Herbicide resistance in Alopecurus myosuroides: The value of routine testing of seed samples submitted by farmers since 1985

In 35 years of routine screening for herbicide resistance, a total of 3758 seed sample/herbicide combinations were assayed. Resistance testing was done in either pots in glasshouses (chlortoluron, fenoxaprop and mesosulfuron-methyl + iodosulfuron-methyl sodium) or Petri-dishes in incubators (sethoxydim, cycloxydim and pendimethalin). With all herbicides, the relationship between herbicide efficacy and year of sampling was linear, with the slope representing the annual loss of efficacy. This was higher for the ALS inhibitors mesosulfuron + iodosulfuron (3.73% year(-1)) and ACCase inhibitors sethoxydim/cycloxydim (1.92% year(-1)) and fenoxaprop (1.36% year(-1)) than for the substituted urea chlorotoluron (0.69% year(-1)) and the dinitroaniline pendimethalin (1.10% year(-1)). These results are consistent with other studies on the relative resistance risk associated with these different modes of action. There was also a surprisingly good correlation between results for random and non-random resistance testing, which has also been found in studies with other weed species in Canada and Australia. This indicates that routine testing of submitted samples can replace, at least partly, the need for random surveys which tend to be both labour intensive and expensive. These results, compiled over 35 years, show the value of routine resistance screening, not only for detecting resistance at the individual field level ('micro' scale), but also the distribution, evolution and impact of resistance country-wide ('macro' scale). However, it is important that standardised testing methods, including appropriate reference populations, are used by different testing centres to ensure consistent results.