Paddy weeds may spread through the manure of cattle feeding on forage rice silage that is contaminated by weed seeds. We investigated the seed viability of several different types of paddy weeds ensiled with forage rice. Weed seeds of six species [Echinochloa crus-galli (L.) Beauv. var. formosensis Ohwi, Bidens frondosa L., Eclipta prostrata (L.) L., Schoenoplectus juncoides (Roxb.) Palla, Cyperus difformis L. and Monochoria vaginalis (Burm. f.) Presl var. plantaginea (Roxb.) Solms-Laub.] mixed in forage rice died completely as a result of ensiling for approximately 90–180 days. However, the viability of Aeschynomene indica L. seeds, which have a hard coat, was not compromised even after ensiling them for approximately 180 days.Therefore, when forage rice silage is contaminated with A. indica L. seeds, there is an increased risk of the excretion of germinable seeds in cattle manure.