Weed control isn’t as easy as it used to be. With the influx of genetic traits, trait stacking and multiple modes of action—not to mention sites of action—keeping weeds under control is no small task.

Add to these factors the rapid spread of herbicide resistance. Data show that many species, including waterhemp and Palmer amaranth, are becoming resistant to more than one type of herbicide.  If you haven’t experienced weed resistance, consider yourself lucky because chances are you will soon.

Bryan Young, Purdue University weed specialist, says waterhemp, Palmer amaranth and marestail are the big three weeds to watch in Illinois this year, although others pose threats, too. The number of weed species with resistance to more than one herbicide mode of action has increased drastically since 1990—and about 50 weed species have multiple forms of herbicide resistance.

Consider Losses

What’s at risk if growers don’t pay closer attention to managing weed resistance? In addition to reduced profitability and yieldloss, there’s the potential to lose some very valuable crop production land and some equally valuable crop management tools. Implementing diverse weed-management practices will ensure that we’ll be able to hang on to the herbicides—and the acres—we have today.

Although a number of factors determine the frequency of resistance in weed populations, research indicates that relying too heavily on one herbicide (or one single site of action) is the main contributing factor to herbicide resistance.

What herbicides should you use? It all depends on the weeds you’re trying to control. Finding the right match is important. Get it wrong and you could reduce the effectiveness of the products, increase production costs and reduce your yields.

Mode of Action vs. Site of Action

According to the United Soybean Board, the distinction between Mode of Action (MOA) and Site of Action (SOA) relates to “how” the herbicide works vs. “where” it works. For example, one mode of action works to destroy chlorophyll in the presence of light. However, different herbicides can do that by blocking different enzymes, or sites, in the plant’s physiology. Each represents a site of action that takes a different path to achieve the same result.

Target the most troublesome and resistance-prone weeds within a field by using different herbicide SOAs in annual rotations. Growers also should use tank mixtures and sequential applications to delay the onset of resistance by minimizing the selection pressure imposed on those weed populations by a particular herbicide SOA.

Interested in learning more about the MOAs and SOAs in your weed control program? Post a question on the forum or lookup more information on the USB’sSite of Action Lookup Tool.

Why Diversity Matters

Using multiple herbicide SOAs in addition to multiple MOAs is one approach to adding diversity to your weed management strategy.

Diversification reduces direct selection pressure by reducing the number of applications of a single herbicide, especially the number of consecutive applications of a single SOA. Applying two different SOAs in sequence also is a sound strategy because the second application may control escapes from the first application, helping to preserve the effectiveness of both products.

More Tips to Take Action

The United Soybean Board, with support from the Illinois soybean checkoff, recently launched a Take Action on Weeds effort to help keep herbicide-resistant weeds from spreading, protect yields and profits.

Here are other tips to help fight resistance:

  1. Know Your Weeds – Know when they grow, when they pollinate and how to stop them before they go to seed. Find out more about the eleven weeds that pose the biggest resistance threat to your farm.
  1. Manage Your Fields – Combine chemical and nonchemical management tactics to diversify selection pressure on weed populations and minimize spread of resistance genes.
  1. Understand Herbicides – Select the right herbicide based on weed(s) in question. Guessing wrong could reduce efficacy, increase production costs and reduce yields. See which weeds are confirmed in your county on ISA’s WeedMap Tool Kit.

Consider the Bottom Line – The costs of managing weeds after herbicide resistance has evolved are often higher than the cost of a program for reducing the risk of resistance in the first place.

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