fed it and if it were kept over for a year we lost 2/3 and after 2 years, dont bother feeding it. Now, with the incredibly tight, net wrapped John Deere and similar balers, I do not beleive there is much waste. We now routinely keep bales 2 years with very little waste and the 3rd year some, but not a whole lot. The key is to use the 5x6 bales and to put 3 turns of wrap, not the 1 1/2 that alot of people use. The more surface area you have, the more waste---that is why 4x4 and 4x5 bales are so expensive, because the percentage of waste is to high.