1. A square baler comprising: a control system; an intake duct; a baling chamber; a sensor for measuring the weight of completed bales, at least one density sensor for measuring the relative density of crop slices within the intake duct and/or the intake region of the baling chamber; and the control system programmed to determine a predicted weight of the slice in the intake duct and/or the intake region of the baling chamber derived from the measured relative density values of this slice and a computed correlation function; and wherein the correlation function is determined by correlating the measured relative density of slices in the intake duct and/or the intake region of the baling chamber with the cumulative weight of these slices after they have been compressed and bound to form a completed bale.