Herbicide-resistant weeds are developing resistance to multiple herbicides and spreading at an accelerating rate. Herbicide-resistant weed species can become a serious problem when a single herbicide or when multiple herbicides with similar modes of action are used repeatedly, as this puts pressure on species to develop resistance. 

However, herbicide resistance is not a new phenomenon; weed scientists have been talking about the problem for decades.

Herbicide-resistant weeds have become a tenacious problem for growers. Without weed control, crop production is negatively affected by reduced yields, leading to decreased profits. According to the Weed Control Guide from the University of Illinois, populated from joint research between  the University of Illinois, The Ohio State University Extension and the Purdue University Extension, losses in U.S. crops due to uncontrolled weeds exceed $8 billion annually, even despite large expenditures on weed control. Uncontrolled weeds compete with crops for water, nutrients and sunlight, and have the potential to harbor insects and diseases, which can infest fields and negatively affect future crops.

According to the Iowa State University Extension, more herbicide-resistant fields are being found in Iowa, including more fields with weeds, mostly waterhemp, that are showing resistance to glyphosate, PPO and HPPD-inhibitor herbicides. PPO resistance is showing up in conventional soybean fields. Waterhemp populations that are resistant to post-emergence applications of HPPD-inhibiting herbicides have been confirmed in Iowa, Illinois and Nebraska.

Specifically within the herbicide resistance issue, the glyphosate-resistant weed population is growing rapidly. For more than 40 years, glyphosate has been an effective weed management tool for soybean growers. However, its popularity and convenience has led to overuse, and by 2014, 15 weed species had developed resistance to glyphosate in the U.S. 

What can you do?

Because of the severity of this issue, growers must create a plan of action to build diversity into their weed management programs to prevent further development of herbicide-resistant weeds. For optimal results in soybean fields, an herbicide with two different modes of action that target the same species should be used to help combat resistance and effectively manage weeds.

It is recommended that growers defend their young soybean plants against weed competition by applying a pre-emergence residual herbicide that contains multiple modes of action. Doing so can protect yield with up to five weeks of residual control of highly competitive broadleaf weeds and grass, allowing young soybean plants to establish their full yield potential in a weed-free environment.

According to the Iowa State University Extension, significant benefits can come from using a pre-emergence herbicide, including better time management in the spring, a wider window of opportunity to make post- treatments, enhanced environmental stewardship and improved profitability. Additionally, pre-emergence herbicides with multiple modes of action reduce selection pressure on the weed community, delaying the development of herbicide-resistant weeds in soybeans.

Syngenta solutions

To effectively fight herbicide resistance, growers must first identify the unique weed spectrum in each field. Whether planting glyphosate-tolerant (GT), conventional or LibertyLink® soybeans, growers should apply herbicides that target difficult-to-control weeds with confirmed resistance or the potential to develop resistance. Utilizing a burndown application like Gramoxone® SL 2.0, where dense weed populations or scattered hard-to-control weeds exist will help reduce early-season weed competition. For continued pre-emergence control, Syngenta offers Boundary® and Prefix® herbicides. Each consists of two leading active ingredients and two non-glyphosate modes of action, to provide fast and effective weed control.

For continued weed control in GT soybeans and for maximum herbicide diversity that relies on four modes of action, follow Boundary with Flexstar® GT 3.5 herbicide applied post-emergence. For control of glyphosate-resistant weeds and to protect the PPO-inhibiting class of herbicides, Flexstar GT 3.5 should be applied post-emergence to weeds 2 to 4 inches tall.

In LibertyLink soybeans, a pre-emergence application of Prefix followed by Liberty® herbicide plus Dual Magnum® herbicide will provide maximum weed control, while protecting the longevity of Liberty and sustaining a valuable weed management tool.

Boundary, Dual Magnum, Flexstar GT 3.5, Gramoxone SL 2.0 and Prefix help sustain both GT and LibertyLink soybean technologies. Offering five different modes of action to control weeds, these brands, when combined with cultural practices such as crop rotation and tillage, help growers reduce the weed seed bank, maximize diversity and fight herbicide resistance.

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