The present invention relates in particular to a method to evaluate the quantity of methane produced by a dairy ruminant. It is notably characterized by the fact that it comprises determination of the ratio between the quantity of fatty acids with 16 carbon atoms or less, called FA<；C16 and the sum of the total fatty acids of the milk, said fatty acids being found in the milk produced by said ruminant, said quantity of methane being defined by the following equation: Quantity CH4=(FA<；C16/Total FAs)*a*(milk production)b in which: quantity CH4 (in g/liter 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； expression of the ratio (FA<；C16)/(Total FAs) as a % of total FAs； milk production=quantity of milk produced in kg of milk/animal and per year； a and b are numerical parameters in which a lies between 10 and 13, and b lies between -0.40 and -0.45.
