Apparatus for dispensing a preservative or other substance, in solid or liquid form, onto a crop material being baled, the substance being dispensed onto the crop material before it is baled at a rate which is dependent on the moisture content of the crop material and the bulk rate of flow of crop material through the baler. The apparatus, which may be mounted on a square baler or a round baler, includes a first sensor for sensing the moisture content of the crop material and producing a first signal, a second sensor for producing an output signal indicative of the bulk rate of corp flow through the baler, and a control circuit responsive to the first signal and the second signal for producing a pulse width modulated signal for controlling the speed of a motor. The motor controls the rate at which the substance is dispensed from a container onto the crop material.