Summary:The vegetative growth of Japanese millet (Echinochloa utilisOhwi et Yabuno) was much lens reduced by pre‐emergent treatments of thiobencarb (S‐eithyl hexahydro‐1,4‐azepine‐l‐thiolcarbamate) and molinate (S‐(4‐chlorobenzyl)‐N.N‐diethylithiolcarbamate] when the herbicides were applied at<4 l a.i. ha−1over rice stubble ash. The effcct was caused by adsorption of the herbicides onto activated carbon in the ash and occurred alike in field and pot experiments.The reduction in herbicide activity was quantified in the pot experiment. There the ash rendered at least 60% of the applied herbicide bio