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Diets of cattle grazing crested wheatgrass [Agropyron desertorum (Fisch.) Schult.] or native range pastures and major forage species in these diets were analysed for crude protein (CP), acid and neutral detergent fiber (ADF and NDF), lignin, and in vitro dry matter disappearance (IVDMD) 1975-1978. Objectives were to determine (1) rate of change in forage quality, (2) effect of weather on rate of change, and (3) degree of selection for diet quality by cattle. CP of 6 forage species declined .03-.23 percentage points/day, while IVDMD declined .06-.90 points/day. CP and IVDMD of western wheatgrass (Agropyron smithii Rydb.), blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Steud.], and scarlet globemallow [Spahaeralcea coccinea (Pursh.) Rydb.] were high in spring, while ADF, NDF, and lignin were low, but quality of grasses decreased much faster than that of the forb. Quality of needleandthread (Stipa comata Trin. & Rupr.) was lower than that of the other two grasses in spring, but CP declined more slowly then, while IVDMD declined at the same rates as that of blue grama and western wheatgrass. Quality of sedges (Carex spp.) was similar to that of western wheatgrass and blue grama in spring, but CP decreased faster while fiber components increased slower than those of grasses. CP of western wheatgrass and blue grama was increased by abundant spring rainfall, while that of blue grama increased after heavy summer rains. As the season progressed, cattle on range selected diets higher in crude protein and lower in cellulose than expected on the basis of botanical composition of the diet and composition of individual species. Quality of forage consumed increased markedly in mid-July when immature blue grama replaced needleandthread in the diet. Quality of crested wheatgrass declined faster than that of range grasses, although it was higher early in the spring. Diets of cattle on crested wheatgrass pasture in early spring were lower in quality than clipped crested wheatgrass, because of consumption of standing dead material, but diets were higher in quality than clipped grass in late spring.