In situ disappearance of neutral detergent insoluble nitrogen from alfalfaand eastern gamagrass at three maturities

In situ digestion kinetics of neutral detergent insoluble nitrogen (NDIN) f rom alfalfa (Medicago sativa I,.) harvested at one-tenth bloom and eastern gamagrass (Tripsacum dactyloides L.) harvested at the boot (GGB), anthesis (GGA), and physiological maturity (GGM) stages of growth were determined wi th nonlinear regression techniques. Whole-plant tissue and associated leaf and stem fractions were incubated in the ventral rumen simultaneously. On a whole-plant basis, potential extents of degradation were particularly high (greater than or equal to 904 g/kg NDIN) for GGB and GGA, relative to thos e of GGM: and alfalfa (772 and 658 g/kg NDIN, respectively). For all plant parts, degradation rates of NDIN were faster (P < .05) for alfalfa than for all gamagrass forages. Degradation rate of NDIN did not differ (P > .05) a cross maturities for any gamagrass tissue type. These results indicate 1) t hat phenological development and lignification do not limit the rate of NDI N degradation in gamagrass forages but do markedly limit the potential exte nt of NDIN availability and 2) that most of the NDIN in these forages is po tentially available in the rumen and can contribute to the ruminal N supply . Our secondary objective was to compare estimates of N escaping ruminal de gradation that were determined on the basis of NDIN degradation kinetics (N DIN method) with those determined traditionally, on the basis of total resi dual N. The NDIN method mathematically eliminates all neutral detergent sol uble N from consideration as part of the pool of dietary N potentially esca ping the rumen intact. Estimates of rumen escape nitrogen determined on the basis of degradation rates of NDIN were consistently less than correspondi ng estimates that were determined on the basis of total residual N. When ru minal escape N that was determined with the NDIN method was regressed on co rresponding estimates with the total residual N method, the slopes of the r egression lines were .53 and .66 for assumed passage rates of .02 and .06 h (-1), respectively. For the forages evaluated in this study, these results indicate that neutral detergent soluble N may make important contributions to the pool of N escaping ruminal degradation.