Once hay temperature rises above 110°F, deterioration of quality begins; and when hay exceeds 160°F, fire poses a threat.
No commercial equipment is available for specifically monitoring hay temperature, but there are several possible procedures to follow. Small, metal thermometers with 6- to 8-inch-long probes are available at auto supply stores and elsewhere, and a commercial device for monitoring grain bins might be adapted for hay.
One alternative way to check the temperature of a stack of bales set on edge is to drive a 1/2-inch conduit pipe down into the center of the stack and lower a standard laboratory thermometer into the pipe.
Be aware that the most common peaks for hay temperature are at around 3 to 5 and 10 to 18 days after harvest. There should be no peaks in temperature after 25 or 30 days in storage.