2018 UW Waterhemp Challenge: Comparison of Soil Residual Soybean Herbicides

by Rodrigo Werle (UW-Madison Extension Cropping Systems Weed Scientist), Dan Smith (UW NPM Southwest Wisconsin Regional Specialist) and Richard Proost (UW NPM Sr. Outreach Specialist)

Waterhemp management in soybeans was a challenge for several farmers in 2018. As we finalize harvesting our crops, it’s important to properly map the fields infested with waterhemp and start developing an effective management plan for 2019.

The use of PRE-emergence herbicides is a foundation for waterhemp control in soybeans. According to our SURVEY conducted earlier this year, the use of a single POST herbicide pass is still a common weed control strategy for several farmers in Wisconsin. Unfortunately, as many learned this year, this strategy will likely not work if glyphosate-resistant waterhemp is present in the field.

In 2018 we conducted a study to evaluate and demonstrate the effectiveness of multiple PRE-emergence soybean herbicides. This was a joint effort between the UW-Madison Nutrient and Pest Management Program (NPM; Dan Smith and Richard Proost) and my team (WiscWeeds; Maxwel Oliveira, Victor Ribeiro, Sarah Striegel, Nikola Arsenijevic, and Ryan DeWerff). The study was conducted at UW Lancaster Ag Research Station, in Lancaster, Grant County, southwest WI in a field with natural and significant waterhemp and common lambsquarters infestation. Treatments consisted of PRE-emergence soybean herbicides containing one, two and three different active ingredients and/or sites of action. Herbicides were sprayed the day after soybeans were planted. Since we wanted to evaluate the residual activity of the PRE-emergence herbicide treatments throughout the season, no POST-emergence herbicides were sprayed to the research plots.

Our intent was not to promote one product versus another, instead, demonstrate the value of using an effective PRE-emergence herbicide program. Three well-attended and well-received plot tours were held at the research site in the summer. Per request of those who attended and also from several of those who could not attend the plot tours, we decided to make our 2018 findings available.

To download the 2018 Preliminary Report of the “UW Waterhemp Challenge: Comparison of Soil Residual Herbicides” (PDF file) click HERE.

While these results should be taken with a grain of salt (only one year of data), they clearly indicate the value of PRE-emergence herbicides and the programs that don’t work. Moreover, the herbicide rates used in the study are the ones recommended by our industry colleagues and supported by us for a typical Wisconsin Silt Loam soil, thus, valuable information for decision-makers. In this publication we also include site and study information, pictures of plots, and the equivalent rate of single active ingredient products in the premixes evaluated (herbicides with multiple active ingredients).

When selecting a PRE-emergence herbicide program, we challenge agronomists and farmers to balance efficacy (using our results and their experience), product cost, and rotation restrictions. We also encourage farmers to compare their use rates with the ones used in our study (recommended by industry representatives and respective product labels), assuming Silt Loam soils. The residual activity of a PRE-emergence herbicide is dependent on the rates applied; cutting rates is not a recommended strategy when attempting to manage troublesome weeds like waterhemp.

This same study will be replicated in at least two Wisconsin locations in 2019. Stay tuned for additional research findings related to our waterhemp control studies and also plot tour opportunities in 2019!

Key Take-Home from the “2018 Comparison of Soil Residual Soybean Herbicides Study”:

  • Several PRE-emergence soybean herbicides evaluated provided good levels of waterhemp and lambsquarters control. The onset of waterhemp emergence in the research site was noticed in the first week of June. Because of excessive rainfall in the spring, soybean planting was delayed and happened on 05/24/2018 at the research site; thus, the application of our PRE-emergence treatments (05/25/2018) matched the time waterhemp started to emerge, explaining the overall satisfactory level of weed control observed in most treatments (perfect timing!).

  • Group 2 herbicides (e.g., Pursuit, Classic, First Rate) applied alone were effective on lambsquarters but NOT on waterhemp. The use of imazethapyr (e.g., Pursuit, Extreme, Thundermaster), which is a common practice in Wisconsin, did not provide satisfactory control of waterhemp. When using imazethapyr (which is an effective herbicide for control of several weed species) as part of the PRE-emergence herbicide program for waterhemp control, the tank mixture with or selection of herbicides that contain other effective active ingredient(s) is recommended.

  • PRE-emergence herbicide programs containing multiple effective sites of action are recommended to broaden weed control spectrum and to lower selection for additional herbicide resistance.

Always read, understand and follow the pesticide label.

Acknowledgements: we would like to thank Mimi Broeske, UW-NPM Senior Editor, for developing the publication layout. Thanks to Doug Wiedenbeck and the UW Lancaster Ag Research Station staff for their support. This study was partially funded by the Wisconsin Soybean Marketing Board.