Resistance to metamitron in Chenopodium album from sugar beet. a tale of the (un)expected?

Metamitron is a key herbicide in modern low rate weed control programs in sugar beet. Fat hen (Chenopodium album, CHEAL) is a common, highly competitive, weed in sugar beet and one of the targets of metamitron. Recently, unsatisfactory control of fat hen has been reported in several sugar beet fields situated in various regions in Belgium. Weather conditions as well as the mere fact of using low rate systems have been blamed for these poor performances. To address the question "Is the recently recorded poor control of C. album due to decreased sensitivity to metamitron", greenhouse bioassays were carried out. A first experiment was conducted applying metamitron (0, 350, 700 and 1,400 g ai/ha) postemergence to three "suspected" C. album populations originating from sugar beet fields with unsatisfactory control by standard metamitron based treatment schemes ('Ligne', 'Outgaarden' and 'Boutersem I' respectively) and to one sensitive population originating from an untreated garden site ('Gent'). In a second experiment seven population, five "suspected" fat hen populations from sugar beet fields ('Boutersem I', 'Boutersem II', 'Postel', 'Vissenaken' and 'Kortessem' respectively), one sensitive reference population 'Herbiseed' and one atrazine-resistant reference population 'ME.85.01', were submitted to metamitron (0, 1, 2 and 4 mg ai/kg air-dry soil) and atrazine (1.5 mg ai/kg air-dry soil) preplant incorporated. All "suspected" C. album populations displayed a significantly lower sensitivity to metamitron compared to the sensitive populations ('Gent' and 'Herbiseed') that never had been exposed to this herbicide. As target site cross-resistance of atrazine-resistant C. album, selected by atrazine in maize, to metamitron has been known for a long time, cross-resistance of C. album populations in sugar beet grown on fields with "maize - atrazine" containing rotations might be expected to appear. The outcome of the experiment with atrazine preplant incorporated was the confirmation of resistance in all "suspected" populations ('Boutersem I', 'Boutersem II', 'Postel', 'Vissenaken' and 'Kortessem'). However, some "suspected" populations came from fields with no background of cropping with maize and use of atrazine. So, the question remains whether these triazine-resistant C. album had been imported, e.g. with slurry, or the rather unexpected possibility that metamitron itself did select for metamitron-resistant biotypes bearing cross-resistance to atrazine, had become reality.