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 intermedium Host) pasture from April 10 through December 7. Sample collections were conducted in May, June, September, and November 1989. Total masticate N decreased (P <.05) and bound N increased (P <.05) with advancing forage maturity. Masticate in vitro OM disappearance was lower (P <.05) in November (40.5%) than in May (62.0%). Organic matter intake (grams/kilogram of BW) decreased(P <.05) at each sampling date (May = 35.7, June = 26.8, September = 20.0, November = 14.9). Correspondingly, particulate passage rate decreased and gastrointestinal mean retention time increased (P <.05) with advancing forage maturity. Extent and rate of NDF digestion were greater (P < .05) during May and June than during September and November. Ruminal NH3 N and total VFA concentrations were greater (P <.05) during May and June than during September and November. Advancing forage maturity did not alter (P >.10) serum growth hormone concentrations; however, nonesterified fatty acid concentrations were greater (P <.05) during September and November than during 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 increased if supplemental energy is provided early in the growing season of intermediate wheatgrass; however, supplemental protein would likely be needed as wheatgrass matures.