2006 Annual Report 4d.Progress report. This report serves to document research conducted under a specific cooperative agreement between ARS and the Department of Microbiology at Cornell University entitled "Ruminal lysine degradation". Additional details of research can be found in the report for the parent project 1907-31000-005-00D "Alternative Mechanisms for Improving Ruminal Fermentation". This project began in late 2004. The objective is to isolate and identify ruminal bacteria responsible for ruminal lysine degradation and to study the effect of the feed additive, monensin, and bacteriocins on lysine degradation. Lysine is an amino acid that often limits milk production, but bacteria responsible for wasteful ruminal lysine degradation had not yet been identified. We enriched mixed ruminal bacteria with lysine in vitro, and isolated strains capable of utilizing lysine as their sole energy source, characterized physiologically and identified via 16S rDNA sequencing as a strain of Fusobacteria necrophorum. However, fermentation acids prevented the enrichment of fusobacteria in vivo and dietary lysine supplementation did not enhance the lysine deamination rate of the mixed ruminal bacteria.