Are herbicide mixtures unaffected by resistance? A case study with Lolium rigidum

Lolium rigidum, a grass weed species infesting winter field crops, has evolved resistance to the largest number of herbicide modes of action. In this study, 140 field populations of L. rigidum were screened with 14 herbicide treatments. Herbicide resistance at the recommended label dosage of pre-emergence (PRE), post-emergence (POST) and binary herbicide mixtures was considered present when plant survival was ≥6%. Plant survival to four acetyl-CoA carboxylase (ACCase) POST herbicides averaged across all populations was approximately 15%, indicating substantial herbicide resistance.

In contrast, the mean survival to the PRE treatments was only 2%, reflecting effective control of L. rigidum. Herbicide mixtures were the most effective treatments, with a significantly lower resistance frequency than stand-alone herbicides. For example, only 12% of the tested samples were resistant to the mixture of clethodim + butroxydim in comparison with 40% and 61% to either butroxydim or clethodim, respectively. Similarly, 8% of the samples were resistant to the mixture of trifluralin + prosulfocarb versus a m uch greater f requency of 36% a nd 51% resistance to prosulfocarb and trifluralin, respectively.

Surprisingly, the binary mixtures of trifluralin + triallate or pyroxasulfone + triallate are not affected by resistance (presently) due to the greater efficacy than that of either stand-alone herbicide. Thus, herbicide mixtures can delay the onset of resistance and mitigate the existing levels of herbicide resistance and cross-resistance in L. rigidum. Systematic screenings of a large number of field populations could identify the most (cost-) effective herbicide mixtures and foster their informed adoption on farm to mitigate the rapid evolution of weed resistance in lieu of expert assumptions or modelling simulations.