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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. Soybeans are difficult to see from this vantage point due to all of the waterhemp plants in this field. Note that the field in the background appears from this distance to be weed-free.
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Weed management struggles in 2015: What happened?

Posted by Angie Peltier - Weeds

In my many hours spent behind a windshield this summer, I have noticed several fields that look like the one in the photos above. Although there are individual marestail and giant ragweed plants that have also poked their way through the soybean canopy, by and large the primary weed species competing with the soybean crop for water, nutrients, and sunlight is waterhemp. As the female flowers receive pollen and set seed, one can't help but think about how the effects of this weedy field will likely last much longer than yield losses observed in 2015.

Each female waterhemp plant is capable of producing more than 1 million seeds. For argument's sake, let's say that there on average 500 waterhemp plants per acre, half of which are female. If each female plant lives up to her potential, more than 250 million waterhemp seeds might be added to that acre's weed seed bank this fall. Now not all of the plants may be this prolific, and not all of the seeds will be viable in 2016 and beyond, but there is no denying that a lot more seeds will be a part of the weed seed bank at the end of this season than were there at the start!

One can't help but speculate about what might have happened in this field. Some possibilities might include:

Perhaps soil-applied residual herbicides were not used. If this was the case, weed seedlings may have gotten a foothold long before soybean seedlings emerged. Rainy weather and wet soils after planting may have kept the sprayer out of the field when they really need to be there to keep ahead of the weeds. This might also give the producer less flexibility – if the weeds grew too quickly, herbicides or rate changes might be required for good control.

Dr. Aaron Hager, University of Illinois Extension Weed Specialist, reminds us that an effective weed management plan can actually save us money in the long run. While the cost of residual and effective post emergence herbicides may cost around $40 per acre, failure to use residual herbicides to manage glyphosate-resistant waterhemp can result in a 25 percent yield loss in soybean. In a field that can typically raise 50 bu/A soybeans this would mean a 12.5 bu/A yield loss due to weed pressure. Under current soybean prices ($9), the out of control weeds could cost us $112.50 in lost yields!

Perhaps the waterhemp population in this field is resistant to the herbicide(s) that were applied. In Illinois, research has confirmed that some waterhemp populations have resistance to at least one of five different herbicide sites of action: ALS inhibitors, photosystem II inhibitors, EPSP synthase inhibitors, PPO inhibitors, and HPPD inhibitors. Some populations have resistance to herbicides from more than one site of action. In fact, in Illinois, populations of waterhemp have been confirmed as having resistance to a combination of four herbicide sites of action.

Perhaps below-label herbicide rates were applied. With commodity prices staying low and production costs remaining high, there is pressure to reduce costs where ever possible. Rates lower than the full label rate may damage weed plants without killing them, resulting in the end in weeds that grow back. Herbicides that have a single active ingredient have recommended label rates that might vary according stage of weed and crop growth, soil pH, etc. Applying full label rates is one good way to slow the speed of herbicide resistant weed populations. However, comparatively-speaking, some of the premix herbicide products contain lower concentrations of individual component herbicides. It is important to check and recheck pesticide labels as formulations may change. It is also important as a reminder of which site of action each herbicide targets and to make sure that weeds are being managed with full label rates of each herbicide.

Regardless of what happened in this field in 2015, additions to the weed seed bank will ensure that weed management will be a challenge in 2016 and beyond. There are several very good online resources that crop producers can use to remain up to date with the latest research regarding weed management and herbicide resistance:

1) Watch for timely articles regarding weed management topics in the Department of Crop Sciences' online publication regarding both pest management and crop development - The Bulletin.

2) There are online courses that summarize some of the latest research regarding weed management at the University of Illinois. To listen and view each course, click the "Begin Course" button under the green box on the right-hand side of the webpage. Certified Crop Advisors in need of credit will need to login.

  1. Confirming Herbicide Resistance – Dr. Aaron Hager
  2. New (and old) Tools for Delaying and Coping with Herbicide Resistance – Dr. Adam Davis

3) The soybean check-off-sponsored website that provides both online tools and links to order complimentary hard copies of materials that help with weed identification, highlight different herbicide sites of action and other herbicide resistance management strategies: Take Action.

4) The 2015 Weed Control Guide for Ohio, Indiana and Illinois.



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