Teenagers are suspected of starting a fire which destroyed about £10,000 worth of hay in a barn at Hawkcliffe Farm, Steeton.
Police are investigating the blaze which tore through the roof of the barn, wrecking almost 1,500 bales.
Farmer David Thompson said: "Police suspect it was young teenagers. Some were seen coming away from the place.
"It is horrendous, especially coming at the end of such a terrible year. There is so little good quality hay about because of the weather. This lot represents 50 acres of grass. The family has farmed here since 1929 and it is the first time we have had a fire."
Mr Thompson still had about 800 bales of silage but he was down to about a third of the feed needed for the winter.
And he now faced the problem of where to house his herd of 150 cattle while the rebuilding work on the barn took place.
Mr Thompson and his son, Daniel, battled throughout the night dragging bales from the barn as firefighters from Keighley, Silsden, Bingley and Shipley fought the blaze.
Keighley Station officer Arthur Atkinson said: "Police are investigating; we suspect it was deliberate.''
He said without the help of Mr Thompson and his son the damage could have been much more extensive.
The blaze, which broke out last night, took about three hours to bring under control and destroyed about 120 tons of hay and extensively damaged the roof and walls of the barn. He expected they would continue to damp down throughout today.
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