Many people have high expectations for the new herbicide resistant traits, but it’s important to remember that these products are not the next “silver bullet” for successful weed management. A group of weed scientists in the eastern United States evaluated glyphosate resistant (GR) waterhemp and Palmer amaranth control with herbicide programs designed for 2,4-D, dicamba and Group 27 resistant soybean (Meyer et al. 2015). I will focus on waterhemp, but both pigweed species responded similarly to the management systems. The findings reinforce the need for diversified herbicide programs and timely herbicide application, regardless of the trait used.
The experiment included 25 herbicide treatments and was conducted at six locations. With such large experiments it is difficult to make concise, take-home messages. However, key conclusions from the study include:
- New traits provided advantages compared to existing technologies;
- Sequential programs with a PRE followed by an early postemergence application (EPOST) were more consistent than a PRE with late postemergence application (LPOST);
- Effectiveness of 2,4-D (Enlist) and dicamba (RR Xtend) based systems on GR waterhemp control was similar;
- Group 4 based systems were similar to Group 27 systems at EPOST, but Group 4 systems were more consistent at LPOST;
- Regardless of the trait and accompanying herbicide, an effective PRE program was essential for successful GR waterhemp management.
Waterhemp control three weeks after the POST applications is shown in Figure 1. While there were differences among the PRE treatments (Trts 2-6), no treatment provided consistent waterhemp control. There were no differences in waterhemp control among the EPOST treatments (Trts 7-16), and there was little difference in the consistency of treatments that included a herbicide active on emerged GR waterhemp (2,4-D, dicamba, glufosinate, fomesafen and mesotrione). Trt 7 relied on glyphosate for post activity on GR waterhemp, thus it was less consistent than the other EPOST treatments.
There were significant differences in performance of the LPOST treatments, and all were less consistent than the EPOST treatments due to the presence of large waterhemp at application. The importance of an integrated system is demonstrated with treatments 18 and 19. Both treatments included the same LPOST herbicides (S-metolachlor, glyphosate and dicamba). The difference in consistency between the treatments is related to their respective PRE treatments. Trt 18 included PRE Trt 2, while Trt 19 used PRE Trt 4. Trt 2 was less effective than Trt 4, resulting in more and likely larger waterhemp at the EPOST application.
In my opinion, the take home message of this study is that while the new herbicide resistant traits provide opportunities for improved waterhemp management, the herbicide system used with these traits is more important than the individual trait or their respective herbicide. This study emphasizes the importance of using both effective PRE and timely POST applications to manage waterhemp, regardless of the herbicide or trait.
Meyer, C. J., J.K. Norsworthy, B.G. Young, L.E. Steckel, K.W. Bradley, W.G. Johnson, M.M. Loux, V. M. Davis, G.R. Kruger, M.T. Bararpour, J. T. Ikley, D. J. Spaunhorst, and T.R. Butts. 2015. Herbicide program approaches for managing glyphosate resistant Palmer amaranth and waterhemp in future soybean-trait technologies. Weed Technol. 29:716-729.