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 an d duodenal (six calves/pasture; beginning BW +/- SE = 267 +/- 6 kg) ca nnulas grazed midgrass prairie rangeland (excellent range condition; M IDGRASS) or plains bluestem (Bothriochloa ischaemum var. Plains) pastu re (BLUESTEM) in mid-May, late-June, mid-August, and mid-October of 19 90 and 1991. Nitrogen in masticate samples collected from MIDGRASS was lowest (P < .05) in June and August across both years. The N in BLUES TEM 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 t hrough August; fall regrowth in October occasionally resulted in a sma ll decrease (P < .05) in fiber content. In vitro OM disappearance (IVO MD) followed a pattern similar to N content. The IVOMD of BLUESTEM mas ticate was greater (P < .05) than, that of MIDGRASS masticate. The rum inal ammonia N concentration (milligrams/deciliter) in cattle grazing BLUESTEM (4.5) usually was greater (P < .05) than in cattle grazing MI DGRASS (3.3). In situ OM and N disappearance was greater (P < .05) fro m BLUESTEM masticate than from MIDGRASS masticate in May, June, and Au gust. The ruminally degraded N:ruminally degraded OM ratio (grams/kilo grams) estimated from in situ digestion suggested that cattle grazing MIDGRASS during the mid-summer of both years and BLUESTEM in August 19 91 may have been marginally deficient in ruminally degraded N. Plains bluestem pasture would complement MIDGRASS by providing better quality grazing during the midsummer.