Since the introduction of highly effective compounds with specific sites of action herbicide resistance has increased. As reliance on these herbicides grows, action is required to safeguard their effectiveness. In contrast to insects and fungi, weed populations are stationary and resistance will persist in infected fields for many years. Reversibility depends on weed species (self- or cross-pollination, seed longevity), farming practice (mainly crop rotation and soil tillage management) and alternative herbicide mode of actions for controlling the resistant weed species. In recent years non-target site resistance has become more frequent. Plant cytochrome P450s catalyse a wide diversity of reactions in plant metabolism. Some cytochrome P450s will metabolize certain herbicides to compounds with reduced or modified phytotoxicity. In weeds, as for insect pests, cytochrome P450-based herbicide resistance is a very threatening resistance mechanism because cytochrome P450 enzymes can simultaneously metabolize herbicides with different modes of action, potentially including never-used herbicides displaying non-target-site cross-resistance across several herbicide modes of action. A major research frontier is to identify the cytochrome P450s conferring resistance in weeds. Grass weeds are the economically most important group of weeds and more prone to non-target site resistance than dicotyledonous plants. Among the grass weeds, silky bent grass (Apera spice-venti) is the most important in Denmark and has not received the same attention in herbicide resistance research as the globally more important species Alopecurus myosuroides and Lolium spp. The first case of resistance in silky bent grass in Denmark was identified in 2010.
