[Excerpt] Introduction: Protein nutrition supports milk production and efficient nitrogen utilization through both metabolizable protein (MP) supply and its amino acid (AA) profile (Schwab and Broderick, 2017). Microbial protein (MiP) and rumen-undegraded protein (RUP) are the major pools underlying MP supply. Traditional MiP AA profiles incompletely represent rumen microbial diversity (Sok et al., 2017). Owing to differences in AA composition among microbial groups, diet and host-associated shifts (Henderson et al., 2015) in community composition may influence MiP AA profiles. Rumen fermentation might alter RUP AA profiles (Edmunds et al., 2013). However, separate bag incubation may not fully represent feed degradation within mixed rumen contents, while conventional AA analysis does not identify individual remaining proteins. The objectives were to use RNA sequencing to characterize MiP AA composition across microbial groups and cows divergent in feed efficiency, and proteomics to explore potential changes in RUP AA composition during fermentation.