For instance, if we talk about herbicide resistance crop whose genes (phenotypic and genotypic features) are moderated in such a manner that if we apply non selective herbicide like glyphosate etc in the entire field, all the vegetation present in the field will be killed except GM crop (herbicide resistance crop HRC). What so ever chemical(non selective herbicide)is being applied in the soil to kill weeds other than crop might be having residual effect in the soil, if it is not degradable by photo or microbial deactivation mechenism....
There is need to modify GM crop in such a manner that chemicals uses are less or negligible...