During the 2018 field season weed samples were submitted to our lab for confirmation of resistance to various herbicides. Rapid DNA assays were deployed whenever possible to provide a report to the individual that submitted the sample within a two-week period to help guide any possible management strategies for the current field season. If a rapid assay was not available or could not be developed, we propagated whole plants in the greenhouse and sprayed with the herbicide in question for resistance determination. New target site mutations that allow for resistance to PPO-inhibiting herbicides in both waterhemp and Palmer amaranth have been identified and reported by our lab over the past year. As much as we can determine at this point, these new resistance mutations are a similar threat as the first, and most common, resistance mutation for PPO-inhibiting herbicides. In other words, these new mutations won’t be anything easier for field management of these weeds; however, management shouldn’t be further complicated too much by these other mutations. We also discovered the presence of metabolism-based waterhemp resistance to ALS-inhibiting herbicides. This has previously been reported in other states such as Illinois, but has never previously been confirmed in Indiana.
The primary weeds submitted in 2018 were waterhemp and giant ragweed. In both instances, the main concern has been suspected resistance to PPO-inhibiting herbicides (e.g. Cobra, Flexstar). Most of the waterhemp samples were confirmed as resistant to PPO-inhibiting herbicides, with additional resistance to glyphosate and ALS-inhibiting herbicides (e.g. Firstrate, Classic). No giant ragweed samples have been confirmed as PPO-resistant to date after two years of testing suspect populations. We can only assume that low herbicide application rates, excessive weed heights, or poor application methods played a role in the lack of weed control with these herbicides. There has been a recent increase in the frequency of giant ragweed populations being confirmed with resistance to glyphosate.
The primary areas of Indiana with confirmed glyphosate- and PPO-resistant waterhemp that continue to expand over years are found in the southwest, northwest, and central regions. Furthermore, our findings would suggest that PPO-resistance in waterhemp will soon be found on any waterhemp that is glyphosate-resistant. This continued increase in multiple resistance in waterhemp has resulted in soybean farmers electing to adopt the Liberty Link (Liberty) or Xtend (dicamba) soybean systems so they have an effective post-emergence herbicide option.
Our effort concerning dicamba stewardship and the use of Xtend soybean was first initiated at the beginning of the 2017 field season that coincided with the first year of commercial use for registered dicamba herbicides in Xtend soybean. Field demonstration trials on weed management with Xtend soybeans compared to other soybean traits/varieties were conducted in 2018 at four Purdue Ag Centers. These demonstrations included Roundup Ready 2 Yield, Roundup Ready Xtend, and Liberty Link soybeans with various herbicide programs that highlight the methods necessary to improve management of problematic, herbicide-resistant weeds. In total, approximately 1,400 individuals who attended training events or agronomy field days viewed these trials. The findings from these demonstrations were incorporated into extension presentations for use in the 2019 winter grower meetings.
An analysis of the weather conditions and label restrictions for performing dicamba applications in Xtend soybeans in 2017 and 2018 was also conducted. In 2017 the focus was on the number of available "spray" hours to perform dicamba applications. In 2018 an evaluation of phone app tools to assist growers in determining if weather conditions would permit a legal dicamba application was also conducted. This research highlighted the importance of measuring the actual weather conditions at the application site over the use of phone app tools that can only predict the conditions at the field site based on weather models. The success of the phone apps to predict a temperature inversion ranged from 84% for the “Engenia Spray Tool” down to 39% for “Pocket Spray Smart”. Analysis of the available "spray" hours were similar for 2017 and 2018, with slightly more hours in the month of June in 2018. However, the number of hours available to spray dicamba in Xtend soybean continues to be one of the greatest limiting factors for successful implementation of weed management and stewardship of herbicide off-target movement in Xtend soybean.
Outcomes:
Herbicide resistance in waterhemp, and to a lesser extent giant ragweed, continues to expand in Indiana and represents the greatest concern from clientele over the past several years. We continue to expand our knowledge as we characterize the specific resistance mechanisms in this species. The discovery of new mutations in waterhemp that allow for resistance to PPO- and ALS-inhibiting herbicides increases our ability to provide sound management recommendations to growers. So why does it matter which resistance mechanism is present in a weed if the plant is still considered to be resistant to the herbicide? Our research demonstrates that some forms of low-level resistance can be overcome by applying the herbicide in a more favorable manner that allows for a high dose of the herbicide on the smallest size weed. More specifically, applying Classic herbicide pre-emergence can control some forms of ALS-resistant waterhemp and yet be mostly ineffective applied post-emergence.
Overall, our research does provide some opportunities to improve weed management, but also highlights the fact that we are not staying ahead of weeds in terms of management and resistance evolution. We desperately need to diversify our management tactics to include non-chemical or cultural practices along with diverse, effective herbicide options that still remain in non-GMO or herbicide-resistant soybean systems.
Management of Xtend soybean was challenging in both 2017 and 2018 with dramatically greater off-target movement of dicamba than anyone predicted. In general, weed management within the Xtend soybean technology was effective, but we need to make significant improvements so that a second application of dicamba is not necessary for complete weed control since this will lead to the evolution of dicamba-resistant weeds and an increased risk of off-target movement of dicamba. Applicators must measure the weather conditions in the field to determine if they allow for a safe application of dicamba.