An agricultural baler for picking up crop material and forming it into a rectangular shaped package, having a bale density control system. The baler has a bale case including four wall members forming a generally rectangular opening, at least one of which wall members includes a tension rail movably mounted to vary the cross sectional area of the rectangular opening. The baler also has a hydraulic control arrangement including a hydraulic cylinder operable for changing the position of the tension rail, a pressure control valve comprising a variable relief valve, and a hydraulic circuit for coupling the hydraulic cylinder to the pressure control valve. The relief valve is selectively controlled by an electronic controller.