A crop chopper for an agricultural machine has a rotor having a plurality of fingers displaceable on rotation of the rotor along the passage, and respective blades interleaved with the fingers and each having a front end pivoted about a horizontal axis and a rear edge formed with a rearwardly open notch so that crop pressed by the fingers between the blades is chopped and moved rearward along the passage. A rocker is pivotal about a horizontal axis and respective actuator plates juxtaposed with the rear edges of the blades are each formed with a rearwardly projecting arm. Respective abutment bodies on the plates are displaceable toward and away from the respective blades and respective springs braced between the bodies and the respective plates urge the bodies forward toward the blades. The plates are each pivotal about a horizontal axis between an active position with the respective body engaged against the respective rear edge and an inactive position with the respective body disengaged from the respective rear edge. Respective springs braced between the plates and the rocker pivot the plates on the rocker about the respective plate axis in a direction pulling the bodies away from the blades and pivot the blade arms downward. An abutment beam extends below the plates, and at least one actuator engaged with the rocker can pivot down the rocker and thereby press the rear arms against the abutment beam and bring the plates into the active positions.