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This study was conducted to determine the effects of supplemental grain feeding on reproductive performance of lactating range beef cows. Results indicated that feeding 3.86 kg grain either before or during lactation, or before and during the breeding season, decreased the fall pregnancy rate in supplemented cows compared to cows on range forage only. The high level of grain feeding reduced grazing time and subsequent forage intake and served as a substitute for the range forage rather than a supplement. A major part of the diet of dams grazing during the early spring period was new growth on western wheatgrass. This forage was available in limited amounts during this early time period, and the moisture content ranged from 65.3 to 82.1%, resulting in a low dry matter intake. Lactating dams lost 1.23 kg daily during this time period, but daily calf gains at this time averaged 0.71 kilograms. Thus, a major part of the nutrients consumed by the dam was being used for production of milk, and at least a portion of the milk was being produced at the expense of body tissue stores of the dam. This work indicates that the period from calving until adequate forage with a sufficient dry matter content is available to produce weight gains in the lactating dams should be considered a critical nutritional period. These findings indicate the need for studies to determine the most satisfactory methods of meeting the nutritional requirements of the lactating dam during this period. 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