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 nutri ent intake and digestion. Forage OM intake (OMI) by cattle grazing MID GRASS or BLUESTEM was similar (P > .05) in June and August. In May and October, cattle grazing MIDGRASS consumed more (P < .05) OM than catt le grazing BLUESTEM. The extent of true ruminal OM digestion was simil ar (P > .05) between forage types except in October 1991, when the ext ent of digestion for BLUESTEM was greater (P < .05) than for MIDGRASS. The N intake by cattle interacted by year and forage (P < .05). Nitro gen intake by cattle grazing MIDGRASS tended to be lower in June and A ugust 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 MID GRASS from May through August; however, duodenal NAN flow in cattle gr azing BLUESTEM was lower (P < .05) in October 1991. Duodenal microbial N synthesis (grams/day) responded quadratically (P < .05) to total ru minal 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, the se data suggest that, in cattle grazing either forage, duodenal NAN fl ow was disproportionately high relative to energy intake.