Agricultural machine arranged so as to be connected to and driven by a tractor and comprising: - at least one working implement capable of being driven from a drive means capable of having a variable drive speed, resulting in a variable working speed for the implement concerned, this working implement being capable of being displaced or transposed at least between a working position or configuration and a transport position or configuration, - at least one movable protective means capable of being displaced at least between an operative position, in which it surrounds and/or covers at least partially the working implement, and an inoperative position, in which the working implement concerned is at least partially accessible, - a device for the displacement of said at least one protective means making it possible to displace the latter at least between the aforesaid operative and inoperative positions, this displacement device comprising at least one adjustment actuator supplied by energy delivered from an external energy source, - a device for locking the protective means, in at least one of the positions of the latter, in which machine: the displacement device comprises a circuit for control of the adjustment actuator or of each adjustment actuator allowing at least transposition of the adjustment actuator concerned in or towards a first configuration, in which the protective means concerned is in its operative position, as well as transposition of the adjustment actuator concerned in or towards a second configuration, in which the protective means concerned may be in its inoperative position, and the control circuit incorporates the locking device for the protective means concerned and is such that it authorises or forces the adjustment actuator concerned to be in its first configuration when the working implement in question is in its working position or configuration, machine characterised in that it is arranged so that the locking device automatically converts a variable input quantity to a control quantity for the adjustment actuator concerned, this control quantity being capable of assuming a first control value used to prevent transposition of said adjustment actuator outside its first configuration, the input quantity has a value which is a function of or dependent on the working speed of said at least one implement concerned, and the control quantity assumes and maintains the first control value as long as the input quantity has a value representative of the working speed of said at least one implement concerned, which is greater than a threshold speed. - at least one working implement capable of being driven from a drive means capable of having a variable drive speed, resulting in a variable working speed for the implement concerned, this working implement being capable of being displaced or transposed at least between a working position or configuration and a transport position or configuration, - at least one movable protective means capable of being displaced at least between an operative position, in which it surrounds and/or covers at least partially the working implement, and an inoperative position, in which the working implement concerned is at least partially accessible, - a device for the displacement of said at least one protective means making it possible to displace the latter at least between the aforesaid operative and inoperative positions, this displacement device comprising at least one adjustment actuator supplied by energy delivered from an external energy source, - a device for locking the protective means, in at least one of the positions of the latter, the displacement device comprises a circuit for control of the adjustment actuator or of each adjustment actuator allowing at least transposition of the adjustment actuator concerned in or towards a first configuration, in which the protective means concerned is in its operative position, as well as transposition of the adjustment actuator concerned in or towards a second configuration, in which the protective means concerned may be in its inoperative position, and the control circuit incorporates the locking device for the protective means concerned and is such that it authorises or forces the adjustment actuator concerned to be in its first configuration when the working implement in question is in its working position or configuration, machine characterised in that it is arranged so that the locking device automatically converts a variable input quantity to a control quantity for the adjustment actuator concerned, this control quantity being capable of assuming a first control value used to prevent transposition of said adjustment actuator outside its first configuration, the input quantity has a value which is a function of or dependent on the working speed of said at least one implement concerned, and the control quantity assumes and maintains the first control value as long as the input quantity has a value representative of the working speed of said at least one implement concerned, which is greater than a threshold speed.