They had to take every bale out of the storage to stop fires from breaking out whenever the damp hay was exposed to oxygen.
The farm is at 3741 Dominion Springs Rd., near Kinburn Side Road and Highway 417 which is about midway between Ottawa and Arnprior.
Fire chief Adrian Dearman said while the cause of the fire is officially not yet determined, he thinks it was spontaneous combustion.
During the rainy summer, many farmers across Ontario put away very wet hay, said Dan Carlow, a manager in the environmental management branch at the Ministry of Agriculture, Food and Rural Affairs.
That wetness speeds along fermentation with the bale, a process that causes a buildup of heat and gas that leads to a chemical reaction and, if unchecked, fire.
Carlow advises farmers to check stored hay; it should be kept below 20 to 25 per cent moisture.
He said all hay needs to be checked for runaway rising temperatures.
"At 55 C and below, you're probably OK, but above 55 you need to be diligent, hunker down and keep an eye on it," said Carlow.