In the June 13th issue of the Wisconsin Crop Manager, I discussed herbicide resistance management for giant and common ragweed. This week’s featured herbicide resistance threats are common lambsquarters, Chenopodium album, and horseweed, Conyza canadensis. Weed scientists across the Midwest and Midsouth have identified eleven species of weeds that are of most concern for herbicide resistance because of their ability to compete with crops and to develop resistance to different herbicide sites of action. In 1979, University of Wisconsin weed scientists identified a population of common lambsquarters resistant to atrazine, a photosystem II inhibitor (www.weedscience.org). Lambsquarters populations in Michigan and Ohio have been found resistant to ALS inhibitors. In 2013, University of Wisconsin researchers identified horseweed plants resistant to glyphosate. Ohio and Delaware have horseweed populations resistant to both ALS inhibitors and glyphosate. Resistance to a single site of action has occurred in over twenty states.
Now is the time to start thinking about fall horseweed management. Emergence typically occurs in the early spring and again in the fall. Long-term no-till systems tend to harbor significant horseweed populations. Scouting in mid to late summer to locate any escapes from spring herbicide applications is important for herbicide resistance management and to decide whether to switch to a horseweed management program that includes both spring and fall control measures. Fall herbicide applications can help to reduce horseweed populations in problem fields. Also, if dandelion is an issue, then there are two reasons to consider fall herbicide applications. University of Wisconsin researchers found that fall dandelion control is best prior to next year’s corn crop. Their results are available as a slide presentation. Kevin Bradley at the University of Missouri has a video, less than five minutes, discussing the importance of application timing for horseweed control.
Common lambsquarters can be difficult to manage because of an early and sustained emergence period, long seed persistence, and competitive ability. A 50 percent reduction of seed in the soil seedbank requires about 12 years and 78 years for a 99 percent reduction. Management goals should include: starting with a clean field, using a pre-emergence residual herbicide, scouting, and applying a post-emergence herbicide if necessary. For specific management recommendations, please consult the TakeAction fact sheet.
Depending upon the herbicide, injury can occur after a pre-emergence application when corn is germinating in cool, wet soils. This year, if corn was planted in mid-April then you may observe some injury. However, it is important to remember that other environmental factors can mimic herbicide injury symptoms such as corn emerging in crusted or compacted soil. For this spring, WCWS has a re-designed online diagnostic tool, available at http://wcws.cals.wisc.edu/herbicide-injury-diagnostic-tool or from the main page, go to ‘Resources’ and then to ‘Tools’. The diagnostic tool asks three basic questions 1) When do injury symptoms appear? 2) Are both broadleaves and grasses affected or just one group? and 3) What are the symptoms and where do they occur? The original web-based tool was developed by Tim Trower and Chris Boerboom to accompany a handy two-page guide. The following changes were made to the new version:
Each page shows your previous answers.
A ‘Start over’ button is located at the bottom of each page.
Simplified guides to symptoms that mimic herbicide injury during and after emergence are included on each mode-of-action page (Fig. 2).
Photo galleries for both corn and soybean injury symptoms are located on the same page (Fig. 3).
Figure 1. Herbicide site-of-action groups, chemical families, active ingredients, and product examples for the seedling shoot growth inhibitor mode-of-action. Specific sections of the larger TakeAction chart are on each mode-of-action page.
Figure 2. Mimics of herbicide injury to corn during or at emergence.
Figure 3. Example of a photo gallery for corn and soybean herbicide injury symptoms.
For pre-emergence applications in corn, the seedling shoot growth inhibitors, particularly the chloroacetamides, may injure seedlings when soils are cool and wet. Injury will not always be apparent aboveground. For example, corn plants with seedling root growth inhibitor damage will display clubbed root tips and grasses will be more affected than broadleaves. To get an idea of injury risk, check out the herbicide tables in “Pest Management in Wisconsin Field Crops” available in pdf and print formats at Cooperative Extension’s Learning Store.
Researchers from the Australian Herbicide Resistance Initiative report on the first confirmed case of a weed, Indian goosegrass (Eleusine indica), resistant to three non-selective herbicides, glufosinate, glyphosate, and paraquat along with several ACCase inhibitor herbicides (Jalaludin et al., 2014). The goosegrass population was initially reported in Malaysia by a vegetable farmer and a planter from an oil palm nursery (Jalaludin et al. 2010).
In the United States, E. indica occurs in forty five of the fifty states. In Wisconsin, populations have been identified in the following counties: Columbia, Dane, Grant, Lafayette, Rock, Kenosha, and Milwaukee. The first documented case of herbicide resistant E. indica was from North Carolina in 1973 and the latest confirmation was in 2011 (Heap, 2015).
The amount of glufosinate to kill half of the tested resistant plants was equivalent to applying 40 fl oz per acre (e.g. Liberty 280 SL). The maximum rate for the season in corn is 36 fl oz per acre. The next generation of plants from the resistant population required 657 fl oz per acre of glyphosate (i.e. Roundup Powermax) to kill half of the tested population. These plants also were twice as resistant to paraquat compared to the susceptible plants. Half of the resistant population survived applications of the ACCase inhibitors- haloxyfop-P-methyl (e.g. Verdict) and fluazifop-P-butyl (e.g. Fusilade).
Heap, I. 2015 The International Survey of Herbicide Resistant Weeds. Online. Available at www.weedscience.org
Jalaludin, A.; Ngim, J.; Baki, BB.; Zazali, A. 2010 “Preliminary findings of potentially resistant goosegrass (Eleusine indica) to glufosinate-ammonium in Malaysia.” Weed Biology and Management 10: 256-260.
Jalaludin, A.; Yu, Q.; Powles, S.B. 2014 “Multiple resistance across glufosinate, glyphosate, paraquat, and ACCase-inhibiting herbicides in an Eleusine indica population” Weed Research 55: 82-89.