I've read a number of papers by both college-types and "real live rancher"-types to the effect that a 3 sided hay shed is one of the easiest of all construction projects to justify from the standpoint of economic payback --- and after seeing what happened to my round bales after an extremely wet 3 or 4 months I believe it. Properly stored hay supposedly loses very little nutritional value. The better and more pricey the hay the more the justification for a shed I think, even with todays higher prices for tin. Being able to store a lot of hay is also good from the standpoint of "insurance" in the event of unexpected drought and the need to feed outside of traditional times of the year. I've had to do that several times in reccent years, when I dind't want to be forced to liquidate any animals (you never can replace them for anywhere near what you got when you sold in a drought stressed situation). If you can't get a shed/barn built and don't want to use tarps then it is a good idea to lay down a good bed of rocks, railroad ties, utility poes, etc. to prevent the bottom of the bales from touching the wet ground