An extended procedure is described for modelling cow weight change over the year for valuing cow weight in breeding objectives. The pattern of change, before and after trait change, is able to be approximated from breeder-described variables for any production system. Examination of seven datasets suggested cow weight changes systematically over age in many herds and breeds. Analyses showed weight loss from pre-calving to mating and cow weight at mating both change in proportion to the weight of the cow pre-calving. The larger change in weight pre-calving than at mating affects feed costs in valuing cow weight. Adoption of the procedure will increase the precision with which feed cost is associated with cow weight in beef cattle breeding objectives.