Interest in soybean (Glycine max L.) as an irrigated crop for the intermountain valleys of the western United States is a recent phenomenon. While the potential for grain production has driven much of this interest, the recent development of improved forage types by the USDA and a prolonged drought in the region also has created some interest in soybean as an annual forage. In 2003 and 2004, the forage potential of three recently developed forage-type cultivars [Derry (MG VI), Donegal (MG V) and Tyrone (MG VII)] were compared to that of six cultivars, representing maturity group 0, I and II soybeans selected for grain production. Cultivars were arranged in a randomized complete block design and grown under irrigated conditions in the Yellowstone River Valley near Huntley, Montana. Forage yield of all cultivars averaged 11.5 metric tons of dry matter per hectare (5.1 tons/acre), and varied from 10.6 to 13.2 metric tons per hectare (4.7 to 5.9 tons/acre). Average protein content and relative feed value of the soybean forage was 14.4% and 124.5, respectively. Higher protein and relative feed values were associated with earlier maturing cultivars where more than 30 percent of the total dry matter produced was derived from reproductive tissue compared to later maturing types where nearly 100 percent of the dry matter was derived from stem and leaf tissue. In 2004, substantially higher nitrate levels were detected in the dry matter of the later maturing forage types where little or no pod development had occurred prior to harvest.