A loader wagon system (1) having a load carrying box (13), a pick-up (11) for gathering halm material and transporting gathered halm material into the load carrying box, a content sensing system (7) for measuring at least one constituent content of the halm material being transported into the load carrying box, a weighing system (3, 4) for measuring weight of halm material present in the loader wagon; and a data processor (2). The data processor is arranged for calculating a quantity of a constituent gathered over a trajectory by calculating a plurality of products of the content data and the associated weight change data resulting from the additional weight of gathered halm material from which the content data have been measured and summing the products over the trajectory from which the halm material has been gathered.
