Phytoestrogens are plant substances capable to affect negatively the fertility of human as well as farm animals. In the organism, both surplus and deficit of phytoestrogens are undesirable. Among others, phytoestrogens are present in lucerne, particularly in the form of coumestrol. The samples of lucerne silage from the three cuttings were analyzed. At the standard process of fermentation, the content of coumestrol was reduced by up to one third within four weeks after the beginning of fermentation. Insilages of lucerne harvested in October with dry matter content lower than 40 %, the coumestrol content was not reduced.