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Low phosphorus (P) levels in Northern Great Plains rangeland forage combined with limited data on the P requirements of range cows (Bos taurus), precipitated 2 studies conducted to compare the performance of P supplemented and no P supplemented (control) beef cows. Phosphorus supplementation levels ranged from 4 to 8 g day-1 depending on estimated P needs at different times of the year. The 2 groups of cows previously had been involved in replacement heifer growing studies, with P treatments established 462 and 402 days, respectively, before initiation of these studies. Winter feed consisted of mixed hay, primarily smooth bromegrass (Bromus inermis Leyss.), with corn silage (Zea mays L.) fed only in 1982 and 1983 from calving to the time cows were turned on summer pasture. Summer pastures contained primarily western wheatgrass [Pascopyrum smithii (Rydb.) A. Love], needleandthread (Stipa comata Trin. and Rupr.), green needlegrass (S. viridula Trin.), blue grama [Bouteloua Gracilis (H.B.K.) Griffiths] and upland sedges (Carex spp.). The P status of cows used in these studies appeared to be estimated more reliably by forage P than by serum or fecal P. According to P levels in hay and pasture, the diets of control cows were below recommended P levels for about 9 months of the year. However, weight change differences between P supplemented and control cows during the first lactation and gestation periods were gradually lost by the end of the studies. Conception rates of control cows were slightly lower (P<0.08) in the first but not the second study. There were no differences in average calving date or calf birth weights, but P supplementationndid increase (P<0.01) calf weaning weights. Cow weight changes, calf weaning weight differences, forage and serum P data, and in the first study conception rate differences indicate that Northern Great Plains forages are marginal to deficient in P for optimal production of beef cows. The most consistent benefit from P supplementation was an increase in calf weaning weights. Data also indicate that energy supplementation for 30 days after calving may increase conception rates. 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