Impact of non-target-site-resistance on herbicidal activity of imazamox on black¬grass ( Alopecurus myosuroides Huds.) in comparison to other ALS-graminicides

Abstract

A black-grass (Alopecurus myosuroides Huds.) resistance-monitoring conducted by BASF in 2010 - 2012 revealed a high number of accessions with resistance against imazamox. However, application of imazamox- based products in a winter crop was limited to winter beans in France and United Kingdom only until the introduction of the Clearfield ® -production system in autumn 2012 in winter oilseed rape. It is therefore assumed that the resistance mechanisms were probably selected by the frequent use of ACCase- and ALS- inhibitors in winter crop rotations during the last 2 decades. Resistance level for each product-biotype combination was calculated according the R-classification system (S, R?, RR, RRR) by directly comparing the product performance on a biotype versus untreated control. Majority of resistant biotypes did not show a target-site mutation at the known codon Pro197 or Trp574. In order to better evaluate the impact of Non- Target-Site-Resistance (NTSR) on the activity of BEYOND (imazamox), ATLANTIS WG (mesosulfuron+iodosulfuron) and ABAK (pyroxsulam), biotypes who have shown an ALS-target-site mutation were removed from further analysis. At the dose rate of 35 g ai/ha BEYOND provided good activity on susceptible biotypes of black-grass almost matching up with ATLANTIS WG and ABAK. However, activity of BEYOND declined stronger on biotypes classified as R? or RR for that product, while ATLANTIS WG and ABAK hardly showed any decline in control on this group of biotypes when applied at the recommended dose rate. It is assumed that the underlying NTSR- mechanism is not effective enough yet to confer resistance to ATLANTIS WG and ABAK, but on BEYOND. In contrast, biotypes classified as R? for ATLANTIS WG did show a stronger impact on the activity of BEYOND and ABAK then of ATLANTIS WG. These differences in control level probably do translate into differences in selection pressure as well.