Methane emissions from ruminants have become an issue over the last 50 years. Previous research has shown that methane emissions are stoichiometrically linked with volatile fatty acid (VFA) profiles in ruminant animals. For example, a shift from acetate to propionate may decrease carbon dioxide (CO2) and hydrogen (H2) production, and in turn, decrease conversion of CO2 and H2 to methane. In vitro methods have been developed to measure the digestibility of feeds, but such methods may not accurately estimate methane or volatile fatty acid (VFA) profile. The development of in vitro methods to accurately estimate gas production and VFA profile in rumen fermentation would enable isolation of fermentation effects from various animal interactions. Therefore, the focus of this dissertation was to develop an in vitro method that will have the same VFA and gas profiles as in the rumen. The objectives of this project are: to develop an in vitro technique that mimics an in vivo rumen environment in order to study VFA profiles and gas production during fermentation, to examine and evaluate the efficacy of selected feed additives (e.g. probiotics) on VFA profiles and gas production, and to develop a mechanistic model of the in vitro fermentation system and the effects of feed supplements on the system. The results indicate that gas profile, VFA profile, and gas production were affected by differing in vitro fermentation conditions (buffering capacity, headspace gas composition, acetate concentration). A review of the literature was conducted to establish the effect of probiotics such as lactic acid bacteria on in vitro and in vivo systems. These findings indicated Enterococcus and Lactobacillus species tended to affect ruminal fermentation parameters. Further in vitro analysis of these probiotics indicated these bacteria tended to affect ruminal fermentation, such as gas and VFA production. A developmental mechanistic model was built to predict whether the effect of probiotics was thermodynamically or kinetically limiting. Future studies will further development of this simple model by using published literature for a meta-analysis that may aid in further interpretation of rumen fermentation regarding thermodynamic limits and maximal efficiency of key rumen fermentation reactions.