Three sets of in vitro rumen fermentation experiments were conducted to determine the effects of diets that included malted barley (MB) and basal diets (grain- and forage-based) on the in vitro gas production, greenhouse gas (GHG) emissions, rumen fermentation profiles, and microbiome changes in the rumen when supplemented with feedlot or dairy rations. The first experiment (Exp. 1) was conducted to evaluate the effects of various levels of MB (0% [referred to as a control], 10%, 20%, and 30%, as-fed basis) supplemented with a grain-based diet in a feedlot ration (2.5 g/bottle) after 48 h ruminal incubation on the in vitro gas production, GHG emissions, and rumen fermentation rate. The second two sets of in vitro experiments (Exp. 2a, b) were conducted to determine (1) the effects of linear dose levels of malted barley (MB
0%, 10%, 20%, 30%, and 40% as-fed) with two different basal diets (grain-based and forage-based) and (2) the effects of different sources of MB (control, Korean, Canadian, and the USA
30% MB, as-fed) in a dairy ration after 24 h incubation on in vitro gas production, rumen fermentation profiles, GHG emissions (methane [CH