Abstract: Chemically modified unpyrolyzed Poaceae fibres having a length of less than 200 mm advantageously comprised between 2 and 100 mm, such as between 2 and 10 mm, said fibres having a water content of less than 40% by weight, and being treated with a treating aqueous dispersion comprising less than 1% by weight of surface treating mixture comprising at least a silanol terminated polydimethylsiloxane, as well as an amino coupling agent.