A square baler is disclosed which comprises a baling chamber, a plunger (16) reciprocable at one end of the baling chamber, a pre- compression chamber (18) within which charges of crop are amassed and pre-compressed by a rotor (32) prior to transfer into the baling chamber, and a pickup roller (30) having radially projecting tines for picking up crop from the ground and advancing the crop to the rotor (32). In the invention, a transducer (40) is provided for producing an electrical output signal indicative of the load on the rotor (32) and a processing circuit is operative to estimate the rate of crop flow into the pre-compression chamber (18) by analysing the output signal of the transducer (40) over a complete filling and emptying cycle of the pre-compression chamber (18).
