First case of resistance to sulfonylurea herbicides reported in Switzerland: a biotype of loosy silk-bent (Apera spica-venti (L.) P.B.).

The first case of resistance to sulfonylurea herbicides detected in Switzerland is described in this paper. It concerns a biotype of loosy silky-bent (Apera spica-venti), one of the most serious grass weed infesting arable fields in Switzerland. The exact level and amplitude of this resistance, as well as its mechanism and genetics, still need to be specified; but preliminary results demonstrate unambiguously its existence. Today, it is justified to strictly implement the rules aiming at avoiding further development of this resistance in fields infested by these weed species; first of all a rotational use of herbicides having different modes of action. During the last decade, ALS-inhibiting herbicides have been registered in an increasing number of crops in Switzerland. Globally, sulfonylurea herbicides play currently a role crucial in the chemical weed control in Swiss agriculture. Therefore, it is particularly important to prevent the spreading of resistances to this class of herbicides. Concerning specifically loosy silky-bent, these species already developed in Switzerland numerous biotypes resistant to isoproturon, a substituted urea herbicide. Therefore, a risk exists of the development of biotypes with multiple resistances. Such an occurrence would lead to very difficult weed situation, especially in cereals.