In a hog production plant the hogs are divided into a series of separate pens within the hog plant based generally on weight such that the lightest hogs closest to the initial weight are arranged in the first pen when the arrive and the heaviest hogs closest to the final market weight are arranged in the last pen. Each pen includes a sorting scale arranged to receive a hog therein and to weigh the hog for selection of one of two outlet paths leading to the next pen of the series or, in respect of the last pen, to a discharge location. A control unit for the system is arranged such that a required number of hogs to be removed from the last of the pens for market, the required number being less than the total number of hogs in the last pen and less than the total number of hogs in each of the other pens of the series is set and the sorter operated to remove the required number of hogs from the last pen and to transfer from each pen to the next of the series substantially the same number so as to maintain the numbers of hogs in the pens substantially constant.