"Farm tractor" means any self-propelled vehicle which is designed primarily for pulling or propelling agricultural machinery and implements and is used principally in the occupation or business of farming, including an implement of husbandry that is self-propelled [...]

(3) "Farm tractor" means any self-propelled vehicle having more than 15 horsepower designed and used primarily as a farm implement for drawing plows, mowing machines, and other implements of husbandry.