Estimation of Effective Protein Degradability of Fresh Forage in situ: Correction for Microbial Contamination using 15N-Labelled Plant Material

It is difficult to make field measurements of the effective degradability (ED) of fresh forage ingested by grazing ruminants. The in situ or nylon bag technique is often used for estimating ruminal protein digestibility of forage samples, but microbial attachment to feed residues in the bags causes true degradability coefficients to be underestimated (Nocek and Grant 1987). In addition, the forage samples are often dried or frozen before being placed in the bags and this may affect microbial attachment. Internal and external microbial markers have been used to correct for microbial 'contamination' (Nocek and Grant 1987). In this study, we have used an alternative approach, i.e. labelling the plant material (perennial ryegrass, 'Lolium perene' L.) with 15N to determine the extent of microbial 14N contamination in situ and, from this, the errors in predicting ED.