To effectively and economically obtain a preventive and an improver for various production diseases of genetically improved domestic animals and domestic fowls, espectally production diseases caused by summer heat and cold stress by including a specific quinone as an active ingredient.

The preventive and the improver comprise at least one compound (e.g. ubiquinone, ubidecarenone) selected from quinones of the formula (Y and Z are each H, CH3 or bonded to form one covalent bond of a double bond; (n) is 1-11) as an active ingredient. Further, vitamin E and the compound are administered in a form of pellet. Preferably, the vitamin E and the compound are orally administered to domestic fowls for about three weeks from hatching and for example, about 1-100 mg/kg of the quinone is added to a feed.