The effects of variations in daily roughage intake on feeding costs are analysed on the basis of experimental data reported elsewhere [see preceding abstr. and DSA 38, 7105], together with model calculations taking into account the entire calving interval. Factors which are critical in determining how much roughage should be offered to each cow are considered, namely: the cow's selection of better-quality roughage; and the possible utilization of the poor-quality roughage residues which are left by the cow because of this selection. Calculations show that when roughage is expensive or in limited supply, feeding costs can be reduced by feeding straw so as to ensure that the roughage intake capacity of each cow is fully utilized. Feeding costs with roughage at different prices, and with straw, are plotted for a typical cow which yields 5000 kg FCM/yr and has an av. daily roughage intake of 10 kg DM, with a 10% coeff. of variation. The cost curves show how much roughage should be offered in each case to minimize feeding costs.