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Angie Peltier


Angie Peltier
Former Extension Educator, Commercial Agriculture



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Hill and Furrow

Current topics about crop production in Western Illinois, including field crops research at the NWIARDC in Monmouth.
Figure. First-year Palmer amaranth infestations are unlikely to look like the one captured by former Extension Educator Robert Bellm in this soybean field near St. Louis. The crop producer intended to mow this field in an effort to minimize additions to the soil weed seed bank.

Evaluate your herbicide-resistance management plan by answering these 5 questions

Posted by Angie Peltier - Weeds

Over time farm sizes have increased as the number of farmers has decreased. This, along with the trend toward soil conservation and the wide-spread adoption of no- or reduced-tillage, has led to the increased reliance on herbicides as the only weed management strategy many employ. Herbicides bind to specific molecules in plants -known as a herbicide's site of action (SOA)- which interferes with a weed's growth and development. Through infrequent but naturally occurring genetic mutations some individuals in a larger population of weeds have genes that allow them to survive a herbicide with a particular SOA. Those individuals will be able to reproduce and set seed, while those that are still sensitive to the SOA will die without setting seed. Through these natural evolutionary processes, repeated use of single SOAs can result in weed populations shifting from herbicide sensitive to resistant, from herbicide effectiveness to ineffectiveness. Research has shown that herbicide resistance can reduce a farmer's per acre return by up to $67 (Livingston et al., 2015). While multiple herbicide SOAs are available for use in corn and soybean production, no new SOAs are scheduled for release in the near future, making stewardship of these valuable tools an urgent priority.

In Illinois there are currently populations of 12 weed species (waterhemp, marestail, Palmer amaranth, cocklebur, giant foxtail, shattercane, giant and common ragweed, smooth pigweed, kochia, lambsquarters and black nightshade) that have been confirmed resistant to one or more of seven different herbicide SOA groups (Group 1: ACCase inhibitors, Group 2: ALS inhibitors, Group 4: auxin receptors, Group 5: photosystem II inhibitors, Group 9: EPSP synthase inhibitor, Group 14: PPO inhibitors and Group 27: HPPD inhibitors) (Heap, 2017).

The University of Illinois Plant Clinic provides a paid service to test waterhemp samples for genetic resistance to glyphosate and PPO inhibitor herbicides. This is a sure-fire way to confirm that you have a waterhemp population that is resistant to one of these SOAs. While this is a nice service, it can be used to confirm resistance to only two SOAs (Group 9 and Group 14) in one weed species (waterhemp). This is where one's experience in weed management successes (or failures) over the years comes into play: Have you started to notice that there are small and growing patches of weed survivors despite the timely application of full label rates of what should be effective herbicides? If so, you may be dealing with an herbicide resistant weed population.

Before the 2017 growing season begins, it would be time well spent to assess the potential effectiveness of your weed management plan from a herbicide resistance perspective. What are your answers to the following five questions?:

Am I deploying herbicides from more than one SOA at a time? A couple of years ago researchers at the University of Illinois set out to determine which factors are most important for fostering the development of glyphosate-resistant waterhemp populations (Evans et al., 2015). To accomplish this they collected information about the management practices, landscape characteristics and weeds on 105 farms in central Illinois and also collected seed from waterhemp plants in each field. They used some powerful statistical tools to analyze the weed, management and landscape data and planted the collected seed to test the effectiveness of glyphosate

Some folks may think that applying more than one herbicide SOA within a growing season, a practice called herbicide rotation, is effective for slowing the speed at which herbicide resistant weeds evolve. This extensive study showed that it wasn't herbicide rotation but rather mixing herbicides from multiple SOAs that slowed the speed of herbicide resistant evolution. Fields with an average of 2 ½ herbicide SOAs per application were 83 times less likely to produce glyphosate resistant waterhemp seeds in the near future than fields with an average of 1 ½ herbicide SOAs per application. This means that herbicide mixing is better able to slow the speed of herbicide resistance evolution, which is important from both resistance management and herbicide SOA stewardship standpoints. . One way to answer this question would be to take the list of herbicides you plan to use and look them up using a tool developed by university and company weed scientists. This tool provides information regarding the SOA group(s) in each stand-alone herbicide and premix product.

Are these SOAs and active ingredients effective against the problematic weeds in each field? Weed scientists from the Ohio State University, Purdue University and Dr. Aaron Hager from the University of Illinois produce a document called the Weed Control Guide for Ohio, Indiana and Illinois. Tables 1, 3 through 6 and 13 through 16 provide weed species-specific efficacy ratings for burndown, pre- or post-emergence herbicides that are labeled for use in corn or soybean. The weed control ratings often also take into account the effectiveness of individual herbicides against the herbicide resistance (or suspected herbicide resistance) that exists in a given population of common or giant ragweed, lambsquarters, marestail, Palmer amaranth, smooth and redroot pigweed, or waterhemp. Comparing among lists of your problem weeds, your 2017 herbicide plan and these tables would be time well spent.

