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 of treatments to determine the effects of the dietary forage:concentrate ratio (96:4 and 60:40) and forage particle size (long hay and coarsely ground hay) on in situ ruminal digestion kinetics of orchardgrass hay DM and cell wall neutral monosaccharides, uronic acids, acetyl groups, and hydroxycinnamic acids. Dacron in situ bags containing orchardgrass hay were fermented for 0, 4, 8, 12, 18, 24, 48, and 96 h. Digestion profiles of DM and cell wall monomers in undigested residues recovered from the rumen were analyzed using a first-order, exponential equation to estimate the indigestible fraction (f(i)), the insoluble, potentially digestible fraction (f(d)), and the fractional rate constant (k(d)) of digestion of f(d). Initial results indicated that f(i) was not consistently influenced by diet fed to the steers; thus, in situ digestion 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 each steer x diet combination (16 total). Estimates of f(i) (percentage of original) for cell wall components were ranked in the following order: galactose (12.6), ferulic acid(13.9), arabinose (14.5), total uronic 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 alterations in the ruminal environment that generally resulted in only small decreases in k(d), but decreasing the forage:concentrate ratio of diets led to decreases in k(d) of 30 to 40% for all cell wall components measured. Decreasing the forage:concentrate ratio of diets generally led to greater reductions in k(d) when steers consumed orchardgrass in the ground form. Our results indicate that dietary forage:concentrate ratio and forage processing did not consistently influence the potential 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 orchardgrass hay in the ground vs long form.