Recommendations for managing herbicide-resistant weeds in wheat

Wheat is an important market for Syngenta; therefore, it pays a lot of attention to wheat production and helping farmers protect yield potential. It is logical that the company would provide opinions and information about managing herbicide-resistant weed problems, and it is obvious that weeds when infesting wheat fields compete with crops and can ultimately impact yield potential and crop quality. 

Growers turn to herbicides as one method of minimizing these threats and protecting their wheat.  Many factors go into an effective herbicide program—application timing, use of full labeled rates, proper use of adjuvants and effective spray application. Even when everything has seemingly been done right to protect wheat fields, growers still find areas where weeds have not been controlled. This lack of control could potentially mean a larger issue is developing: herbicide resistance. 

Herbicide resistance is the inherited ability of a weed to survive the rate of herbicide which would normally give effective control. According to The International Survey of Herbicide Resistant Weeds, there are 14 herbicide-resistant broadleaf and grass weed species found in U.S. wheat crops today. Wheat growers have observed a particularly widespread occurrence of herbicide-resistant kochia and wild oat. Like other weeds, they compete with the crop for valuable sunlight, water and soil nutrients needed for robust development and strong yield.

“Once resistance has been confirmed, it can remain in the population for decades. If growers wait until herbicide resistance is confirmed in their field, the ability to manage their weeds may become more difficult,” said Nathan Popiel, agronomic service representative for Syngenta in North Dakota. “Implementing a proactive approach helps combat herbicide resistance before it has the chance to impact an entire farming system.”

To stay ahead of this issue and retain maximum yield potential, Syngenta recommends growers consider the following proactive management techniques.

Manage weeds to prevent seed bank build-up

Weeds are often prolific seed producers, meaning that when weeds emerge and mature, they can develop and contribute seeds back into the soil, creating a large seed bank. Depending on the weed species, seeds may be viable in the seed bank for many years and can emerge when least expected. In terms of herbicide resistance, these are the seeds that are capable of building a resistant population.

Limiting resistance starts with anticipating or recognizing the issue in the field by evaluating the plants that survive a herbicide application, and managing these weeds escapes to prevent an increase in next year’s weed seed bank. Once growers understand the issue, they can take steps to limit weed production by implementing cultural and herbicide management practices, including:

  • Scout fields early and often to manage weed escapes
  • Identify weeds properly to select a product that addresses the weed spectrum in their fields.
  • Clean equipment as machinery can easily spread seed into other fields.
  • Alternate tillage practices at a reasonable interval to change the environment and destroy weeds that can contribute back to the soil seed bank.
  • Do not allow weeds to reproduce by seed or to proliferate vegetatively.
  • Manage weeds by applying an effective burndown herbicide.¬†

Managing the seed bank is a year round activity. As the season progresses and crops mature, growers may begin to let their guard down. Throughout the season, diligent record keeping is also critical for growers to know which herbicides were applied to which fields. Later in the season is still an important time to continue scouting fields, implement agronomic practices and to consider an effective herbicide application.

It’s all about herbicide timing

Many variables impact herbicide application timing, including planting date, soil temperature, and amount of rainfall that occurs both before and after planting. By utilizing a post-emergence treatment, growers ensure overlapping coverage that keeps fields protected season long.

When applying herbicides, ensure good spray coverage of target weeds is obtained for complete control. According to Purdue University, it is essential to apply herbicides at the correct stage of wheat growth to avoid crop injury. When wheat has not yet reached the jointing stage, most herbicides labeled can be safely applied, while others have sensitive application windows that need to be avoided at early wheat growth stages. As the wheat growth stage advances past jointing and approaches the boot stage, herbicide choices become much more limited.

Full, recommended herbicide rates help delay onset of resistance

Help prolong the effectiveness of available herbicides and delay the onset of resistance in wheat fields by changing herbicides with different modes of action often and using the full recommended rates. Full, recommended herbicide rates provide consistently high levels of weed control and eliminate biotypes with a low level of natural resistance. With repeated exposure to reduced rates of herbicides, more biotypes with moderate levels of natural resistance are more likely to survive, therefore increasing the population of resistant weed biotypes.

