New recommendations for the ruminal in situ determination of indigestible neutral detergent fibre

Three different experiments were conducted aiming at evaluating different aspects of the methodology for ruminal in situ determination of ash-free indigestible neutral detergent fibre (iNDFom) with reference to a near infrared reflectance spectroscopy (NIRS) application. Sample grind size, in situ bag pore size and cloth characteristics were compared for different feed samples in the first trial. In the second trial, the concentration of iNDFom in silage was compared with the value of the parental grass crop, and finally in a third trial four different steps in the methodology of iNDFom determination were evaluated between two different laboratories. The determinations of the concentration of iNDFom using 12-μm Saatifil PES bags were generally greater compared to iNDFom determined in the Sefar Petex bags. To avoid potential errors from particle losses or impaired microbial activity we recommend a grind size of 2.0 mm and bags made from Sefar Petex 07-6/5 or 07-11/5 to be used in ruminal in situ determination of iNDFom. When the iNDFom concentration was measured from grass and subsequent silage samples, the values were similar revealing that iNDFom concentration of the plant material was not altered during the ensiling process. Further, the NIRS spectra generated from the grass and subsequent silage samples were very similar. Based on the results of the current study, one common calibration dataset is enough for NIRS predictions of iNDFom in grass silages despite feed value determination from the herbage material. Last, of the four different methodological steps that were tested, bag type and cow within site affected the determinations, while washing procedures and the analysis of ash-free neutral detergent fibre concentration in the incubated residues did not contribute to any differences between the two laboratories. When iNDFom values from multiple laboratories are used, it is recommended to choose one biologically validated reference laboratory, and apply laboratory specific corrections for the others.