Influence of weed time of emergence and removal on crop yield loss

The paper briefly reviews the aspects related to influence of the relative time of weed emergence and removal on crop yield loss. The importance of early plant growth in determining the intensity and outcome of the subsequent weed-crop competition is highlighted. The variability, in space and time, of the relationships between weed time of emergence and removal, weed density and crop yield loss area is discussed. The concept of the critical period is debated and it is concluded that the identification of a period alone does not provide information on how many or when control practices should be performed. Given the time dependence of the weed competitive effect, treatments carried out within this period cannot be regarded as having the same net margin and therefore there has to be an optimum time for one or more weed control tactics. The time density equivalent (TDE) approach could provide a solution to this problem. The need for more basic information on weed-crop competitivity and on environmental data in third world countries is stressed.