A new approach to determine when to control weeds

A methodological approach to determine when to control weeds is presented. It integrates aspects of weed biology, weed-crop competition and to compare different weed control strategies. It is based on the concept of Time Density Equivalent (TDE): for weeds with a given time of emergence and removal, TDE is defined as the density of plants that germinate with the crop and compete until harvest that determines the same yield loss as caused by those weeds. The model accounts for the pattern of weed emergence and permits the determination of the timing of weed control that minimizes the economic loss due to weeds emerging both before and after the treatments. The outcomes of the model are presented with two examples, corn in competition with velvetleaf and soybean in competition with Amaranthus cruentus. For both crops, six different weed control strategies involving preemergence, chemical and mechanical postemergence treatments are considered. The results obtained with the model are compared with the classic calculation of Net Margin, based on the assumption of a contemporary emergence of crop and weeds. Different control strategies are compared considering not only the maximum Net Margin but also its dependence on time of control. The flexibility of the different treatments is discussed in terms of time of application and it is suggested that this should be used as an additional criterion for selecting the best weed control strategies.