THREE-QUARTERS of a ton of hay went up in smoke on the A27, closing the eastbound carriageway for two-and-a-half hours.
Fire crews had to use pitchforks and tools to stop the blaze inside an agricultural hay baler at 8.45pm on Tuesday.
The quick-thinking driver of the tractor towing the baler managed to detach it before it spread to his vehicle.
Two crews from Shoreham attended and called two more fire engines from Hove when it became clear that they would run out of water.
The fire happened between the A283 Steyning Road junction and the A270 Upper Shoreham Road junction, where there are no fire hydrants.
A special water carrier and a command unit arrived to help tackle the fire, with flames reportedly leaping to as high as 30ft.
Roy Barraclough, district commander for Worthing, said it was likely that the machinery of the baler had overheated and that the fire was fuelled by machine oil and the wind as it travelled along the road.
He said: “It’s an unusual incident to get bailer fires on the road.
“It’s a bit out of the ordinary but we dealt with it.”
A West Sussex Fire and Rescue Service spokeswoman said the fire started accidentally and nobody was hurt.
Sussex Police said about 100 vehicles were escorted back along the A27 to use the flyover to find other routes.
This had happened by 10.15pm but the road remained closed for another four hours while its surface was assessed for any damage that may have been caused by the blaze.