Selection can also occur on the vegetative stage, through hand weeding. Weeding often takes place when the crop plant is very young, and most vulnerable. "Echinochloa oryzoides", a species of grass which is found as a weed in rice ("Oryza sativa") fields, looks similar to rice and its seeds are often mixed in rice and difficult to separate. This close similarity was enhanced by the weeding process which is a selective force that increases the similarity of the weed in each subsequent generation.