In vitro degradability of three forages: fermentation kinetics and gas production of NDF and neutral detergent soluble fraction of forages

The fermentation of the neutral detergent-soluble (NDS) fraction of three forages (alfalfa hay, Italian ryegrass+oats hay and corn silage) was measured using a curve subtraction technique with in vitro gas production data from the whole forage (WF) and the isolated neutral detergent fibre (NDF). NDF disappearance and volatile fatty acid (VFA) production were determined. There was no significant difference between the VFA patterns from the whole forage and the isolated NDF. There were significant (P < 0.001) linear correlations between the volume of gas and moles of VFA produced and the mass of fibre digested in the NDF samples. Using the monophasic curve model, maximum gas production rates (R,,) for the whole forages and the isolated NDF and NDS fractions can be calculated. For all three forages we obtained R-M-NDS > R-M-WF > R-M-NDF. Trends in the rates of gas and VFA production were similar in the whole forages. The NDF showed a high superimposition between substrate degradability and VFA production. Our experiment confirmed the curve subtraction technique as a simple method to obtain information on the size and digestion kinetics of the NDS fraction, which helps in understanding the nutritional significance of this important fraction of the forages studied.