Standard weed control in glyphosate-tolerant (GT) crops has become a headache for growers in the United States. Many are wondering why glyphosate, an herbicide that has always been extremely effective, is beginning to fail them.

"This is becoming a trend more so in the United States than in the rest of the world" says Ian Heap, director of the International Survey of Herbicide Resistant Weeds. "More GT crops are planted in the U.S. than anywhere else, and since it is possible to use nothing but glyphosate on these crops, many growers are doing just that."

Growers may contemplate why they should switch to other herbicide modes of action when they've had so much success with glyphosate in the past. The answer is simple. Over time, weeds develop a tolerance to any herbicide that is applied continuously and eventually become immune to it. Once this occurs, it is impossible to reverse.

Without new products in the pipeline, the loss of glyphosate technology would be costly to the farming industry. However, glyphosate weed resistance can be delayed if problem weeds are recognized in a timely manner, and proper measures are taken. As the list of glyphosate-resistant weeds continues to grow, early detection of these intruders is becoming more critical than ever. Here's what to do to help identify potentially tolerant or resistant weeds before they become a major economic problem.

Growers should closely monitor weed species on which glyphosate-based herbicide performance is already variable, beginning with waterhemp, horseweed, giant ragweed, lambsquarters and Palmer pigweed. The same holds true for any species that has already been identified as resistant or tolerant to glyphosate in the United States. Populations that have been confirmed resistant to other herbicide modes of action should also be monitored closely. Waterhemp is a prime example of this, where resistance to several modes of action and variable tolerance to glyphosate are already prevalent across much of the Corn Belt.

1. Look for stubborn weeds, unexpected flourishes or flushes and species that were not a serious problem before. Glyphosate resistance is genetically inherited, and usually develops gradually. It is also important to remember that some weed populations may be genetically programmed to be more resistant to a certain herbicide than others. And, though there are weeds resistant to multiple modes of action, it does not mean they are automatically resistant to all MOAs.
   
   "Resistance levels will vary from population to population," Heap says. "Because of this, it is essential that growers pay close attention to weeds that were once easily controlled with one herbicide, but are evolving to become more difficult to manage with that same herbicide. We take those types of cases very seriously."
   
   That doesn't mean that other cases are simply set aside. Instead, they are red flags for researchers because this shows that glyphosate resistance is actually developing due to herbicide overuse rather than already being present when the herbicide is applied for the very first time.
    
2. If resistance is indeed developing in a field, applying more glyphosate only accelerates the problem. The most common misconception seen by researchers is that upping the rate of glyphosate will produce better results. While that may work once or twice at most, it will only cause weeds to develop a higher tolerance in the long run. Instead, Syngenta recommends the Resistance Fighter code for effective management 2-1-2: no more than two applications of glyphosate on one field over a two-year period. This and other tips on management and delaying resistance can be found at www.resistancefighter.com.
    
3. You can't tell if a weed is glyphosate-resistant just by looking at it. There are many factors that can hinder the performance of a herbicide. Environmental conditions, insect damage or spraying errors can all be factors. However, if a weed persists, check it out. Collect a few random samples of the weed and its seeds and call the county agent or crop consultant about testing.
    
4. Test results, whether positive or negative, can help keep control costs (and frustration) in check. Nothing negative will derive from testing a suspected weed. Controlled greenhouse studies will verify if a problem does indeed exist, measure the level of tolerance and indicate whether resistance traits are passed on genetically. And, if it turns out that resistance is not an issue, researchers can usually determine the actual problem. Pinpointing the issue can avoid wasting money on the wrong rate or the wrong herbicide with every extra pass next season.

In order to preserve glyphosate, growers should also adhere to the following best management practices in addition to following the 2-1-2 program: rotate glyphosate-tolerant crops with conventional crops, use alternative modes of action and employ burndown, pre-emergence and post-emergence herbicides in weed control programs. Syngenta recommends using Gramoxone Inteon(TM) mixed with 2-4,D for burndown. In addition, residual herbicides, such as LUMAX® and Lexar®, are recommended for pre-emergence or early post-emergence weed control in corn. Sequence® or Boundary® are residual herbicides suggested for soybeans. The most important rule to follow is to avoid continuous glyphosate applications in order to delay the further development of resistance.

For more information on the 2-1-2 program and resistance management, visit www.resistancefighter.com.

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GRAMOXONE Inteon, Lexar and LUMAX are Restricted Use Pesticides.