Podbel’skogo, St. Petersburg, 196608 Russia, e-mail t.gagkaeva@mail.ru Supported in part by Russian Science Foundation (project ¹ 14-16-00114) A b str a c t Zearalenone, the fungi metabolite with an estrogenic effect, can lead to significant economic losses in livestock due to fodder contamination. <...> Pasture herbage and fodders from dried pasture plants can really cause intoxication. <...> However, the reasons and mechanism of abnormally high accumulation of zearalenone in these fodders are not still found out. <...> In this article we summarized the data on investigation of frequency and the level of zearalenone contamination in wild herbage and in hay. <...> Also the ability to produce this mycotoxin was studied in vitro in 13 strains from laboratory collection. <...> A total of 211 samples of wild , reed canary grass, hedgehog, fire, Moscow Province were prepared from 120 hay batches herbage including ( , dogstail grass, bluegrass, fescue, bentgrass, bluegrass, timothy, foxtail) were collected in Kashirskii, Noginskii and Ruzskii regions of Moscow Province in 2014 from June to September. <...> A total of 427 samples of pasture grasses were also repeatedly collected in North Karelia, Leningradskaya, Tverskaya and Astrakhanskaya provinces from 1998 to 2014 at different frequency for different plant species. <...> In these samples, along with commonly grown grasses such as , bluegrass, fescue, bluegrass, timothy, some local plants were found, particularly reed, reed canary grass, hedgehog, bent grass, reeds, Leymus, dogtail grass, oats, ryegrass, clover, rank, peas, vetch, rye, pampasskaya grass, foxtail, alfalfa. <...> An uneven zearalenone contamination of pasture grasses with rare overcontamination cases was shown to occur to the end of vegetation. <...> In hay an increased frequency and more high level of contamination were found. <...> The mycotoxin was detected in 54 samples of 120 those tested, and more than a half of them contained zearalenone at the level below 100 g/kg, while the rest samples contained toxin at 1000 g/kg, and in 13 % of the samples tested the contamination reached 1000 to 10000 g/kg. After 7 day cultivation of , , , , and on semi-synthetic agar medium more that 80 % of strains produced zearalenone, and the rate of strains with the high production reaching more than 10000 ng/ml <...>