A management method for biomass at plant harvest according to the present invention includes the steps of: measuring the fatty acid content in a plant; acquiring the proportion of linolenic acid with respect to the total fatty acid content; and estimating the biomass at plant harvest on the basis of the acquired proportion of linolenic acid.本發明之收穫時生物量之管理方法係包含測量植物中之脂肪酸的含量、取得次亞麻油酸相對於全部脂肪酸含量的比率、基於取得的次亞麻油酸比率而推測植物之收穫時生物量的推測步驟。