Forage intake by and site and extent of digestion in beef cattle grazing midgrass prairie rangeland or plains bluestem pasture throughout the summer.

Eight beef steers fitted with esophageal (four steers/pasture) and 12 beef calves fitted ruminal and duodenal (six calves/pasture; beginning BW = 267 +/- 6 kg) cannulas grazed either midgrass prairie rangeland (excellent range condition; MIDGRASS) or plains bluestem (Bothriochloa ischaemum var. Plains) pasture (BLUESTEM) during mid-May, late June, mid-August, and mid-October of 1990 and 1991 in order to compare nutrient intake and digestion. Forage OM intake (OMI) by cattle grazing MIDGRASS or BLUESTEM was similar (P > .05) in June and August. In May and October, cattle grazing MIDGRASS consumed more (P < .05) OM than cattle grazing BLUESTEM. The extent of true ruminal OM digestion was similar (P > .05) between forage types except in October 1991, when the extent of digestion for BLUESTEM was greater (P < .05) than for MIDGRASS. The N intake by cattle interacted by year and forage (P < .05). Nitrogen intake by cattle grazing MIDGRASS tended to be lower in June and August than in May and October. The N intake by cattle grazing BLUESTEM peaked (P < .05) in August during 1990; however, N intake was lowest (P < .05) in August during 1991. Duodenal non-ammonia N (NAN) flow was higher (P < .05) in cattle grazing BLUESTEM than in cattle grazing MIDGRASS from May through August; however, duodenal NAN flow in cattle grazing BLUESTEM was lower (P < .05) in October 1991. Duodenal microbial N synthesis (grams/day) responded quadratically (P < .05) to total ruminal OM digestion (kilograms/day). Extent of true ruminal N digestion of both forages decreased (P < .05) as forage became more mature and lower in total N. Midgrass prairie seemed superior to BLUESTEM in May and October because of higher energy intakes and BLUESTEM seemed to be a good alternative to MIDGRASS during June through August, suggesting that these forages would make excellent complements. Furthermore, these data suggest that, in cattle grazing either forage, duodenal NAN flow was disproportionately high relative to energy intake.

Diet quality and ruminal digestion in beef cattle grazing midgrass prairie rangeland or plains bluestem pasture throughout the summer.

Beef cattle fitted with esophageal (four steers/pasture) or ruminal and duodenal (six calves/pasture; beginning BW +/- SE = 267 +/- 6 kg) cannulas grazed midgrass prairie rangeland (excellent range condition; MIDGRASS) or plains bluestem (Bothriochloa ischaemum var. Plains) pasture (BLUESTEM) in mid-May, late-June, mid-August, and mid-October of 1990 and 1991. Nitrogen in masticate samples collected from MIDGRASS was lowest (P < .05) in June and August across both years. The N in BLUESTEM masticate peaked (P < .05) in August 1990, but N was lowest (P < .05) in August 1991. The detergent fiber content of masticate from both forages increased (P < .05) as the grazing season advanced from May through August; fall regrowth in October occasionally resulted in a small decrease (P < .05) in fiber content. In vitro OM disappearance (IVOMD) followed a pattern similar to N content. The IVOMD of BLUESTEM masticate was greater (P < .05) than that of MIDGRASS masticate. The ruminal ammonia N concentration (milligrams/deciliter) in cattle grazing BLUESTEM (4.5) usually was greater (P < .05) than in cattle grazing MIDGRASS (3.3). In situ OM and N disappearance was greater (P < .05) from BLUESTEM masticate than from MIDGRASS masticate in May, June, and August. The ruminally degraded N:ruminally degraded OM ratio (grams/kilograms) estimated from in situ digestion suggested that cattle grazing MIDGRASS during the mid-summer of both years and BLUESTEM in August 1991 may have been marginally deficient in ruminally degraded N. Plains bluestem pasture would complement MIDGRASS by providing better quality grazing during the mid-summer.

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) 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.