Abstract: An adaptor for a support member includes a body having a mounting assembly for mounting the adaptor to the support member, and a rotor coupled to the body for rotation relative to the body, wherein the rotor is provided with a fixing assembly for fixing a disc coulter to the rotor. The mounting assembly may include one or more threaded shafts for mounting through apertures of the support member. Alternatively, the mounting assembly may include a tapered sleeve for mounting the adaptor by interference fit to a tapered fitting of the support member. A method of fitting a disc coulter to a cultivator tine, includes the steps of removing a tillage point from a point mount of the tine, mounting an adaptor to the point mount, and fixing a disc coulter to the adaptor.