“Using full rates is important as some of the early stages of resistance will survive half rates of the chemicals. If growers allow the plants treated with half rates to produce seeds, those seeds will carry some of the resistant traits”, said Kirk Howatt, weed scientist at North Dakota State University.  “By keeping the rate of the chemical up to its fully labeled capacity, we are putting our best foot forward to fight resistance.”

Even injured low‐level resistant weeds still have the ability to reproduce, which can allow resistant weed populations to spread rapidly. Weeds can be exposed to “low rates” due to spraying plants larger than those recommended on the label, inadequate coverage of weeds because of size, density and/or crop cover, inaccurate sprayer calibration faulty or ineffective equipment or mixing errors and intended use of below label rates.

Diversify modes of action to reduce the risk of herbicide resistance

In addition to using full rates, it’s also important to diversify herbicide modes of action (MOA). Syngenta offers a robust portfolio of cereal herbicides that include multiple grass, broadleaf and cross‐spectrum options with varying MOAs. The MOA is the way in which the herbicide controls susceptible plants. Over‐reliance on a single herbicide MOA places heavy selection pressure on a weed population and may eventually result in the evolution of resistant weeds. Over time, resistant weeds will multiply and become dominant in a field, resulting in herbicides that are no longer effective for weed control.

“Syngenta understands the challenges cereal growers face and works diligently to provide the solutions that will make cereal growing simpler and more profitable,” said Gigi Arino, cereal herbicides product lead for Syngenta. “We are constantly working to ensure we are providing wheat growers with a comprehensive portfolio of top-performing herbicides.”

This spring, Syngenta recommends controlling weeds by making an effective herbicide application and provides control of  multiple grasses and broadleaf weeds.

Pending registration for the 2017 growing season, Talinor herbicide will be the newest cereals broadleaf herbicide from Syngenta. It will offer selective, post-emergence broadleaf weed control in barley, spring wheat (including durum), and winter wheat. It contains the newest active ingredient in cereal herbicides, bicyclopyrone, combined with bromoxynil, to deliver standalone control of resistant and other tough-to-control broadleaf weeds, including kochia and Russian thistle.

“Talinor will be an important tool for controlling troublesome weeds, particularly those that have become resistant to ALS-inhibitor and auxin herbicides,” said Don Porter, herbicide technical product lead for Syngenta. “Upon registration, Syngenta anticipates Talinor will control more than 50 broadleaf weeds, including kochia, mayweed chamomile, Russian thistle and wild buckwheat. This new broadleaf herbicide will be an ideal fit for growers in North Dakota, and will also be available to growers in the surrounding Northern Plains states, as well as the Pacific Northwest.”

Diversify crops for future harvests

In addition to alternating herbicide MOAs, diversifying crop management practices by planting rotational crops can help minimize the development of herbicide resistance in wheat fields. Crop rotation is a key technique for mitigating the risk of herbicide resistant weeds from ever developing. Different crops are prone to different weed infestations. One benefit of crop rotation is that it allows growers to change the weed population complex in their fields, and therefore change-up the MOA used in a particular field. This minimizes the pressure of different weed species that can develop or may already be present in the fields.

Rotational crop options vary greatly based on geography and economics. Researchers suggest rotating wheat with a broadleaf crop to help minimize the pressure of typically resistant grass weeds in wheat fields. However, good and consistent crop rotation requires an investment. Each crop requires different equipment and a variety of management techniques, but the investment can pay off with healthy crops and effective weed control.

“Waiting for resistant weed populations to appear can cause herbicide resistance to spread faster across an entire field,” Popiel said. “By practicing best management techniques early and continuing them throughout the season, growers can fight the onset of resistance and ensure productive wheat fields with high quality and yield potential at harvest.”


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