A convertible mower deck has a plurality of cutting chambers with a rotary cutting blade in each chamber and a side discharge opening from one of the cutting chambers, pivoting gates between the adjacent cutting chambers, and a pivoting side discharge door between a cutting chamber and the side discharge opening. Each of the gates and side discharge door are movable between a discharge or collection position and a mulching position. A mulch on demand actuator includes a switch in an operator station that causes a pivot control plate on a shaft to rotate and move linkages between the gates and door between the discharge or collection position and mulching position.