Approaches to early detection of herbicide resistance in Apera spica-venti regarding intra- and inter-field situations

Herbicides are still the most effective way of weed control. Evolved resistance to herbicides may become a serious and escalating problem in crop production systems worldwide. The challenge in avoiding the dissemination of resistant populations is an early exploration of resistance. Our study aimed at answering the questions: (1) Can the intra-field distribution of an Apera spica-venti population be used to indicate the first steps in the evolution of resistance? (2) Is the selection of field populations based on casual observations of farmers an approach to get a reasonable overview on the resistance status of field populations in monitoring? To answer these questions an intra-field study of one population in Germany and the results of a two years monitoring in north-eastern Germany regarding the actual resistance status of A. spica-venti were used. Dose-response bioassays with isoproturon showed considerable differences in the susceptibility of samples collected within the same field. Herbicide efficacy was not plant density-dependent, but it was related to spatial plant distribution depending on the processing direction in the field. The inter-field situation survey confirmed resistance to at least one herbicide in 15 out of 38 populations. The occurrence of resistance was associated with a mixed tillage system (ploughing and not-ploughing in alternation) and normal winter wheat sowing dates. Fourteen farmers were correct in their assumptions regarding absence of herbicide resistance in their fields. However, resistance was only confirmed in 56% of the fields in which farmers suspected resistance indicating that poor herbicide performance can be caused by other reasons as for example poor environmental conditions.