FORAGE LEVEL AND PARTICLE-SIZE EFFECTS ON ORCHARDGRASS DIGESTION BY STEERS .2. RUMINAL DIGESTION KINETICS OF CELL-WALL COMPONENTS

Four steers (502 +/- 49 kg) with ruminal cannulas were used in a 4 x 4 Latin square experimental design with a 2 x 2 factorial arrangement o f treatments to determine the effects of the dietary forage:concentrat e ratio (96:4 and 60:40) and forage particle size (long hay and coarse ly ground hay) on in situ ruminal digestion kinetics of orchardgrass h ay DM and cell wall neutral monosaccharides, uronic acids, acetyl grou ps, and hydroxycinnamic acids. Dacron in situ bags containing orchardg rass hay were fermented for 0, 4, 8, 12, 18, 24, 48, and 96 h. Digesti on profiles of DM and cell wall monomers in undigested residues recove red from the rumen were analyzed using a first-order, exponential equa tion to estimate the indigestible fraction (f(i)), the insoluble, pote ntially digestible fraction (f(d)), and the fractional rate constant ( k(d)) of digestion of f(d). Initial results indicated that f(i) was no t consistently influenced by diet fed to the steers; thus, in situ dig estion profiles were analyzed to estimate single f(i) and f(d) values common to all steers and diets and different estimates of k(d) for eac h steer x diet combination (16 total). Estimates of f(i) (percentage o f original) for cell wall components were ranked in the following orde r: galactose (12.6), ferulic acid(13.9), arabinose (14.5), total uroni c acids (15.4), glucose(19.8), xylose (28.4), p-coumaric acid (34.6), and acetyl groups (35.8). Grinding of forage fed to steers led to alte rations in the ruminal environment that generally resulted in only sma ll decreases in k(d), but decreasing the forage:concentrate ratio of d iets led to decreases in k(d) of 30 to 40% for all cell wall component s measured. Decreasing the forage:concentrate ratio of diets generally led to greater reductions in k(d) when steers consumed orchardgrass i n the ground form. Our results indicate that dietary forage:concentrat e ratio and forage processing did not consistently influence the poten tial extent of digestion of orchardgrass cell wall components. However , rate of digestion of cell wall components was decreased by reducing the forage:concentrate ratio of diets fed to steers and by feeding orc hardgrass hay in the ground vs long form.