Prediction of NDF degradation characteristics of grass and grass/clover forages based on laboratory methods

The degradation profiles of neutral detergent fibre (NDF) for typical Danish forages (25 grass and grass/clover samples) were evaluated using rumen nylon bag incubations. In vitro digestibility of organic matter (OM) and NDF was performed by two different standard laboratory methods, one based on Tilley and Terry rumen fluid (T and T) and one on enzymes (ENZ). Concentrations of NDF, acid detergent fibre (ADF) and acid detergent lignin (ADL), ash and crude protein (CP) in feeds were determined. The ability to predict in situ potential NDF degradability and in situ fractional rate of NDF degradation based on in vitro digestibilities and on chemical analyses was tested using multiple regression analysis. A considerable variation was found in NDF degradation between samples. The indigestible NDF (INDF) proportion was determined after 504 h in situ incubation and ranged from 0.047 to 0.246 of NDF. Fractional rate of NDF degradation (c) varied from 0.022 to 0.150 per hour. The potential degradable NDF fraction (b) showed high correlation with ADL content and ADL/NDF ratio. Fractional rate of degradation (c) showed the highest correlation with in vitro enzymatic NDF digestibility (ENZ_NDF) and NDF and ADL content. This shows that potential degradability of NDF is mostly correlated to lignification of fibre, whereas fractional rate of degradation is mostly correlated to digestibility. It was possible to describe 0.87 of the variation in the potentially degradable NDF fraction (b) and 0.83 of the variation in fractional rate of degradation (c) using all available information on the feeds. Equations based on in vitro OM digestibility and chemical analyses described 0.83 and 0.85 of the variation in b and c, respectively, and equations based only on NDF content and calculated in vivo OM digestibility (sheep) were able to describe approximately 0.80 of the variation in b and c.