PROBLEM TO BE SOLVED: To provide an agent capable of suppressing grassy smell called abnormal flavor which is one of quality deterioration of the milk of the livestock by reducing concentration of hexanal in milk of livestock (such as cow, sheep or goat), and orally administrated or fed to exert satisfactory effect； and to provide a method or the like.SOLUTION: Trehalose is orally administrated or fed to livestock as an active ingredient to reduce the concentration of hexanal in milk of livestock (for example, the concentration of the hexanal in the milk is less than 40% relative to that when trehalose is not administrated or fed) to suppress the grassy smell of the milk of the livestock, so that the quality of the milk of the livestock is improved and stabilized.
