Geese folks give me a break. Its cow hay. If it gets a bit of mold so what. All the good horse hay is in the barn on pallets.
This cow hay pile is under a tarp on pallets with good ventilation and yes for the first week I check for temperature spike and had none. These are bales I would not put in the barn. The pile consists of older bales, odd size bales, bales left in baler from last run, etc. Good for cows, covering plants, bale gardens, and composting as I have done in the past etc.
BTW - if you want to compost bales, soak with water and put under a black plastic tarp. They will start cooking in a few days, then remove tarp and keep damp. This needs to be done far away from buildings as they will start smoking at some point (a good thing), but if kept damp will not combust To speed this up, throw on some nitrogen fertilizer. Turn after 3 months, then every month after. but keep the pile damp and ariated. Makes excellent compost. Before use in garden, cover again with blast plastic tarp under full sun for a week to kill any seeds that might still be in it. I had 50 bales that got rained on (soaked) a few years back and an old timer showed me how to do it. Worked great.