There are many rationing models used commercially for evaluating diets fed to dairy cows. A new model – BioParaMilk – uses a unique protein degradation model to determine microbial protein synthesis, based upon the in vitro gas production technique (IVGPT). Optigen®, a slow release, blended, non-protein nitrogen source, can partially replace soyabean meal (SBM) in a dairy diet. The partial replacement of soyabean meal and rapeseed meal with Optigen® has been shown to increase fibre digestion and may improve volatile fatty acid (VFA) and microbial nitrogen (N) flow in the rumen (Sinclair et al., 2008). The purpose of this study was to evaluate the protein degradation curve from Optigen® compared to SBM for use in this new model, using IVGPT.