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Protein supplementation is widely used to enhance the nutritional status of cattle on rangeland. The effects of protein supplementation on particulate and fluid passage rates, rumen fermentation, and weight gain were evaluated on cattle grazing dormant blue grama rangeland. Twelve rumen-cannulated steers were randomly allotted to 3 equal supplement groups (4/treatment): cottonseed cake (CSC), pelleted alfalfa (ALF), or no supplement (CON). Supplements were individually fed every other day at isonitrogenous levels (1.7 kg/hd CSC vs 3.6 kg/hd ALF). Particulate passage rate was measured during 2 collection periods. Concurrently, 102 yearling heifers were allotted to the same treatment groups for measurement of average daily gain. Cattle were fed these treatments from January through April 1983. In a second trial, 9 rumen-cannulated steers were randomly allotted to the same treatment groups as trial 1. An intraruminal dose of Co-EDTA was used to estimate fluid passage rates. Rumen ammonia, volatile fatty acids, and pH were also measured. In trial 1, average daily gain did not differ between ALF and CSC supplemented heifers; however, both ALF and CSC gained more than CON heifers. Passage rate estimates were not different among treatment groups. In trial 2, rumen fluid dilution rate, volume, and outflow rate were not different among treatments. Rumen ammonia-N was different at 11 h postsupplementation when the CSC steers had higher levels than steers in other treatment groups. Rumen pH was not influenced by supplementation. Molar proportions of acetate and propionate at 8 and 11 h after supplementation differed among treatment groups. Acetate was lowest in ALF, intermediate in CSC, and highest in CON supplemented steers. Propionate followed the reverse trend. Overall, protein supplementation improved livestock performance but the mechanism involved was not elicited. The current data suggest shifts in fermentation patterns and meeting dietary demands for gain are the factors involved in improving performance. 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