An agricultural square baler equipped with a pre-compression chamber and configured to guide crop material towards and into the baling chamber where rectangular bales are formed. At least one of the upper and lower walls of the pre-compression chamber is formed by a conveyor belt, arranged to move from the supply towards the baling chamber. The conveyor belt has at least two portions configured to exert different friction forces on a batch of crop material, so that the high friction portion is capable of moving the batch into the baling chamber, whereas the low friction portion exerts a low force on the batch, thereby preventing material to be prematurely moved towards the baling chamber.