Forage level and particle size effects on orchardgrass digestion by steers. I. Site and extent of organic matter, nitrogen, and cell wall digestion

Four steers (502 +/- 49 kg) with ruminal, duodenal, and ileal 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 dietary forage:concentrate ratio (96:4 [96F] and 60:40 [60F]) and forage particle size (long hay [L] and coarsely ground hay [G]) on site and extent of digestion of OM, N, and plant cell wall monomeric components. Orchardgrass hay was the forage source used and was the sole source of cell wall material fed to steers (DMI = 88.6 g/kg BW(.75)). Diurnal variation in ruminal pH was greater when steers consumed 6OF vs 96F (P < .01) and greater for G vs L (P < .10). Ruminal fluid volumes (liters) and daily outflows (liters/day) were greater (P < .05) when steers consumed 96F or L. True ruminal OM digestion (percentage of intake) was greater (P < .05) when steers consumed 6OF (69.0) vs 96F (59.5) or L (65.8) vs G (62.7). Efficiency of net bacterial CP synthesis (grams of N/kilogram of true ruminal OM disappearance) was greater (P < .05) when steers consumed 6OF (33.8) vs 96F (30.8). Total tract digestibilities of NDF and ADF were 63.3 and 53.0% when steers consumed 96F and were decreased (P < .05) to 52.3 and 43.8% when steers consumed 60F. Total tract digestibilities of plant cell wall arabinose, galactose, glucose, xylose, and uronic acids were 78.7, 69.7, 70.8, 67.6, and 79.7% when steers consumed 96F and were decreased (P < .05) when steers consumed 60F. Ruminal digestion accounted for greater than 90% of total tract digestion of all cell wall monosaccharide components when steers consumed 96F but accounted for 6 to 20 percentage units less of total digestion when steers consumed 60F. Total tract disappearances of cell wall acetyl groups, ferulic acid, and p-coumaric acid were 62.4, 79.4, and 50.6% when steers consumed 96F and were decreased (P < .05) to 49.7, 73.2, and 42.2% when steers consumed 60F. Addition of concentrate to diets, but not forage processing, decreased total tract digestibilities of cell wall components and shifted site of digestion to the lower gastrointestinal tract.