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Results of a 2-year study conducted on the mixed prairie near Sidney, Montana, indicated that high rates of nitrogen (N) fertilization accompanied by phosphorus (P) were necessary to obtain maximum levels of energy fixation. Total energy fixed over a 2-year period (1969-70) by the above-ground portion of native vegetation was 1136 kcal/ m2 or 2384 lb./acre yield equivalent. Single applications of 100, 300, and 900 lb./acre of N increased the level of energy fixation 1.6-, 2.2-, and 2.0-fold, respectively, when applied without P; 1.7-, 3.2-, and 2.8-fold, respectively, when applied with 100 lb. P/acre; and 2.0-, 3.0-, and 3.3-fold, respectively, when applied with 200 lb. P/acre. The high N-P treatment decreased the grass plus sedge portion of total yield from 77 to 70% in 1969, but increased it from 61 to 98% in 1970. Increased growth of individual plants and changes in species composition accounted for the high levels of energy fixation by the fertilized vegetation. Precipitation-use efficiency for the 1970 growing season was 110 lb./acre/inch on the unfertilized plots and 336 lb./acre/inch on the high N-P treatment plots. This material was digitized as part of a cooperative project between the Society for Range Management and the University of Arizona Libraries. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020