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A 3-year study evaluated weight gain, blood urea nitrogen (BUN), and fecal nitrogen (FN) of beef steers fed 0.00, 0.34, or 0.68 kg/hd/day of cottonseed meal (41% CP) while grazing mesquite (Prosopis glandulosa var. glandulosa Torr.)/tobosagrass (Hilaria mutica [Buckl.] Benth.) range between April and July. Mixed breed beef steers (avg wt 230 kg) were allocated to three 6-pasture grazing cells and group-fed prorated amounts of supplement 3 days a week. Individual weights were recorded every 21 days. Crude protein in clipped forage samples remained above 7.0% except in July, 1985 (6.5%). Gain response varied among periods within year but the primary effects occurred in the first 40 to 60 days of grazing. In 1985, daily gains over 92 days were 0.38, 0.44, and 0.67 kg/hd/day for the 0.00, 0.34, and 0.68 kg supplement groups, respectively. In 1986 and 1987, daily gains during 85-day trials were 0.65, 0.66, and 0.71 kg/hd/day and 0.98, 1.08, and 1.07 kg/hd/day, respectively. Blood and feces were collected from 10 steers in each treatment group on each weigh date during the first 2 years. The 0.68 kg/hd/day supplement maintained higher (P<0.05) BUN and FN than the control group but response to 0.34 kg supplement was inconsistent. Performance and BUN data suggested that protein concentrate was not the appropriate supplement for steers grazing tobosagrass in the spring and summer. 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