Police believe between 300 and 400 bales were deliberately set alight on farm land off Habberley Lane in Kidderminste at around 5pm on Sunday.
The blaze resulted in £9,000 of damage to crops, and police are now appealing for information about the cause and people spotted nearby.
Eyewitness Richard Leadbetter, from Dudley, said he was driving nearby at around 7pm when he saw a "huge ball of smoke".
"We ended up driving straight past it so we pulled up quite close to make sure everyone was okay," he said.
"The fireman said to my partner that it was a hay fire and would probably be burning for days now. The farmer had just removed all the hay he could salvage.
"Another fire unit then turned up to assist and we left. The heat was still very intense."
Police have said witnesses described seeing three youths in the area shortly before the fire.
Two were male - one wearing black, and one in grey on a bike - along with a female wearing a grey hooded top and grey short.
Anyone with information is asked to contact police via 101 quoting reference 00491_i_19092021, or report anonymously via Crimestoppers on 0800 555 111.