A loader wagon system comprising: a load carrying box for carrying harvested stalk material; a pick-up for gathering stalk material and transporting gathered stalk material into the load carrying box; a content sensing system for measuring at least one constituent content of the stalk material being transported into the load carrying box; and a data processor connected to the content sensing system for receiving content signals indicative of the at least one constituent content; characterized by a weight sensing system (3, 4) for measuring weight of stalk material present in the loader wagon; wherein the data processor is connected to the weight sensing system (3, 4) for receiving weight signals indicative of the weight of stalk material present in the loader wagon; wherein the data processor is arranged for registering, during gathering, content data determined from the received contents signals in combination with weight data determined from the weight signal resulting from the weight of gathered stalk material from which the content signal has been measured, for calculating associated weight change data resulting from the additional weight of gathered stalk material from which the content data have been measured, and for calculating a quantity of the at least one constituent gathered over said trajectory by calculating a plurality of products of said content data and said associated weight change data resulting from the additional weight of gathered stalk material from which the content data have been measured and summing said products over the trajectory from which the stalk material has been gathered. a load carrying box for carrying harvested stalk material; a pick-up for gathering stalk material and transporting gathered stalk material into the load carrying box; a content sensing system for measuring at least one constituent content of the stalk material being transported into the load carrying box; and a data processor connected to the content sensing system for receiving content signals indicative of the at least one constituent content; characterized by a weight sensing system (3, 4) for measuring weight of stalk material present in the loader wagon; wherein the data processor is connected to the weight sensing system (3, 4) for receiving weight signals indicative of the weight of stalk material present in the loader wagon; wherein the data processor is arranged for registering, during gathering, content data determined from the received contents signals in combination with weight data determined from the weight signal resulting from the weight of gathered stalk material from which the content signal has been measured, for calculating associated weight change data resulting from the additional weight of gathered stalk material from which the content data have been measured, and for calculating a quantity of the at least one constituent gathered over said trajectory by calculating a plurality of products of said content data and said associated weight change data resulting from the additional weight of gathered stalk material from which the content data have been measured and summing said products over the trajectory from which the stalk material has been gathered.