Am I planning to apply the full label rate? There are several mechanisms by which a weed can have reduced sensitivity or full-blown resistance to a particular herbicide SOA. In some cases there can be multiple copies of a particular herbicide site of action (the molecular target). In this case, while some of the SOAs may be impaired by the herbicide, enough of them remain unaffected resulting in the herbicide being effectively ineffective. In other cases, the mechanism of herbicide resistance can be through enhanced metabolism or the plant's ability to degrade the herbicide before it can reach its site of action. A full label rate of the corresponding herbicide SOA is sometimes still effective in weeds that have a lower level of these types of resistance.

How might the full label rate of an active ingredient in the premixed product I plan to use compare to the full label rate of a single SOA herbicide? It is important to not only apply full label rates of multiple and effective herbicide SOA groups, but to do so with resistance management in mind. Sometimes the label rate of individual active ingredients in a pre-mix herbicide is much lower than the label rate of the same active ingredient on its own. This can become an important factor to consider, particularly when one of the herbicide SOAs in a premix is already known to be ineffective due to either herbicide resistance or weed insensitivity. For example, in a soybean field with ALS inhibitor (Group 2) resistant waterhemp, the product Authority Assist (Group 2: imazethapyr and Group 14: sulfentrazone) would be relying exclusively on the Group 14 active ingredient. In this case, applying the full label rate of the pre-mix product would be like applying 34% of the maximum rate of the single active ingredient product Spartan. Given what we know about some of the mechanisms of resistance, putting this much pressure on a single site of action group might not be the best plan from a resistance management standpoint. To help in quickly making apples to apples comparisons, on pages 15 through 21 of their 2017 herbicide guide, weed scientists at the Iowa State University provide tables in which they include the "active ingredients found in prepackage mixes, the amount of active ingredients applied with a typical use rate, and the equivalent rates of the individual products" (Owen and Hartzler, 2016).

Am I also incorporating non-chemical methods to manage weeds? Herbicides aren't the only weed management strategy to which weeds can evolve resistance. Dr. Adam Davis, a USDA weed ecologist with the University of Illinois often cites an example that can help to illustrate this point. He was called several years back by an organic soybean farmer in Northern Illinois with a field so thick with giant ragweed that one couldn't see the soybeans. He collected giant ragweed seeds from plants that were in the soybean field and from plants that were growing in a nearby streambank. He counted the seeds and planted the same number of seeds from the field and from the streambank and then monitored their germination pattern. He noted that the majority of those seeds that originated from the streambank germinated early in the growing season, while those originating from the soybean field germinated throughout the entire cropping system. Giant ragweed had evolved to overcome the primary means of control on this organic farm – cultivation. In this case, those seeds that germinated later on were able to survive, reproduce and add their seed to the seed bank, while those that germinate early were controlled with cultivation. The lesson here is that weeds can evolve to overcome any single weed management practice that we employ and that diversifying our management strategies is key.

Diversifying weed management to include non-chemical means of control (row spacing and plant populations that foster a thick canopy, tillage or cultivation or even walking fields with a weed hook) along with applying full label rates of multiple, effective herbicide SOAs in an overlapping and timely fashion are important factors if one is to develop an effective multi-year weed management plan.

Disclaimer: Please note that an illustrative example was used in this document and wasn't meant to imply that any one product mentioned or unmentioned is superior or inferior to any other product.

Literature Cited

Evans, J.A., Tranel, P.J., Hager, A.G., Schutte, B., Wu, C., Chatham, L.A. and Davis, A.S. 2015. Managing the evolution of herbicide resistance. Pest Management Science. 72:74-80.

Heap, I.TheInternational Survey of Herbicide Resistant Weeds. Online. Internet. Monday, February 27, 2017.

Livingston, M., Fernandez-Cornejo, J., Unger, J., Osteen, C., Schimmelpfennig, D., Park, T. and Lambert, D. 2015. The economics of glyphosate resistance management in corn and soybean production. ERS Report #184. USDA-ERS

Loux, M.M., Doohan, D., Dobbels, A.F, Johnson, W.G., Young, B.G., Legleiter, T.R. and Hager, A. 2016. Weed Control Guide for Ohio, Indiana and Illinois. Pub #WS16/Bulletin 789/IL15. The Ohio State University.

Owen, M.D. and Hartzler, R. 2016. 2017 Herbicide Guide for Iowa Corn and Soybean Production. WC-94. Iowa State University Extension

United Soybean Board. 2016. Take action against herbicide-resistant weeds. Online.

University of Illinois Plant Clinic. Herbicide Resistance Testing and Molecular Weed ID: Information & Sample Submission Form.



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