A flexible cutterbar header for an agricultural plant cutting machine, includes a plurality of elongate, telescoping support arms pivotally connected to a floor of the header and supporting a flexible cutterbar for upward and downward, and fore and aft movements relative thereto, apparatus extending between at least some of the support arms configured and operable for controlling the telescoping thereof substantially uniformly, and a preload adjusting system configured and operable for applying a preload force against the cutterbar through the support arms in a manner to automatically increase as the support arms telescopically increase in length, and automatically decrease as the support arms telescopically decrease in length.