Long term application of a certain herbicide let a variation of weed populations in crop fields.It increases the difficulty controlling weeds effectively.Therefore,it is a hot problem for us to solve.Studies on the niche of weeds in crop fields can reveal the ecological demand similarity of different weeds.It can be applied to predict the change trend of crop weed community on the basis of the references about the sensitivity of various weeds to the herbicide applied.In this paper,the rape fields of Middl...