An ancillary implement for an agricultural working machine comprising: a hydraulic pump unit adapted to deliver a hydraulic fluid at a predetermined working pressure, a drive unit connected to the hydraulic pump unit to drive same to provide the working pressure, a hydraulic drive means (40, 50) having a drive fluid inlet (41, 51) in fluid communication by way of a pressure line (P1, P2) with a pump fluid outlet (13, 23) of the hydraulic pump unit to feed hydraulic fluid under working pressure to the hydraulic drive means (40, 50), and a drive fluid outlet (42, 52) in fluid communication by way of a feed line with a pump fluid inlet (12, 22) of the hydraulic pump unit to feed hydraulic fluid output by the hydraulic drive means (40, 50) to the hydraulic pump unit, and a fluid exchange device having a fluid feed line in fluid communication with the feed line and opening into a feed connection for the controlled feed of hydraulic fluid from a hydraulic circuit of the working machine and a fluid discharge line in fluid communication upstream of the fluid feed line with the feed line and opening in a discharge connection for the controlled discharge of hydraulic fluid from the feed line into the hydraulic circuit of the working machine, characterised in that the fluid exchange device has a flow regulating valve integrated into the fluid feed line to provide a desired feed volume flow of hydraulic fluid from the hydraulic circuit of the working machine into the feed line and a preload valve which is integrated into the fluid discharge line to compensate for an increase in pressure caused in the feed line by the feed volume flow by delivering a discharge volume flow of hydraulic fluid from the feed line into the hydraulic circuit of the working machine. a hydraulic pump unit adapted to deliver a hydraulic fluid at a predetermined working pressure, a drive unit connected to the hydraulic pump unit to drive same to provide the working pressure, a hydraulic drive means (40, 50) having a drive fluid inlet (41, 51) in fluid communication by way of a pressure line (P1, P2) with a pump fluid outlet (13, 23) of the hydraulic pump unit to feed hydraulic fluid under working pressure to the hydraulic drive means (40, 50), and a drive fluid outlet (42, 52) in fluid communication by way of a feed line with a pump fluid inlet (12, 22) of the hydraulic pump unit to feed hydraulic fluid output by the hydraulic drive means (40, 50) to the hydraulic pump unit, and a fluid exchange device having a fluid feed line in fluid communication with the feed line and opening into a feed connection for the controlled feed of hydraulic fluid from a hydraulic circuit of the working machine and a fluid discharge line in fluid communication upstream of the fluid feed line with the feed line and opening in a discharge connection for the controlled discharge of hydraulic fluid from the feed line into the hydraulic circuit of the working machine, characterised in that the fluid exchange device has a flow regulating valve integrated into the fluid feed line to provide a desired feed volume flow of hydraulic fluid from the hydraulic circuit of the working machine into the feed line and a preload valve which is integrated into the fluid discharge line to compensate for an increase in pressure caused in the feed line by the feed volume flow by delivering a discharge volume flow of hydraulic fluid from the feed line into the hydraulic circuit of the working machine.