Prediction of indigestible NDF in South African and Australian forages from cell wall characteristics

A portion of the forage cell wall is unavailable to microbial digestion in ruminants and in previous work this fraction was characterized by the relationship between lignin (sa) and neutral detergent fiber (NDF) as 2.4 × lignin and termed the indigestible NDF (iNDF). Further, this relationship was considered to be a static relationship between lignin (sa) and NDF with no genetic variation or observed effects of growing conditions on the plant. The objectives of this work were to further characterize the relationship between lignin and iNDF and to evaluate the concept of a fixed value, assuming that the relationship is more dynamic and dependent on agronomic and environmental factors. A further objective was to predict iNDF from cell wall composition. More than two hundred samples of several forage species from Australia and South Africa were analyzed for NDF, acid detergent fiber, lignin and iNDF. Forages were classified according to origins, type and species. The ratios of iNDF to ADL ranged between 1.6 and 8.0, demonstrating the dynamic nature of the relationship. Further, an attempt to predict iNDF based on the relationship between ADL/NDF and the ratio was made and the equations provided R2 values between 0.82 and 0.95, indicating that a prediction is possible, especially when considering one species at a time and more information within forage group and single forages on agronomic and environmental conditions would explain the extra variation. A single value of the ratio iNDF to ADL does not allow proper estimation of the size of the potentially digestible NDF and its rate of digestion, but species-specific equations can be developed and be effective in predicting iNDF. Long-term fermentations should however be preferred to correctly quantify these values and, if not available, data should allow for specific equations or near infrared spectroscopy calibration equations.