Effect of Chemical Control on the Weed Community in Tobacco Fields

The experiments on the herbicides were carried out in tobacco fields from 1997 to 2000.All herbicides used could control the broad -leaved weed such as Ranunculus chinensis Bge., Polygonum amphhibium L. and Cardamine flexuosa With., which had more than 88 percent of controlling effect.The use of the herbicides year by year caused the shift of the weed community.Once the dicotyledonous weeds, mainly Ranunculus chinensis Bge. had been prevalent in the field, gradually the community was changed into a mixture of the monocotyledonous and dicotydonous weeds in which Digitaria sanguihallis(L.) Scop., Echinochola crusgalli(L.) Beauv.var.hispidula(Retz.) Hack., Ageratum conyxoides L., Polygonum amphhibium L.and Ranunculus chinensis Bge.were donminant.