Four ruminally and duodenally cannulated beef steers (388 +/- 12 kg) were used to evaluate effects of advancing season on forage quality, intake, site of digestion, and microbial efficiency while grazing mixed-grass prairie in western North Dakota. Five 11-d sample collections were conducted from late June to mid-November. Chromic oxide (8 g) was dosed twice daily at 0700 and 1900 h via gelatin capsule from d 2 to 11 of each collection period, and duodenal and fecal collections were performed on d 7 to 11. Masticate samples were collected for each sampling period. Dietary N declined linearly (P = 0.01), from 1.95% in June to 1.15% in November, whereas NDF increased linearly (P = 0.01), 72.4% in June to 85.1% in November. Total OM intake (g/kg of BW) decreased linearly (P <or= 0.01) from June to November, from 26.2 to 11.4 +/- 2.7 g/kg of BW, respectively, and ruminal fill increased linearly (P <or= 0.01) from June to November, from 45.4 to 65.9 +/- 3.3 kg, respectively. True ruminal OM and N digestion declined linearly (P <or= 0.01) with advancing season. Total tract OM, N, and NDF digestion declined linearly (P <or= 0.01). Microbial efficiency (g of microbial N/kg of OM truly fermented) increased linearly (P = 0.02) from late June to late November. Undegradable intake protein (UIP) intake (g/d) declined (P <or= 0.06) linearly and cubically from 374.4 in June to 215.9 g in November. Degradable intake protein (DIP) intake was 735.5 g/d in June and declined linearly (P <or= 0.01) to 99.5 g/d with advancing season. Likewise, microbial CP supply at the duodenum (g/d) declined linearly (P <or= 0.01) as season advanced. As expected, DIP (% of CP) decreased (P <or= 0.01; 66.1 to 31.9 +/- 5.8% for June to November, respectively), whereas percentage of UIP increased (P <or= 0.01; 33.9 to 68.1 +/- 5.8% for June to November, respectively) with advancing season. The portions of DIP and UIP (% of CP) were not different in late August and mid-September. These data imply that mixed-grass range forage consumed by cattle after late September is deficient in N, particularly DIP, and that forage intake may decrease and may be too low to support adequate cow performance. Supplementation could be implemented to overcome these deficiencies; however, additional research is needed to determine supplemental type and frequency needed to optimize nutritional and economic efficiency.