An agricultural baler having a pre-compression chamber (2) to gather crop material, form a slice (3) thereof and push the slice towards a baling chamber (1) into a first segment (4) thereof is disclosed. The baling chamber comprising a plunger (5) for compressing slices of crop material into a bale, wherein a slice-presence-detecting sensor (7) is provided in said first segment, an output of the sensor being operationally connected to a controller (9) which is adapted to adjust a synchronization between the periodically forming and pushing of the slice in the pre-compression chamber and the reciprocal movement of the plunger.