Influence of advancing season on dietary composition, intake, site of digestion, and microbial efficiency in beef steers grazing season-long or twice-over rotation native range pastures in western North Dakota.

Eight ruminally and duodenally cannulated beef steers (374 +/- 11 kg) were used to evaluate effects of advancing season and grazing treatment (season-long; SL or twice-over rotation; TOR) on dietary composition (Exp. 1 and 2), intake, site and extent of digestion, and microbial efficiency (Exp. 2) of native range. In Exp. 1, six 11-d sample collections were conducted from early June to mid-November 2000 and 2001. In vitro OM disappearance decreased (P < or = 0.04) for both years and both treatments with advancing season. Dietary N declined (P < or = 0.07), whereas fiber content increased (P < or = 0.05) during both years in both treatments, with the exception of NDF (P = 0.55) during yr 2 (YR2) on the TOR, as season advanced. In Exp. 2, three 11-d sample collections were conducted from late July to mid-September 2000 on SL and TOR. Organic matter intake (g/kg of BW) was not altered (P = 0.28) by grazing treatments or advancing season. Total tract OM and apparent ruminal OM digestion were not different (P > or = 0.12) between treatment and decreased (P < or = 0.04) with advancing season. Grazing treatment x season interactions (P = 0.06) were present for true ruminal OM digestibility with TOR being greater (P < or = 0.10) than SL in late August and mid-September but not late July. Microbial efficiency was greater (P = 0.07; 15.1 vs. 10.8 +/- 1.6 g of microbial N/kg of OM truly fermented) in SL than TOR, respectively. Degradable intake protein (g/d) was less (P = 0.05) in TOR than SL during late July to early August and not affected by treatment in late August or mid-September. However, undegradable intake protein was unchanged (P > or = 0.54) between treatment and across season. These data suggest that mixed-grass range forage consumed by cattle after late September is deficient in N, particularly degradable intake protein, and that forage intake may be insufficient to support adequate performance in lactating cows independent of grazing management strategies evaluated. Knowledge of diet quality and forage intake should aid producers in meeting the nutritional needs of livestock grazing these forages.