NOVELTY - Process for evaluating the quantity of methane produced by a dairy ruminant, comprises determination of ratio between quantity of fatty acids with less than or equal to 16C, and the sum of total fatty acids of the milk, where the fatty acids are present in the milk produced by the ruminant.    USE - The process is useful for: evaluating the quantity of methane produced by a dairy ruminant; and reducing and controlling the quantity of methane produced by a dairy ruminant.    ADVANTAGE - The process is easy to implement. The determination of fatty acids from milk does not require sophisticated facilities and/or is inexpensive.    DETAILED DESCRIPTION - Process for evaluating the quantity of methane produced by a dairy ruminant, comprises determination of ratio between quantity of fatty acids (FA) with less than or equal to 16C, and the sum of total fatty acids of the milk, where: the fatty acids are present in the milk produced by the ruminant; the quantity of methane is defined by the equation (methane (CH4) quantity is equal to (FA less than 16C/total FA)x ax (milk yield)b); CH4 quantity (in grams per liter of milk) is equal to amount of produced methane; FA less than 16C is equal to quantity of fatty acids with 16 carbon atoms or less; total FA is equal to total quantity of fatty acids; the expression of the ratio (FA less than 16C)/(total FA) is in % of total FA; milk production is equal to quantity of milk produced in kg of milk per animal per year; a is 10-13; and b is 0.40-0.45. An INDEPENDENT CLAIM is included for a process for reducing and controlling the quantity of methane produced by a dairy ruminant, comprising providing a diet that meets at least one of following criteria: diet that excludes any fatty substance of animal origin; diet that limits the exogenous supply of vegetable oil containing more than 30% of saturated FA, in the saponified or hydrogenated state of at most 15 grams per animal and per 100 kg of live weight; or diet that contains at least one lipid source rich in omega-3-alpha-linolenic acid (ALA), where 30% of total FA is in the form of omega 3 FA.