The blaze ripped through a single-storey barn at Burn Farm in Willington on Monday evening.
Farmer Charles Hedley said it had been a ’bumper year’ of crops and the building was stacked full of hay and straw. He fears his rape seed may also be too damaged to sell.
Mr Hedley managed to remove his tractor but a baler and trailer were destroyed by the flames.
About 50 firefighters worked to bring the blaze under control in the early hours of Tuesday morning.
The fire was discovered by Mr Hedley while he was working in a field behind the shed.
Anyone with any information is asked to call the police via 101.