The occurrence of herbicide-resistant Avena fatua (wild oats) populations to ACCase-inhibiting herbicides in Ireland

Following growers’ reports of herbicide control problems, populations of 30 wild oats, Avena fatua, were
collected from the south-east main arable counties of Ireland in 2016 and investigated for the occurrence and
potential for herbicide resistance to acetyl-CoA carboxylase (ACCase) inhibitors pinoxaden, propaquizafop
and cycloxydim, as well as acetolactate synthase (ALS) inhibitor mesosulfuron + iodosulfuron. Plant survival
≥20% was considered as the discriminating threshold between resistant and susceptible populations, when
plants were treated with full recommended field rates of ACCase/ALS inhibitors. Glasshouse sensitivity screens
revealed 2 out of 30 populations were cross-resistant to all three ACCase inhibitors. While three populations
were cross-resistant to both pinoxaden and propaquizafop, and additionally, two populations were resistant to
propaquizafop only. Different degree of resistance and cross-resistance between resistant populations suggest
the involvement of either different point mutations or more than one resistance mechanism. Nevertheless, all
populations including the seven ACCase-resistant populations were equally susceptible to ALS inhibitor. An
integrated weed management (cultural/non-chemical control tactics and judicious use of herbicides) approach is
strongly recommended to minimize the risk of herbicide resistance evolution.