FORAGE INTAKE AND DIGESTION BY CATTLE GRAZING MIDGRASS PRAIRIE RANGELAND OR SIDEOATS GRAMA SWEETCLOVER PASTURE

In mid-May, beef cattle fitted with esophageal (four steers/pasture) o r ruminal and duodenal cannulas (six heifers/pasture; 274 +/- 6 kg BW +/- SE) grazed midgrass prairie (excellent range condition; PRAIRIE) o r a seeded mixture of sideoats grama (48% of pasture DM) and sweetclov er (6% of pasture DM; Bouteloua curtipendula [Michx.] Torr./Melilotus officinalis [L.] Lam.; PASTURE). Masticate NDF, ADF, and in vitro OM d isappearance did not differ ( P > .11) between forage types (average = 66.6, 36.1, and 58.8% of OM). Only N differed (P = .02) between PRAIR IE (2.1) and PASTURE (2.4% of OM). Extents of in situ OM and N disappe arance were greater (P < .05) and rate of N disappearance between 12 a nd 36 h was slower (P < .10) from PRAIRIE than from PASTURE masticate. Based on in situ data, the ruminally degraded N:ruminally degraded OM (grams/kilogram) ratio differed (P < .05) between PRAIRIE (22) and PA STURE (25). Ruminal ammonia N concentration (milligrams/deciliter) was less (p < .02) for PRAIRIE (2.8) than for PASTURE (3.8). Forage OM in take and fecal OM output did not differ (P > .72; average = 8,207 and 3,380 g/d), but duodenal OM flow tended (P = .13) to be greater (PRAIR IE = 4,892, PASTURE = 5,170 g/d) in cattle grazing PASTURE. Apparent a nd true ruminal OM digestion did not differ between forage types (P > .18; average = 38.3 and 48.5%). Nitrogen intake, nonammonia N, and for age N flow at the duodenum were greater (P < .04) for PASTURE (198 vs 171, 242 vs 210, and 162 vs 135 g/d) than for PRAIRIE. Microbial N flo w (average = 78 g/d) and microbial efficiency (average = 20 g of micro bial N/kg of OM truly fermented) did not differ (P > .25) between fora ge types. Apparent and true ruminal N digestion did not differ (P > .6 5; average = -26.6 and 19.3%) between forage types. Flow of nonammonia N was in excess relative to digestible OM intake; hence, digestible O M intake seemed to be first-limiting for performance by cattle grazing either forage type.