STEERS GRAZING INTERMEDIATE WHEATGRASS AT VARIOUS STAGES OF MATURITY - EFFECTS ON NUTRIENT QUALITY, FORAGE INTAKE, DIGESTA KINETICS, RUMINAL FERMENTATION, AND SERUM HORMONES AND METABOLITES

Six ruminally cannulated Holstein steers (average BW of 495 +/- 5 kg) grazed a 32-ha, nonirrigated, intermediate wheatgrass (Thinopyrum inte rmedium Host) pasture from April 10 through December 7. Sample collect ions were conducted in May, June, September, and November 1989. Total masticate N decreased (P <.05) and bound N increased (P <.05) with adv ancing forage maturity. Masticate in vitro OM disappearance was lower (P <.05) in November (40.5%) than in May (62.0%). Organic matter intak e (grams/kilogram of BW) decreased(P <.05) at each sampling date (May = 35.7, June = 26.8, September = 20.0, November = 14.9). Corresponding ly, particulate passage rate decreased and gastrointestinal mean reten tion time increased (P <.05) with advancing forage maturity. Extent an d rate of NDF digestion were greater (P < .05) during May and June tha n during September and November. Ruminal NH3 N and total VFA concentra tions were greater (P <.05) during May and June than during September and November. Advancing forage maturity did not alter (P >.10) serum g rowth hormone concentrations; however, nonesterified fatty acid concen trations were greater (P <.05) during September and November than duri ng May and June. Serum insulin-like growth factor I decreased (P <.05) with advancing forage maturity. Seasonal changes in forage intake and ruminal fermentation suggest that animal performance might be increas ed if supplemental energy is provided early in the growing season of i ntermediate wheatgrass; however, supplemental protein would likely be needed as wheatgrass matures.