In Nevada, we'd use a rule of thumb that once the temp deep inside a bale went over 140, it was time to get very serious about checking the rest of the stack. Once the internal temps go above about 140 to 150 - somewhere in that range, the process runs away on you very quickly. So we used 140F as being the point where we had to get serious immediately - either feed the hay right there, right then, or get the hot bales out of the stack that day.

I've shipped hay that heated from 120 to 130 in two days, and warned the customer about it. It fed well, the customer liked it, but agreed with me that another week and it would have been a wreck. Having good relationships with your broker, customers, etc - means that if you see hay heating quickly, you have a way to get rid of it before it become a fire hazard and still get some income from it.