A square baler having a baling chamber, a plunger reciprocable at one end of the baling chamber, a pre-compression chamber where charges of crop are amassed and pre-compressed by a rotor prior to transfer into the baling chamber. A transducer is provided for producing an electrical output signal indicative of the load on the rotor and a processing circuit is operative to estimate the rate of crop flow into the pre-compression chamber by analyzing the output signal of the transducer over a complete filling and emptying cycle of the pre-compression chamber.