moisture, and provides both high yields and quality (Karnezos et al., 1994) as well as fixing 82 to 254 kg N Producers in the irrigated steppe of the southern Rocky Mountains ha Ϫ1 yr Ϫ1 (Heichel and Henjum, 1991). Many producers are seeking ways to improve the summer productivity of their established cool-season grass pastures, commonly tall fescue (Festuca arun-have incorporated this species into their irrigated cooldinacea Schreb.). From 1994 to 1997, a study was conducted under season grass pastures, grazing them during the spring irrigation at the New Mexico State University Alcalde Sustainable and fall and harvesting 1 to 2 cuttings of hay during the Agriculture Science Center, in which dry matter yield of monoculture summer to use as supplemental winter feed. However, tall fescue was compared with that of swards containing tall fescue there are some limitations to alfalfa's sustainability in in mixtures with alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus regard to increased inputs to prevent bloat (Karnezos corniculatus L.), cicer milkvetch (Astragalus cicer L.), and kura clover et al., 1994; Majak et al., 1995; Marten et al., 1990; (Trifolium ambiguum M.B.) in a randomized complete block design Peterson et al., 1992), historically poor persistence under with three blocks. Percent tall fescue in the sward and the yield of grazing (van Keuren and Matches, 1988), reestablishtall fescue in mixtures declined from 1994 to 1996 and then increased ment only by seeding, and autotoxicity in reestablishin all mixtures except tall fescue-kura clover in 1997. Dry matter yields of alfalfa and cicer milkvetch increased until 1996 then declined, ment (Melton et al., 1988). For these reasons, interest in while birdsfoot trefoil equilibrated in 1996 and 1997 and kura clover alternative forage legumes is building and, while several continued to increase. Combined dry matter yields followed a trend have been shown to be well adapted to the higher precipsimilar to that of the legume yields except that, in 1996 and 1997, itation areas of the eastern USA (Heichel and Henjum, yields of the tall fescue-cicer milkvetch mixture were comparable to 1991; Hoveland and Richardson, 1992; Taylor and those of monoculture tall fescue, which were 5.76 and 6.72 Mg ha Ϫ1 for Smith, 1998) and to the drier but cooler climates of the 1996 and 1997, respectively. Over the life of the study, tall fescue-cicer central and northern Great Plains and Rocky Mountains milkvetch yielded less than one-half that of tall fescue-alfalfa while of the USA and Canada (Berg, 1990; Farnham and
