The aim of this new methodology is to quantify reduction of methane (CH4) emissions from enteric fermentation in beef cattle, as well as impacts on emissions during manure handling. The methodology focuses on the application of feed supplements to reduce enteric methane production in the rumen of livestock. The methodology provides two approaches for quantification of emissions from enteric fermentation. Approach 1 requires on-site measurements whereas Approach 2 applies regression models or IPCC Tier 2 equations integrating data from peer-reviewed publications. New models may also be used if proof of validity and applicability is provided.