In order to obtain maximal crop productivity with minimal environmental pollution, the amount of nutrients applied to the soil should be matched to the site-specific crop needs. In the case of liquid manure application, this means that manure flow must be controlled according to the local soil fertility, the crop needs, the manure composition and the driving speed. Since each of these parameters varies during injection, a fast and accurate flow control system is needed. In vacuum manure tankers, the flow is controlled by opening and closing a manure valve, which is actuated hydraulically While magneto-rheological flow sensors are the best choice for manure flow measurement, their time delay and lag inhibit fast and stable feedback control with conventional equipment. Therefore, a new control strategy with fast and accurate control of the manure valve in combination with on-line flow gain estimation is presented. Two estimation algorithms have been developed in simulation and validated in practice: recursive least squares and sliding observer. Although the recursive least-squares algorithm performed well, the sliding-mode observer had less overshoot, which resulted in a settling time of about 5 s. Since the desired flow is expected to vary smoothly in practice, it is expected that the performance of this combination of a valve position controller and a sliding-mode observer will be sufficient for precise manure application purposes. (c) 2007 lAgrE. Published by Elsevier Ltd. All rights reserved.