Firefighters will continue to battle 100 tonnes of burning hay through the night.
Greater Manchester fire service rushed to a farm in Rosemary Lane, Over Hulton, at around 1.45pm on Sunday.
Around 45 firefighters have been at the scene near Bolton over the course of the afternoon.
They tackled the fire which is believed to have started inside a barn stacked tightly with around 100 tonnes of hay, according to Greater Manchester Fire and Rescue Service area manager Ben Levy.
The barn is around 30 metres by 30 metres in size.
The fire service has asked drivers to drive slowly and be careful on the A6 as hose lines are currently out in the road pumping water to the burned out barn.
Area manager Levy told the MEN: "We got the first report of a fire at around 1.45pm this afternoon.
"We had about 10 emergency calls to that fire. We sent the emergency fire engines to the scene, which then sent for assistance from other fire stations.
"At the fire's peak, there were six fire engines and a number of support vehicles dealing with a bar with approximately 100 tonnes of hay well alight.
"It is going to be smouldering for a long time. It is very difficult to put out a hay fire and it was stacked quite tightly. It will burn deep inside."
"The fire has been causing quite a bit of smoke. It it low lying and drifting across the local area.
"We are advising anyone living in the area affected to keep windows and doors closed."
A team of firefighters will remain in Rosemary Lane overnight as they continue to douse the smouldering building and ensure the hay is safe.
Mr Levy said: "We are expecting to be there overnight dealing with this fire but we are expecting the smoke to subside quite significantly.
"We will be making sure it does not catch again, our firefighters will be working at the scene for a number of hours yet."
Mr Levy says a fire investigation is underway to establish the cause of the blaze.
