Soybean herbicide application decisions are evolving daily in 2020. The Extend system has offered an excellent tool in managing some important glyphosate resistant weed species. The Enlist system offers improved control on many of these weeds as well, however kochia is a notable exception. Kochia management with 2,4-D is not a good option. The fortunate thing with Enlist is that is comes with both RoundUp and Liberty traits. The question then becomes which of these products are OK to mix during a single application. There have been several products made in past years that have both RoundUp and Liberty premixed, however, many customers were not happy with performance and there is currently no such product in the U.S. The issue is concern over antagonism between the two chemistries.

The 2020 growing season will be the third year of testing different combinations of RoundUp, Liberty, and Enlist at the CREC. After reviewing the results, the one thing that is clear is that the results are not. Most of the time combining the products results in control that matches what either product does alone. In other words, there has usually been no increase in control by combining the products. There have been several instances where combining the products has reduced control, compared to separate applications. Notably, grass control was reduced when Liberty was added to RoundUp. A similar phenomenon was noted in 2018 in common lambsquarters, but not in 2019. An important management note here is that the Liberty appears to antagonize the RoundUp more so than the other way around.

How does this change when we talk about glyphosate-resistant species?

If the products were combined, conditions need to be optimized for Liberty efficacy. This means 20 GPA volume and finer droplet size. To be clear, this needs to be viewed as a Liberty application. Figure 1 shows some of the 2020 results of the combinations. In this case, the control of kochia followed what the Liberty application was capable of (applying to kochia 2” or less), which was about 95% control. About 40% of the plants at this site were resistant to RoundUp.

What is the recommended practice?

Split applications are best to avoid any potential antagonistic problems. Antagonism is not as severe as initially thought, but can be present, and can ultimately lead to future weed resistance problems. If products were combined it is critical to apply to optimize Liberty activity.    

A snapshot of trial results at the NDSU Carrington Research Extension Center comparing Liberty, RoundUp and a combination of the two.

Mike Ostlie, Ph. D.
Mike.Ostlie@ndsu.edu
Research Agronomist