A crop harvesting header comprising: a main frame structure extending across a width of the header for movement in a forward direction generally at right angles to the width across ground including a crop to be harvested; a mounting assembly for carrying the main frame structure on a propulsion vehicle; a cutter bar across a front of a table arranged to move through the crop in a cutting action for cutting the crop as the header is moved forwardly; a ground engaging element arranged so that the cutter bar is supported adjacent the ground for cutting crop close to the ground as the ground engaging element moves over the ground; the mounting assembly comprising left and right float elements each for applying controlled left and right lift forces to the header; the float elements (241, 242) being arranged to allow lifting of each end of the header relative to an opposed end so as to change an angle of tilt of the header relative to the propulsion vehicle; characterized in that it further comprises: at least one sensor (116, 117) for detecting the angle of tilt of the header so as to detect when the left end is raised relative to the right end and when the right end is raised relative to the left end; and a control system for decreasing the left lift force when said at least one sensor (116, 117) indicates that the left end is raised relative to the right end and for decreasing the right lift force when said at least one senor sensor (116, 117) indicates that the right end is raised relative to the left end. a main frame structure extending across a width of the header for movement in a forward direction generally at right angles to the width across ground including a crop to be harvested; a mounting assembly for carrying the main frame structure on a propulsion vehicle; a cutter bar across a front of a table arranged to move through the crop in a cutting action for cutting the crop as the header is moved forwardly; a ground engaging element arranged so that the cutter bar is supported adjacent the ground for cutting crop close to the ground as the ground engaging element moves over the ground; the mounting assembly comprising left and right float elements each for applying controlled left and right lift forces to the header; the float elements (241, 242) being arranged to allow lifting of each end of the header relative to an opposed end so as to change an angle of tilt of the header relative to the propulsion vehicle; characterized in that it further comprises: at least one sensor (116, 117) for detecting the angle of tilt of the header so as to detect when the left end is raised relative to the right end and when the right end is raised relative to the left end; and a control system for decreasing the left lift force when said at least one sensor (116, 117) indicates that the left end is raised relative to the right end and for decreasing the right lift force when said at least one senor sensor (116, 117) indicates that the right end is raised relative to the left end. at least one sensor (116, 117) for detecting the angle of tilt of the header so as to detect when the left end is raised relative to the right end and when the right end is raised relative to the left end; and a control system for decreasing the left lift force when said at least one sensor (116, 117) indicates that the left end is raised relative to the right end and for decreasing the right lift force when said at least one senor sensor (116, 117) indicates that the right end is raised relative to the left end.