Enhanced production of water‐soluble cinmethylin metabolites by Lolium rigidum populations with reduced cinmethylin sensitivity

Abstract

Background Cinmethylin, a pre‐emergence herbicide inhibiting fatty acid thioesterase activity, has recently been introduced to Australian cereal cropping for the control of Lolium rigidum Gaud. (annual ryegrass). To date, there have been no confirmed cases of cinmethylin resistance identified in this species, but some populations exhibit reduced sensitivity to this herbicide. To explore the mechanism which contributes to reduced sensitivity of annual ryegrass to cinmethylin, the extent and nature of cinmethylin metabolism, using carbon‐14 (14C)‐labelled herbicide, were analysed in three reduced‐sensitivity annual ryegrass populations, alongside a susceptible population and cinmethylin‐tolerant wheat as controls. Results All samples showed the same metabolite profile, with the extent of production of a specific water‐soluble metabolite being correlated to the level of herbicide sensitivity. Application of the cytochrome P450 inhibitor phorate caused a decrease in water‐soluble metabolite production as well as seedling growth in the presence of cinmethylin, indicating that reduced cinmethylin sensitivity in annual ryegrass could be wholly or partially due to oxidative modification of cinmethylin. Conclusion Because annual ryegrass has the potential to metabolize cinmethylin in the same way as wheat, careful stewardship is required to ensure the longevity of this herbicide. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Sensitivity of herbicide-resistant rigid ryegrass (Lolium rigidum) populations to cinmethylin, a new herbicide site of action

“…The minimal effect of temperature on the response of L. rigidum to cinmethylin in the closed agar system, where evaporation of the herbicide was minimized, compared with the more significant effect in the open soil system, indicates that volatility losses of cinmethylin have more effect on cinmethylin response in these populations than any nascent resistance mechanism. This is supported by the results of a previous study in which population P22 was shown to have no greater ability to detoxify cinmethylin than did population S (Goggin et al [2022], in which population P22 was designated "R1").…”

“…The mechanism for reduced cinmethylin sensitivity in the populations used in the current study has not been investigated. However, in two populations with even lower sensitivity, the capacity for metabolic detoxification of cinmethylin was inversely correlated with the level of sensitivity (Goggin et al 2022). As mentioned earlier, parental population P22 did not display significantly higher levels of cinmethylin metabolism than population S in the previous study, but the higher LD 50 of its cinmethylin-selected progeny could potentially be linked to an incremental selection for enhanced cinmethylin metabolism.…”

“…Because cinmethylin is a new herbicide site of action recently introduced into Australia, selection pressure for resistance evolution in L. rigidum has been limited, and the inadvertent application of below-label rates due to volatility loss could contribute to selection of less-sensitive weed populations. The potential for rapid evolution of cinmethylin resistance in L. rigidum populations is presently unknown, but L. rigidum is capable of low levels of metabolic detoxification of cinmethylin, and this capacity is greater in populations with reduced cinmethylin sensitivity (Goggin et al 2022).…”

Inhibition of acyl‐ACP thioesterase as site of action of the commercial herbicides cumyluron, oxaziclomefone, bromobutide, methyldymron and tebutam

“…Indeed, it was shown that multipleherbicide resistant biotypes of Lolium rigidum or Alopecurus myosuroides biotypes with resistance to flufenacet can be effectively controlled by cinmethylin. [38][39][40] Identification of alternative FAT inhibitors for use in wheat or other crop systems will strengthen and diversify the tools for weed management. The herein identified new FAT inhibitors lay the groundwork for the development of new herbicides with this newly identified SoA.…”

“…For the control of increasing numbers of herbicide resistant weeds with target site or non‐target site resistances, there is a dire need for new active ingredients and ideally SoAs. Indeed, it was shown that multiple‐herbicide resistant biotypes of Lolium rigidum or Alopecurus myosuroides biotypes with resistance to flufenacet can be effectively controlled by cinmethylin 38–40 . Identification of alternative FAT inhibitors for use in wheat or other crop systems will strengthen and diversify the tools for weed management.…”