A header for harvesting a crop, the header comprising: a flexible cutterbar assembly extending lengthwise in a lateral direction relative to a normal direction of travel of the header; a plurality of laterally spaced apart support arms (74,86) configured to support the flexible cutter bar assembly, with each support arm (74,86) including - a first end attached to the flexible cutterbar assembly, and a second end pivotal about a laterally extending axis so that the flexible cutterbar assembly is configured to flex in response to changes in terrain as the header is advanced; a plurality of load sensors, with each load sensor being located at the first end of one of the support arms (74,86) and configured to sense a load on the first end and generate an electronic load signal; charactereised in that the header further comprises a plurality of hydraulic cylinders (80,88), with each hydraulic cylinder being located at the second end of one of the support arms (74,86) and actuatable to adjustably raise and lower the first end of the support arm (74,86); and a controller configured to - receive the electronic load signals generated by the load sensors, determine whether the load signals indicate that actuating one or more of the hydraulic cylinders (80,88) is warranted due to a changing load at the first ends of one or more of the support arms (74,86), and if so, send a control signal to change a hydraulic pressure in the one or more hydraulic cylinders (80,88) to raise or lower the first ends of the one or more support arms (74,86) to offset the changing load. a flexible cutterbar assembly extending lengthwise in a lateral direction relative to a normal direction of travel of the header; a plurality of laterally spaced apart support arms (74,86) configured to support the flexible cutter bar assembly, with each support arm (74,86) including - a first end attached to the flexible cutterbar assembly, and a second end pivotal about a laterally extending axis so that the flexible cutterbar assembly is configured to flex in response to changes in terrain as the header is advanced; a first end attached to the flexible cutterbar assembly, and a second end pivotal about a laterally extending axis so that the flexible cutterbar assembly is configured to flex in response to changes in terrain as the header is advanced; a plurality of load sensors, with each load sensor being located at the first end of one of the support arms (74,86) and configured to sense a load on the first end and generate an electronic load signal; charactereised in that the header further comprises a plurality of hydraulic cylinders (80,88), with each hydraulic cylinder being located at the second end of one of the support arms (74,86) and actuatable to adjustably raise and lower the first end of the support arm (74,86); and a controller configured to - receive the electronic load signals generated by the load sensors, determine whether the load signals indicate that actuating one or more of the hydraulic cylinders (80,88) is warranted due to a changing load at the first ends of one or more of the support arms (74,86), and if so, send a control signal to change a hydraulic pressure in the one or more hydraulic cylinders (80,88) to raise or lower the first ends of the one or more support arms (74,86) to offset the changing load. receive the electronic load signals generated by the load sensors, determine whether the load signals indicate that actuating one or more of the hydraulic cylinders (80,88) is warranted due to a changing load at the first ends of one or more of the support arms (74,86), and if so, send a control signal to change a hydraulic pressure in the one or more hydraulic cylinders (80,88) to raise or lower the first ends of the one or more support arms (74,86) to offset the changing load.