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Combatting Resistance and the Future of Organic Herbicides

October 1, 2019

By Scott Ockey, Western US Field Development Manger, Certis Biologicals

Herbicides have been under increased scrutiny with many of the challenges appearing at the forefront of multiple news cycles. With recent legal issues, from drift concerns to performance issues and/or herbicide resistance, one might wonder if there is anything positive to discuss concerning herbicides.

The good news is there is hope for growers who are in need of reliable weed control. Registration of herbicides by the EPA through the USDA -funded National Organic Program (NOP) has listed several products that answer both performance and resistance concerns, giving growers the tools to help address these concerns and gain peace of mind.

Herbicide resistance has been documented in nearly 250 cases across a wide array of chemistries worldwide, with 160+ cases in the United States with 90 cases alone in California. To help educate growers on how to avoid or significantly reduce the risk of herbicide resistance, the Herbicide Resistance Action Committee (HRAC) and most University Extension Services publish Best Management Guidelines/Tools. The Primary tactics common to most guidelines include: identifying the modes of action of the herbicides being used and to rotate to different modes of action on subsequent sprays, rotate crops for the most successful and robust weed control programs, and employ mechanical and cultural weed management strategies in an herbicide spray program. Report suspected herbicide resistant weeds to your local extension service so that they can be tested, and if resistance is found, a control plan can be initiated. Diligent adherence to these guidelines will ensure long-term efficacy of your most useful herbicides.

We’re also seeing our options limited, with some of the non-selective herbicides that are often used to manage resistant weeds being placed under increased scrutiny by the EPA. This is mainly due to concerns surrounding toxicity, which has resulted in new labeling rules that restrict application to only certified applicators and requires additional recurring and specific training for those applicators. With increased restrictions and a reduction in these options, growers are left searching for viable solutions to compliment the efforts of the HRAC and University Extension Services.

Effective organic herbicides are now available for inclusion in programs as those mentioned above to help safeguard against herbicide resistance by introducing additional modes of action into weed management programs. As the name states, they are also vital for weed management programs in organic farming systems. Currently available organic herbicides are non-selective and act through catastrophic disruption of the plant cuticle, which results in exposure and destruction of cellular contents. Because of the nature of this mode of action, it is highly unlikely that plants will develop resistance to these weed control products. The incorporation and timing of applications to preserve the use of selective herbicides is an ideal use for organic based bioherbicides.

Primary organic herbicide categories that are available today include fatty acids, soaps, plant extracts, and acetic acids. These broad-spectrum contact herbicides have strong attributes, but each has characteristics that require a bit of education for optimal performance.

Since organic herbicides rely on destruction of the plant cuticle, coverage is paramount. Increasing application volume directly correlates with increased weed control. Treating weeds when they are young is also advised. It’s more manageable to control 2-5” weeds vs. 6”-12” weeds. This timing is more effective as full coverage is achieved with lower water volumes and the plants have not yet been able to lignify tissues or produce a substantial root system to support regrowth.

Fatty acids and soaps often benefit from application under higher temperatures and low pH. Acetic acid efficacy increases in water sources with low carbonates and low pH. Acetic acid is corrosive and may damage application equipment. Immediate rinsing of spray tank and washing all equipment contacted by the acetic acid solution is advised. Plant extracts vary in requirements to optimize efficacy; however, purity due to the plant source and processing/formulation for these extracts can often impact efficacy. It is advisable to consult local distributors and/or University Extension Service reports to make the decision of which plant extract is appropriate.

While we will certainly continue to face current and future challenges as it relates to weed management, it’s good to know that we have some great guidance and viable solutions. When used correctly and according to the guidelines from those with knowledge of these products, our future is bright and full of crops, and free of weeds.

This article appeared in the October 2019 issue of CAPCA Adviser Magazine. You can read the original here.

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