The liveweight of a free-ranging cow is constantly changing. If she is weighed automatically upon entering a corral before drinking water, a liveweight profile results which can be used to manage her individual performance. Using a manual procedure and weighing the same cow every 28 days also results in a liveweight profile. To determine how the amount of information contained in the 2 weighing procedures differed, the same cows of an experimental herd were weighed using both procedures. The resulting data referred to 1281 manual and some 40 000 automatic weighings, which were then classified into the phenological stage of the grazed plants and the physiological stage of the cow. By assuming that a cow is biologically the same within a 7-day period, and with as little as 1 weeks' data, an 11% change in a cow's liveweight can be detected 90% of the time with only one additional automatic weighing. Real changes in liveweight as little as 5% can be detected at the same rate if more than a single weighing is observed. Therefore, automatic weighing not only can be used to estimate the effects of past management practices, but also can be useful in making sound day-to-day livestock management decisions on individual animals.