A baling machine for forming large cylindrical bales of hay has a variable volume baling chamber defined by moving belts with a crop inlet on the underside of the machine. When a bale has been completed it is wrapped with surface wrapping material stored in a holder on the back of the baler. The wrapping material is advanced by feed rolls through a cutting mechanism below the holder, and is thus guided by a plate extending along the underside of a run of the belts to the baling chamber. In order to avoid wasting wrap, a cutter is triggered by a peg carried by a metering wheel movable between an adjustable "home" position against an abutment or stop to a trigger position in which it engages and lifts a release lever to release the cutter, and this distance is a measure of the length of wrap material required for the completed bale. In respective embodiments, the metering wheel is in the form of a friction wheel, a worm wheel and a ratchet wheel with all wheels being selectively driven by one of the feed rolls.