AbstractNitrogen fertilization has been used to increase forage production of range grasses in the northern Great Plains; however, little information is available as to how N fertilization affects minor element composition of range forage. Our objectives were to determine the effects of N fertilization on Zn, Cu, Fe, and Mn concentrations in vegetative and floral tillers of western wheatgrass (Agropyron smithii Rydh.) and green needlegrass (Stipa viridula Trin.) throughout the growing season. Nitrogen fertilizer (0,40,80,160,320, and 640 kg N/ha) was applied in a completely randomized design to each grass species growing on a calcareous silt loam (Typic Haploborolls). Plants were collected on 10 and 11 sampling dates in 1973 and 1974, respectively. Nitrogen fertilization significantly increased the level of Zn, Cu, and Mn in green needlegrass and Zn and Mn in western wheatgrass forage. Absolute changes in mineral composition caused by N fertilization were very small and of little practical significance. Forage Zn and Cu levels declined throughout the growing season and were generally below recommended allowances (less than 20 to 30 πg/g Zn and 4 πg/g Cu) for proper cattle nutrition by mid‐June. Therefore, supplemental Zn and Cu may be required in diets of cattle being fed or grazing western wheatgrass or green needlegrass grown in the northern Great Plains after mid‐June. Forage Fe and Mn levels decreased as the growing season progressed until maximum forage yield was reached in June. Iron then started accumulating in the forage while Mn remained relatively constant during the remainder of the growing season. Iron (>80 πg/g) and Mn (>35 πg/g) were well above the recommended minimum levels for proper cattle nutrition. Floral tillers of both grasses had lower concentrations of Zn, Fe, Mn, and Cu than did vegetative tillers. In general, the earlier the harvest, the better the forage quality of western wheatgrass and green needlegrass will be with respect to Zn and Cu nutrition of cattle. Maintaining the grasses in the vegetative rather than floral growth stage would also improve forage quality