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Sixteen ruminally cannulated steers grazing Northern Great Plains native winter range were assigned to 4 treatments in a 2 X 2 factorial arrangement. Main effects were protein and monensin. A soybean meal-barley pellet (P; 26% crude protein) was fed at 0 and .8 kg head-1 d-1. Steers either received no monensin (M) or M was released at 101 ml/day via a ruminal delivery device. Forage intake, rmninal fermentation, in vivo organic matter (OM) digestibility, and ruminal fluid passage and particulate digesta kinetics were measured during trials in November and January. Esophageally fistulated steers were used to collect diet samples during each trial. Dietary crude protein was greater (P<0.01) in November (8.3%) than January (4.9%). Forage OM intake was not (P>0.10) influenced by either P or M. In vivo OM digestibility was increased (P<0.05) by P (60.6 vs 57.4%) and not affected (P>0.10) by M. Particulate passage rate increased (P<0.05) when P was combined with M. Ruminal fluid flow characteristics, fluid volume and pH were not affected (P>0.10) by either P or M. Ruminal ammonia-N was increased (P<0.01) by P (2.9 vs .6 mg/100 ml) and not affected (P>0.10) by M. Total ruminal volatile fatty acid concentrations, along with molar proportions of ruminal propionate and butyrate, were not affected (P>0.10) by P or M. Ruminal acetate was decreased (P<0.10) by P and not influenced (P>0.10) by M. We conclude that supplemental protein, through ruminal modifications, has beneficial effects on OM digestibility, and can thereby provide cattle grazing winter range with additional energy at a time when it is most crucial. 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