The fire, in a barn at a farm on Coles Lane in Eling, was started deliberately at some time between 6.15pm and 9pm on Thursday, Hampshire Constabulary said.
More than 500 bales of hay caught alight in the fire.
In total, eleven units encompassing fire engines, a telehandler, water carriers and support vehicles were called to the blaze, before seven relief crews from as far away as Basingstoke took over throughout the night.
150 cows housed nearby had to be transported to a safe location.
Three tractors were also caught up in the blaze, which was tackled by firefighters in breathing apparatus using hose reels and jets.
Police closed Marchwood Road in both directions between Jacobs Gutter Lane and Bury Lane overnight due to the amount of vehicles in attendance.
Now, police have launched an arson investigation and are appealing for witnesses.
A spokesperson said: “We are appealing for anyone with information about the fire to come forward. Were you in the area on Thursday evening? Did you see any vehicles in the area, or anyone acting suspiciously?
“If you have any information to assist please contact us online or on 101 quoting 44240541406.”
