In the semi-arid Intermountain West, irrigation water is both an increasingly limited resource and a significant input cost. This study was designed to determine the rooting depth and water use of pasture species relative to herbage dry matter production, with the goal of increasing the effective rooting depth of pasture mixtures. Nine plant species were grown as single plants in 10-cm-dia, 90-cm-deep PVC containers, harvested three times, and then destructively harvested to determine root surface area. Plant species included the legumes alfalfa, birdsfoot trefoil and white clover, the cool-season grasses orchardgrass and tall fescue, the warm-season grasses big bluestem and blue grama, and the forbs Lewis flax and small burnet. Plant species from these four functional groups were also grown as mixtures in 20-cm-dia, 60-cm-deep PVC containers. Mixtures contained either two plants each of orchardgrass and white clover (OG-WC) or orchardgrass and birdsfoot trefoil (OG-BT), or one plant each of orchardgrass, white clover, small burnet and blue grama (OG-WC-SB-BG) or one plant each of orchardgrass, birdsfoot trefoil, small burnet and blue grama (OG-BT-SB-BG). By the third harvest, there was higher herbage DM for mixtures with four rather than two plant species, and herbage DM was predicted by the sum of DM for plants grown alone for mixtures containing birdsfoot trefoil. However, mixtures containing white clover had higher DM at the third harvest than predicted by the sum of DM of individual plants.