A machine is provided including a shredder formed by at least two parallel series of contrarotating knives, alternating with counter-knives. The knives are carried by shafts and the edge of each counter-knife facing the opposite knife associated thereto has an arcuate portion. At least one counter-knife has a portion extending above the axis of the shaft and forming an arcuate edge centered on the axis of the other shaft of the shredder, this arcuate edge completing the arcuate portion. The arcuate portion has a notch formed on it, opening into the arcuate edge formed by this arcuate portion, and forming a tip oriented toward the upstream side of the counter-knife.