AbstractBoth quantity and quality must be considered in selecting forages for livestock. This study was conducted to determine effects of N and irrigation treatments on the quality of tall fescue (Festuca arundinacea Schreb.) and smooth bromegrass (Bromus inermis Leyss.) forage in the Southern High Plains. âFawnâ tall fescue and âSouthlandâ smooth bromegrass were grown on Pullman clay loam (fine, mixed thermic, Torrertic Paleustoll) under N rates of 0, 168, 336, 504, and 672 kg haâ1 yearâ1 on three water regimes: Wâl, a moderate (about 50âcm) and Wâ2, an adequate (about 100âcm) irrigation water level, both distributed for seasonâlong production, and Wâ3, a moderate (about 60âcm) irrigation level distributed for coolâseason forage production. Forage was harvested and analyzed for N, P, K, Ca, Mg, NO3â and in vitro dry matter disappearance (IVDMD). Nitrogen fertilization increased N, K, and NO3ââN in both forages. The increased K levels increased the K/(Ca + Mg) ratios. Less marked effects of N included slight increases in IVDMD in both grasses, increases in Ca and Mg, and decreases in P in tall fescue but not in smooth bromegrass forage. The total N and NO3ââN were lower and the Ca and Mg were higher in forage from Wâ2 than in that from Wâl and Wâ3. With N rates through 336 kg/ha, NO3ââN remained below the 2,000âppm level considered harmful to livestock. Forage P and IVDMD were not affected by differences in irrigation. Although smooth bromegrass was superior to tall fescue in N, K, and Ca concentrations and in IVDMD, N fertilization at levels high enough to produce satisfactory yields increased the K/(Ca + Mg) ratios to levels considered critical for induction of grass tetany. With pure stands of smooth bromegrass, supplemental feeding of Mg would be necessary. Tall fescue contained more Mg than did smooth bromegrass, and its K/(Ca + Mg) ratios were favorable. Without P fertilization, feeding supplemental P would be necessary with both forages. Surfaceâapplied feedlot waste, at N rates equivalent to those applied as NH4NO3, was less than oneâhalf as efficient as NH4NO3 in increasing N to forage, but it increased P to levels satisfactory for nutrition of mature cows. Both irrigation adequacy and grass species must be considered in fertilizing for maximum production of high quality forage.