One important issue facing many Nebraska farms this growing season is the challenge of dealing with herbicide resistant weeds. While not a new issue, herbicide resistance in weeds has increasingly become a struggle for many parts of the United States and in Nebraska. The discovery of multiple weed species with resistance to glyphosate, or the active ingredient in Roundup, has challenged weed control in many fields.
Fig 1. Common waterhemp in soybean field
No one weed species better shows these challenges than common waterhemp (Figure 1), which was first confirmed to have developed resistance to glyphosate in Nebraska in 2013. In addition, separate populations of common waterhemp have been found that are resistant to 2,4-D, atrazine, ALS inhibitors, and HPPD inhibitors, with some populations expressing resistance to multiple herbicide modes of action. Given common waterhemp’s quick growth, long germination period, and the ability to cross pollinate, resistance issues will only intensify.
With weed resistance not going away, the question then becomes, what can be done to slow its spread? A recent study conducted by the USDA/ARS and the University of Illinois drew some interesting conclusions on dealing with weed resistance. This multi-year study identified the relationship between the development of glyphosate resistant waterhemp in 105 Illinois fields and 66 different variables related to weed management and the environment.
One conclusion of the study was that management practices were the single biggest factor in the development of glyphosate resistant waterhemp. Researchers were more likely to find resistance in fields where glyphosate was used more frequently and where fewer different herbicides or modes of action were used. Neither of these should come as any surprise, but this shows that using best management practices on your own fields can help slow weed resistance. In fact, the proximity of a field to other herbicide resistant fields was not an important predictor of resistance. Often, weed problems are blamed on neighboring weedy fields, but if appropriate diversified weed management strategies are used, it should be possible to effectively control weed populations.
One other interesting conclusion was that fields with 2.5 modes of action per herbicide application were 83 times less likely to develop glyphosate resistant waterhemp within 4-6 years than fields with only 1.5 modes of action per application. This shows that the use of effective tank mixes with more than one mode of action at each herbicide application is critical to slowing resistance. In short, relying on a single herbicide mode of action for post-emergence control is likely to eventually lead to herbicide resistance, even if pre-emergence herbicides are applied.
Growers facing weed resistance issues have lots of resources available to them, including UNL’s 2015 Guide for Weed Management, which contains 300 pages of information covering a wide variety of topics related to weed, insect, and disease management. This publication is available for purchase at your local Extension office or online at UNL’s Marketplace.
Another excellent resource is the Take Action program, which offers resources to help combat the spread of herbicide resistance nationwide. Resources are available online at takeactiononweeds.com, with the option for growers and consultants to request print versions of the herbicide site of action chart (Figure 2) and information on resistant weeds.
Another excellent resource at the site is the online site of action lookup tool, which is available as a mobile web site for use with smart phones and tablets.
To learn more contact me at the Nebraska Extension in Colfax County office by calling 402-352-3821, emailing , or by visiting croptechcafe.org.
