Sierra Sue (movie, 1941)

To fight a poisonous weed, ranchers are burning their land. Gene is the Inspector brought in and he recommends spraying. The spraying goes well until the Larabee ranch is reached. When Larrabee refuses to allow the equipment on his land, Gene has it sprayed by airplane. Cattle must stay off recently sprayed land and when a Larrabee man shoots down the plane, the crash sends the cattle stampeding toward the newly sprayed land.