Effects of nitrogen fertilization and maturity of bromegrass on in situ ruminal digestion kinetics of fiber.

Four ruminally cannulated, nonlactating Holstein cows (593 kg) were fed bromegrass hay diets within a 4 X 4 Latin square. Treatments were arranged factorially and consisted of bromegrass fertilized with 0 or 89 kg N/ha and harvested in the late-boot or full-head stage of maturity. Total tract digestibility of DM (57.6 vs 51.1%), NDF (68.8 vs 63.0%), ADF (51.1 vs 46.0%) and cellulose (63.4 vs 53.5%) decreased with increasing maturity (P less than .05). These forages also were placed in dacron bags for determination of in situ digestion kinetics. Rate, extent of digestion, and lag times were determined for DM and fiber components using linear regression of log-transformed data or a nonlinear fitting procedure. Treatment effects generally were consistent between methods of calculating kinetic data; however, numerical results often differed greatly. Extents of in situ ruminal digestion (72 h residue) for NDF, hemicellulose and cellulose were lower (P less than .05) for full-head than for late-boot-stage bromegrass. Extent of digestion calculated using the nonlinear method followed similar trends. Nitrogen fertilization had no effect on extent or total tract digestibilities of fiber components of DM. In situ disappearance rates (calculated using both methods) for fiber fractions and DM generally were almost always affected less by maturity than by fertilization. Particles of N-fertilized bromegrass had more (P less than .05) bacterial N associated with them during the first 12 h in situ than did particles of unfertilized brome. Surface area equations based on lignification of NDF accurately estimated fiber digestion and suggested that lignification was the primary reason DM and NDF digestibilities decreased with advanced maturity. Due to the relatively low DM intakes of these diets (11 kg/d), rate of passage, not rate of digestion, was of principle importance in determining extent of digestion of these diets.