The Austin Fire Department responded to a farm south of Austin Tuesday morning for a fire involving hay bales in a shed.

A resident noticed smoke coming from the shed while letting a dog out around 7:52 a.m. on the farm at 53548 173rd Street.

“Went outside to let a puppy out and the mom saw there was smoke,” Austin Fire Chief Jim McCoy said. “She thought it was fog and went to check it out and called us.”

The cause of the fire is being ruled as spontaneous combustion.

“The bales are put away a little bit wet and gets packed tight,” McCoy said. “It heats up and catches fire.”

Flames were visible when firefighters arrived and had been smoldering long enough to burn through boards along the base of the shed where the fire started. Of the 75, 1,200-pound bales stored in the structure, McCoy estimates they lost about 30 bales.

Though damaged, the structure is still usable. There were no injuries.