An object of the present invention is to improve the milk yield and/or milk quality of a ruminant for milk production, to prevent or treat a perinatal disease of a ruminant, and to improve the reproductivity of ruminant. In order to solve the above-mentioned problem, the following is provided. An agent for improving at least one of milk yield and milk quality of a ruminant, comprising at least one of cashew nut shell liquid, heat-treated cashew nut shell liquid, anacardic acid, cardanol, and cardol. An agent for preventing or treating a perinatal disease of a ruminant, comprising at least one of cashew nut shell liquid, heat-treated cashew nut shell liquid, anacardic acid, cardanol, and cardol. An agent for improving reproductivity of a ruminant, comprising at least one of cashew nut shell liquid, heat-treated cashew nut shell liquid, anacardic acid, cardanol, and cardol.