An improved belt tensioning system for an agricultural round baler that increases belt tension to reduce slippage between the belts and the driving rolls during the initial stage of a bale forming cycle when the bale chamber is empty and as the belts are beginning to be rolled. The system includes an accumulator that is pressurized by the tensioning system as a bale is being formed. The accumulator pressure may then be directed to a hydraulic cylinder included in a bale density system to increase tension in the belts after a completed bale is ejected and the bale chamber is being reset for the next bale.