Resistance profile of herbicide-resistant Alopecurus myosuroides (black-grass) populations in Denmark

Alopecurus myosuroides Huds is one of the most important grass-weeds in North-western Europe and is also the most important herbicide-resistant weed species in European agricultural systems. Fifty-three Danish A. myosuroides populations, previously confirmed to be fenoxaprop-P resistant, were evaluated for five and two known mutation points within the ACCase and ALS genes, respectively. The resistance pattern of 28 out of the 53 populations was investigated to four herbicides using a seed bioassay technique. A whole plant dose response experiment was conducted on seven populations in 2012 and 2013 to evaluate the accuracy of the seed bioassay results. Two resistant populations from the UK and a susceptible population from Denmark were included as reference populations in all experiments. Of the 53 populations, nine (17%) populations were ACCase target site resistant (TSR), all with a mutation at the Ile-1781 position. No mutations conferring TSR to ALS inhibitors were detected in the 53 populations. The seed bioassay results showed that all populations had varying degrees of resistance to fenoxaprop-P. In contrast, all populations were susceptible to cycloxydim suggesting that non-target site resistance (NTSR) was present in all populations including the ones where TSR were found. The seed bioassay was not a suitable method for detecting resistance to the residual herbicides pendimethalin and prosulfocarb. The whole plant dose response experiment results confirmed the results of the seed bioassay for fenoxaprop-P and cycloxydim. Three and two out of seven populations were also resistant to flupyrsulfuron and pendimethalin, respectively, while all resistant populations were susceptible to mesosulfuron + iodosulfuron. The widespread occurrence of particularly NTSR is a severe challenge to the effective management of A. myosuroides. In Denmark this challenge is even more prominent due to few modes of action being available for A. myosuroides control mainly due to national regulation on groundwater protection.