An agricultural tractor comprises lifting devices to which are hitched rotary supports in the form of rollers. The lifting devices push the rollers downwardly relatively to the rest of the tractor. This transfers some or all of the weight of the tractor to the rollers which reduces or eliminates pressure on the underlying surface, and consequent soil compaction, applied by wheels of the tractor. The rotary supports may be drivable and improve traction. If all of the weight of the tractor is placed on the rotary supports, the wheels are raised from the ground.A system is also described for automatically detecting and eliminating or correcting for skidding of the wheels of the tractor.