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

Abstract

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.