An agricultural round baler comprising: a main frame; a bale forming chamber supported by said main frame and defined in a space between opposing side walls by a plurality of side-by-side continuous belts trained around a plurality of transversely elongate guide rollers including fixed rollers (43, 44, 45, 46, 47) and at least one moveable roller (53, 54), said bale forming chamber configured to form a generally spirally wound, cylindrically shaped bale of increasing diameter oriented transverse to said frame by motion of said plurality of belts and rotation of said plurality of guide rollers; a tailgate movable by a tailgate actuator between open and closed positions for allowing a formed bale to be ejected from said bale forming chamber; a tension arm pivotally moveable between opposing empty and full positions and having a return spring to bias said tension arm toward said empty position, said at least one moveable roller (53, 54) connected thereto for movement therewith; and a bale tensioner connected to said tension arm and moveable between generally opposing first and second positions as said tension arm moves between said empty and full positions, movement of said bale tensioner from said first position toward said second position resisting movement of said tension arm toward said full position to maintain tension in said plurality of belts as said bale increases in diameter; and a pre-tensioning device selectively acting on said bale tensioner to apply a bias increasing force on said tension arm toward said empty position thereby increasing tension in said plurality of belts to a tension greater than the tension applied by said return spring, wherein said bale tensioner comprises a hydraulic cylinder having first and second pressure sides, characterized in that said bale tensioner further comprises a closed-loop hydraulic circuit connecting said first and second pressure sides, and a pressure setting device (104,105) for setting a first pressure in said hydraulic circuit, and said pre-tensioning device comprises a pressure accumulator having a second pressure connected to said hydraulic circuit by a control valve. a main frame; a bale forming chamber supported by said main frame and defined in a space between opposing side walls by a plurality of side-by-side continuous belts trained around a plurality of transversely elongate guide rollers including fixed rollers (43, 44, 45, 46, 47) and at least one moveable roller (53, 54), said bale forming chamber configured to form a generally spirally wound, cylindrically shaped bale of increasing diameter oriented transverse to said frame by motion of said plurality of belts and rotation of said plurality of guide rollers; a tailgate movable by a tailgate actuator between open and closed positions for allowing a formed bale to be ejected from said bale forming chamber; a tension arm pivotally moveable between opposing empty and full positions and having a return spring to bias said tension arm toward said empty position, said at least one moveable roller (53, 54) connected thereto for movement therewith; and a bale tensioner connected to said tension arm and moveable between generally opposing first and second positions as said tension arm moves between said empty and full positions, movement of said bale tensioner from said first position toward said second position resisting movement of said tension arm toward said full position to maintain tension in said plurality of belts as said bale increases in diameter; and a pre-tensioning device selectively acting on said bale tensioner to apply a bias increasing force on said tension arm toward said empty position thereby increasing tension in said plurality of belts to a tension greater than the tension applied by said return spring, wherein said bale tensioner comprises a hydraulic cylinder having first and second pressure sides, characterized in that said bale tensioner further comprises a closed-loop hydraulic circuit connecting said first and second pressure sides, and a pressure setting device (104,105) for setting a first pressure in said hydraulic circuit, and said pre-tensioning device comprises a pressure accumulator having a second pressure connected to said hydraulic circuit by a control valve.