Disclosed is a method of evaluating the quantity of methane that will be produced by a dairy ruminant, comprising analysing milk from the ruminant to determine what percentage of the total fatty acids in the milk consists of fatty acids with 16 carbon atoms or less (FA<；C16) using the following equation: Quantity CH4 = (FA<；C16/Total FAs)*a*(milk production)b wherein: quantity CH4 (in g/litre of milk) = quantity of methane produced； FA<；C16 = quantity of fatty acids with 16 carbon atoms or less； Total FAs = total quantity of fatty acids； milk production = quantity of milk produced in kg of milk per animal per year； a and b are numerical parameters, with a being between 10 and 13, and b being between -0.40 and -0.45.
