title: Forage intake and digestion by cattle grazing midgrass prairie rangeland or sideoats grama/sweetclover pasture.
abstract: In mid-May, beef cattle fitted with esophageal (four steers/pasture) or ruminal and duodenal cannulas (six heifers/pasture; 274 +/- 6 kg BW +/- SE) grazed midgrass prairie (excellent range condition; PRAIRIE) or a seeded mixture of sideoats grama (48% of pasture DM) and sweetclover (6% of pasture DM; Bouteloua curtipendula [Michx.] Torr./Melilotus officinalis [L.] Lam.; PASTURE). Masticate NDF, ADF, and in vitro OM disappearance did not differ (P > .11) between forage types (average = 66.6, 36.1, and 58.8% of OM). Only N differed (P = .02) between PRAIRIE (2.1) and PASTURE (2.4% of OM). Extents of in situ OM and N disappearance were greater (P < .05) and rate of N disappearance between 12 and 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 PASTURE (25). Ruminal ammonia N concentration (milligrams/deciliter) was less (P = .02) for PRAIRIE (2.8) than for PASTURE (3.8). Forage OM intake 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 (PRAIRIE = 4,892, PASTURE = 5,170 g/d) in cattle grazing PASTURE. Apparent and true ruminal OM digestion did not differ between forage types (P > .18; average = 38.3 and 48.5%). Nitrogen intake, nonammonia N, and forage 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 flow (average = 78 g/d) and microbial efficiency (average = 20 g of microbial N/kg of OM truly fermented) did not differ (P > .25) between forage types. Apparent and true ruminal N digestion did not differ (P > .65; average = -26.6 and 19.3%) between forage types. Flow of nonammonia N was in excess relative to digestible OM intake; hence, digestible OM intake seemed to be first-limiting for performance by cattle grazing either forage type.
authors.affiliation: Oklahoma Agricultural Experiment Station, Stillwater 74078-0102.
authors.forename: S A
authors.initials: SA
authors.lastname: Gunter
authors.forename: F T
authors.initials: FT
authors.lastname: McCollum
authors.forename: R L
authors.initials: RL
authors.lastname: Gillen
authors.forename: L J
authors.initials: LJ
authors.lastname: Krysl
mesh_terms: D000821:Animal Feed; D000818:Animals; D002417:Cattle / Q000502:physiology*; D004063:Digestion; D004386:Duodenum / Q000378:metabolism; D004435:Eating; D007887:Fabaceae; D005260:Female; D005285:Fermentation; D006863:Hydrogen-Ion Concentration; D008297:Male; D009584:Nitrogen / Q000378:metabolism; D010945:Plants, Edible; D010946:Plants, Medicinal; D006109:Poaceae; D012417:Rumen / Q000378:metabolism
chemical_list: D009584:Nitrogen