WSSA: Reducing the Risks of Herbicide Resistance

Executive Summary

From Weed Science Society of America.  Click the link at the end of the summary for the whole paper.

Herbicides are the foundation of weed control in commercial crop-production systems. However, herbicide-resistant (HR) weed populations are evolving rapidly as a natural response to selection pressure imposed by modern agricultural management activities. Mitigating the evolution of herbicide resistance depends on reducing selection through diversification of weed control techniques, minimizing the spread of resistance genes and genotypes via pollen or propagule dispersal, and eliminating additions of weed seed to the soil seedbank. Effective deployment of such a multifaceted approach will require shifting from the current concept of basing weed management on single-year economic thresholds.

Programs for herbicide-resistance management must consider use of all cultural, mechanical, and herbicidal options available for effective weed control in each situation and employ the following best management practices (BMPs):

1. Understand the biology of the weeds present.
2. Use a diversified approach toward weed management focused on preventing weed seed production and reducing the number of weed seed in the soil seedbank.
3. Plant into weed-free fields and then keep fields as weed free as possible.
4. Plant weed-free crop seed.
5. Scout fields routinely.
6. Use multiple herbicide mechanisms of action (MOAs) that are effective against the most troublesome weeds or those most prone to herbicide resistance.
7. Apply the labeled herbicide rate at recommended weed sizes.
8. Emphasize cultural practices that suppress weeds by using crop competitiveness.
9. Use mechanical and biological management practices where appropriate.
10. Prevent field-to-field and within-field movement of weed seed or vegetative propagules.
11. Manage weed seed at harvest and after harvest to prevent a buildup of the weed seedbank.
12. Prevent an influx of weeds into the field by managing field borders.

The long-term economic benefits of avoiding additional costs associated with managing HR weeds are clear. Nevertheless, widespread adoption of these BMPs must overcome several real barriers. In particular, growers' focus on immediate economic returns must be overcome as well as their beliefs that the evolution of herbicide resistance in weeds is unavoidable and that continued availability of novel herbicide technologies will solve the problem. There is, at present, no single database collating information on weed management practices employed by U.S. growers, so the extent of the adoption of BMPs for HR weeds must be inferred by combining data from multiple sources. Available survey data show that, although many U.S. soybean [Glycine max (L.) Merr], corn (Zea mays L.), and cotton (Gossypium hirsutum L.) growers employ at least some BMPs, a significant proportion of growers are not practicing adequate, proactive herbicide-resistance management. Two key recommendations, in particular, must be more widely implemented: diversifying weed management practices and using multiple herbicide MOAs. Growers need to be educated about MOAs and be made aware that discovery of new herbicide chemistries is rare, that the existing herbicide resource is exhaustible, and that indiscriminate herbicide use leading to rapid evolution of HR weeds may result in the loss of herbicide options for all.

To address the increasingly urgent problem of herbicide resistance, we make the following recommendations:

1. Reduce the weed seedbank through diversified programs that minimize weed seed production.
2. Implement a herbicide MOA labeling system for all herbicide products and conduct an awareness campaign.
3. Communicate that discovery of new, effective herbicide MOAs is rare and that the existing herbicide resource is exhaustible.
4. Demonstrate the benefits and costs of proactive, diversified weed-management systems for the mitigation of HR weeds.
5. Foster the development of incentives by government agencies and industry that conserve critical herbicide MOAs as a means to encourage adoption of best practices.
6. Promote the application of full-labeled rates at the appropriate weed and crop growth stage. When tank mixtures are employed to control the range of weeds present in a field, each product should be used at the specified label rate appropriate for the weeds present.
7. Identify and promote individual BMPs that fit specific farming segments with the greatest potential impact.
8. Engage the public and private sectors in the promotion of BMPs, including those concerning appropriate herbicide use.
9. Direct federal, state, and industry funding to research addressing the substantial knowledge gaps in BMPs for herbicide resistance and to support cooperative extension services as vital agents in education for resistance management.

In some instances, short-term costs may not favor implementation of BMPs that provide insufficient immediate economic benefit, even though their adoption will delay the evolution of HR weed populations over time. In such cases, consideration should be given to providing incentives and expert advice for growers to develop and implement risk-reducing weed management plans, following the precedent set by similar incentives for the conservation of soil and water resources in agriculture.

http://wssajournals.org/doi/full/10.1614/WS-D-11-00155.1