The rumen is considered an anaerobic environment. Laboratory studies with bovine rumen samples using mass spectrometry have shown that passing oxygen over the surface of the rumen fluid reduces methanogenesis. Similarly, in vitro trials have demonstrated that the inclusion of certain probiotics inhibits methanogenesis. We carried out an in vivo experiment to quantify the reduction in methanogenesis using super-oxygenated water and a probiotic combination offered to cattle fed a finisher diet.