Feed it to goats or cattle.
Hay stored outdoors where it sits on the ground or gets rained on grows mould and other fungus. Only ruminant animals have a hope of digesting it without difficulty, and even then I prefer to use clean hay.
The cost of hay for guinea pigs is trivial. Any effort in trying to "clean" bad hay is totally wasted. It's like trying to improve manure by washing it. Or trying to improve bad wine by mixing some good wine into it.
When we had guinea pigs, we found that they would go crazy with joy if we gave them lettuce.
If you're never read "Pigs Is Pigs" by Ellis Parker Butler, try to find it, and share it with your fellow guinea-pig fans.