As part of “Rumen-Up”, an EU Framework 5 project, a total of 500 plants and plant extracts were screened in vitro for their effect on general rumen fermentation parameters, in particular their ability to influence microbial protein production, protozoal numbers, methane production, bloat and acidosis. Many of the plants investigated were found to have potentially beneficial effects with respect to ruminant production. The activities of 25 plants were patented, including two that The University of Reading identified as maintaining a significantly higher fermentation medium pH - common lettuce (Lactuca sativa) and stinging nettle ( Urtica dioica) with the aim that these plants (or their derivatives) would be incorporated as feed supplements to reduce the incidence of rumen acidosis. The results reported here are from an in vitro dose titration study conducted to identify possible dietary inclusion levels prior to conducting a large-scale animal production study.