A shredder attachment for a round bale hay baler which will effectively shred corn stalks or stubble, milo stubble and the like and discharge the shredded stubble into a round bale hay baler which will form a bale of stubble which has been shredded which can be used for feed, bedding and the like. The shredding attachment can be easily and quickly attached to a round bale hay baler at the front lower area thereof and is drivingly connected to the power system of the hay baler and includes a rotatable shaft having a plurality of pivotal flail elements or knives mounted thereon for shredding stubble or stalks and discharging the shredded material rearwardly and upwardly into the hay baler in the same manner that hay enters the hay baler with the shredded stubble being rolled into a hay bale in the same manner that hay is baled by the round bale hay baler. The shredder attachment provides short, chopped material which facilitates baling and provides a one pass operation thus eliminating separated shredding and raking operations.