Tekippe, J. A., Hristov, A. N., Heyler, K. S., Zheljazkov, V. D., Ferreira, J. F. S., Cantrell, C. L. and Varga, G. A. 2012. Effects of plants and essential oils on ruminal in vitro batch culture methane production and fermentation. Can. J. Anim. Sci. 92: 395-408. In this study, plants (14) and essential oils (EO; 88) from plants that are naturalized to, or can be successfully grown in North America were evaluated in a batch culture in vitro screening experiments with ruminal fluid as potential anti-methanogenic additives for ruminant diets. Essential oils were tested at four inclusion levels: 0 (blank), 10, 50, and 100 mg L^-1 and plants were tested at 313, 1250, 2500, and 5000 mg L^-1 final incubation medium concentration. Compared with the blank, two of the EO increased acetate concentration (8 to 10%), 11 EO increased propionate concentration (9 to 23%), 10 EO increased butyrate concentration (24 to 29%), and three EO reduced methane production [20 to 30%; Anethum graveolens (dill weed oil), Lavandula latifolia, and Ocimum basilicum #7 accession]. Four EO decreased and one increased neutral detergent fiber (NDF) degradability. Three plants increased acetate concentration (8 to 12%), two increased propionate concentration (16%), and one (Origanum vulgare) decreased methane production (31%). Eight of the plants increased NDF degradability at various inclusion levels. Overall, these results indicate that some EO, or EO-producing plants could have a potential anti-methanogenic effect. Further research is needed to verify these results in vivo in long-term experiments.