During the 2017 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. The primary weeds submitted in 2017 were waterhemp and giant ragweed. In both instances, the main concern has been suspected resistance to PPO-inhibiting herbicides. Most of the waterhemp samples were confirmed as resistant to PPO-inhibiting herbicides (e.g. Cobra, Flexstar). No giant ragweed samples have been confirmed as PPO-resistant to date. 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 (Figure 1). 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 or Xtend soybean systems so they have an effective postemergence herbicide option.

This project was initiated at the beginning of the 2017 field season which coincided with the first year of commercial use for registered dicamba herbicides in Xtend soybean. Field demonstration trials on weed management were conducted in 2017 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 have been incorporated into extension presentations for use this winter at grower meetings.

An analysis of the weather conditions and label restrictions for performing dicamba applications in Xtend soybeans in 2017 was also conducted. The number of available spray hours for legal applications of dicamba in Xtend soybean were very limited. For example, in June 2017 the weather in Lafayette, IN would have provided fewer than 200 hours for applications of dicamba. Under the same weather patterns, the revised labels that will be enforced in 2018 will provide only 44 hours of available spray time. The amount of time to make safe, legal applications of dicamba in Xtend soybean and still provide timely weed management remains a major concern as we move into the 2018 growing season.