A control system for an implement (10) mounted on a tractor (11) via a three point linkage (10) and which has one or more depth control wheels (13) which contact the ground and which support at least part of the weight of the implement. The three point linkage has an upper link (17) and two lower links (14, 15), the upper link including an adjusting means such as an hydraulic cylinder (18) to vary its effective length and the lower links being provided with raising means (16) to raise the implement. The control system has a control unit (23) which received signals indicative of the level of slippage of one or more wheels of the tractor and which in response to a wheel slippage level above a first level (X) operates the adjusting means (18) to shorten the effective length of the upper link (17) to transfer some of the weight of the implement (10) onto the tractor to reduce wheel slippage. The control unit (23) also operates the lower link raising means (16) to raise the lower links (14,15) and hence the implement, if the wheel slippage level exceeds a second higher level (Z).