Jacobaea aquatica (=Senecio aquaticus, Asteraceae), marsh ragwort, grows regionally in low management intensity wet grasslands in various European countries. The plant can be regarded as a noxious weed as it contains pyrrolizidine alkaloids (PAs), which cause health problems in livestock. In the Waldviertel region of Northern Austria, marsh ragwort has established dense populations as the fertilizing and production intensity of the meadows changed during the last decades. Ragwort biomass production and alkaloid concentration in J. aquatica were recorded during three consecutive years at three sites to study the effects of two treatments: cutting twice a year either early in June plus September, or in July plus August, respectively. Six PAs (Z-erucifoline > senecionine > integerrimine > seneciphylline > acetyl-erucifoline and acetyl-seneciphylline) were evaluated. The alkaloid contents were highest in July and August when 600-1,400 mg/kg dry matter (DM) total PAs occurred in most ragwort samples. Consequently, the forage can easily be contaminated with 40-80 mg/kg DM of PAs and may present a health risk for domestic animals and also affect human health as these alkaloids are transferred into milk.