Abstract: The density of plant growth in front of a harvester machine is measured, to control the operating rate of the machine, by producing an ultrasonic field immediately forward of the cutters on the machine, which field comprises at least one ultrasonic beam of frequency modulated ultrasonic impulses. The apparatus employed comprises two transducers consisting respectively of an ultrasonic transmitter and an ultrasonic receiver respectively located on opposite sides of the machine immediately forward of the cutting blades and oriented on a line therebetween disposed orthogonally to the direction of travel of the machine, and at least one further ultrasonic transducer disposed adjacent one of said two transducers but oriented on a line relative to the other of said two transducerswhich line is inclined relative to the direction of travel of the machine.