Water available for irrigation in the Inter-Mountain Western USA is frequently limited and varies with time and location. Our objective was to evaluate the relative forage production of grass and legume monocultures compared with grass legume mixtures at five irrigation levels and with minimal addition of fertilizer. Five distinct alfalfas, birdsfoot trefoil (BFTF), tallfescue, meadowbrome, and each legume in a mix with each grass were evaluated for three years and four harvests to simulate grazing. The five water levels received an average of 123, 237, 350, 386, and 599 mm for the season. Average seasonal production was 7.89, 5.57, 4.94, 8.02, and 5.99 Mg/ha for the alfalfa's, BFTF, grasses, alfalfa grass mixtures, and BFTF grass mixtures respectively. Increase in production of the grass-alfalfa mixtures over grass monocultures ranged from 42% at the lowest water level in the meadowbrome-alfalfa mixture to 88% at the highest water level in the tallfescue-alfalfa mixture. Increase in production of the grass-BFTF mixtures ranged from 6% at the low water level to 70% at the highest water level in the BFTF-tallfescue mixture. Yields of the mixtures that included alfalfa tended to be higher than the alfalfa component except at the third harvest when the grasses were dormant at the lower water levels. Yields of the mixtures that included BFTF were two to three times higher than the BFTF component at harvest one but lower at all other harvests. Regardless of amount of irrigation a significant production advantage can be realized by including a legume with grasses in a pasture, particularly in low fertility situations.