Welcome to the second posting of the Weeders of the West (WOW). Let’s start with kudos to Drew Lyon for organizing this blog and recruiting several researchers from throughout the PNW to join the team. Clearly multi-state extension efforts are the way forward, as we have seen with the success of the Pacific Northwest Handbooks since the 1980’s.
Last time, the blog began with a discussion of herbicide resistance, and Dr. Lyon suggested the role that glyphosate-resistant soybean and corn have played. Personally, I think Drew was right, but I would like to add a few details to expand that thought. Here is my thought process: Certainly, the glyphosate-resistant crops are part of the problem. GR-crops exacerbated the selection of new cases of resistance because glyphosate was often employed as the only weed management strategy. The first case of glyphosate-resistance was documented in rigid ryegrass from an orchard in Australia that had been treated with glyphosate two to three times per year for 15 consecutive years (1).
We can deduce at least two important things from that report. First, that the orchard was not glyphosate resistant, even though the authors never mentioned what species the orchard was planted to. Second, glyphosate resistance occurred because of the repetitive use of glyphosate, not because the crop was glyphosate resistant. To further support that, the authors stated that rigid ryegrass displayed resistance to most of the major herbicides used in Australia. I suspect that major herbicides implies the ones most often used in that region of Australia at that time.
The evidence leads us to conclude that the repetitive use of a single weed control method was the cause of resistance development. Diversified weed control methods are the solution to herbicide-resistance . Yet, we continue to struggle with this problem since the 1950’s. Resistance development is also a problem in disease and insect management.
Let me offer my own theory of why resistance is a problem. Agriculture is a biological system managed by economical reasoning. Because weed control is a production cost, the default solution is to rely on the cheapest and most effective control methods. Repeated use of a single weed control method over a long period of time is cost effective. I might have blamed low commodity prices, except that herbicide resistance is a problem even in specialty crops like wine, where crop prices are not necessarily low. For instance, glyphosate-resistant weeds are a problem in vineyards of Napa Valley (2), where the price of the average wine bottle is probably over $75.
The lack of new herbicide modes-of-action has added to the problem. Rotation of modes-of-action and herbicide tank-mixtures is a practice to manage herbicide-resistance in weeds. This approach has its limitations; cases of resistance continue to increase and options for rotation and tank-mixtures have become limited. This is about to change; reports of new herbicide modes-of-action are coming from multiple sources.
During the 2020 Weed Science Society of America meeting, a series of presentations reported on a new pre-emergent herbicide to be registered in cereal crops soon (3). The new active ingredient, cinmethylin, belongs to the new mode of action WSSA group 30, or inhibition of fatty acid thioesterase (4). Group 33 is another new herbicide mode-of-action; cyclopyrimorate is one example (5). For an updated view of the most current classification of modes-of-action, please see a report by the Herbicide Resistance Action Committee (6).
As new tools become available, the management of herbicide resistance will become easier, but as long as weed management relies on a single tool, we will revisit this topic.
Works Cited
- Powles, Stephen B., et al. “Evolved resistance to glyphosate in rigid ryegrass (Lolium rigidum) in Australia.” Weed science 46.5 (1998): 604-607.
- University of California Agriculture and Natural Resources Weed Control Blog.
- Weed Science Society of America (2020) 2020 Meeting program (pdf).
- Campe, Ruth, et al. “A new herbicidal site of action: Cinmethylin binds to acyl-ACP thioesterase and inhibits plant fatty acid biosynthesis.” Pesticide biochemistry and physiology 148 (2018): 116-125.
- Kahlau, Sabine, et al. “Aclonifen targets the solanesyl diphosphate synthase, representing a novel mode of action for herbicides.” Pest Management Science (2020).
- Herbicide Resistance Action Committee Mode of Action Classification (pdf).
