Are You PRE-pared for the Soybean (and Waterhemp) Growing Season?

4 min read

by Rodrigo Werle (UW-Madison Extension Cropping Systems Weed Scientist)
and Shawn Conley (UW-Madison Extension Soybean and Small Grain Specialist)

Spring has arrived rather quickly in Wisconsin and the planting season is right around the corner (or perhaps it has already started for some growers). Before getting their soybeans in the ground, farmers should make sure they have solid weed management plans for the growing season, particularly if waterhemp is present in their operations.

According to our 2018 SURVEY, several Wisconsin soybean fields get only treated with a one-pass POST-emergence herbicide program (keep in mind that pre-plant tillage is a common practice adopted in Wisconsin cropping systems and that helps farmers “start clean”). Because of the widespread occurrence of waterhemp populations resistant to glyphosate and/or other POST-emergence herbicides (e.g., ALS- and PPO-inhibitors; see article: “Herbicide Resistance in Wisconsin: An Overview”) combined with its extended emergence window (mid-May through July), a one-pass POST-emergence program in soybeans is likely not a viable strategy in fields infested with waterhemp.

Moreover, research conducted across the Midwest has demonstrated the importance of keeping the soybean crop weed-free from establishment through the V3 growth stage (3rd trifoliate). Weeds emerging after the V3 growth stage will likely not impact soybean yield; however, they should still be proactively managed to prevent them from reproducing and replenishing the seedbank (remember “no seed, no weed”). Thus, effective PRE-emergence herbicides can help farmers maintain their fields weed-free during initial establishment of the crop (= achieve full yield potential) and also reduce the selection pressure on POST-emergence herbicides (due to fewer weeds to be controlled POST-emergence), helping on the fight against herbicide resistance.

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Several PRE-emergence herbicides are available for soybeans. Most of them should be applied pre-plant up to 3 days after planting (see product label). To maximize their residual activity in-season, PRE-emergence herbicides should be sprayed at or shortly after planting. PRE-emergence herbicides need moisture for incorporation and activation in the soil. If there are established weeds at the time of PRE-emergence application and no additional pre-plant field cultivation will take place, it’s important to have an effective burndown herbicide in the tank-mix. A PRE-emergence herbicide containing 2 or more effective sites of action (SOA) will likely provide control of a wider range of weed species when compared to the use of a single SOA. Using multiple effective SOA during each pass is also a proactive strategy for herbicide resistance management.

In 2018 we conducted a study to evaluate and demonstrate the effectiveness of multiple PRE-emergence soybean herbicides (see results: 2018 UW Waterhemp Challenge: Comparison of Soil Residual Herbicides). 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. This study will be replicated at multiple locations during the 2019 growing season. Stay tuned for our field days!

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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 use the labeled rate for their soils. The residual activity of a PRE-emergence herbicides is dependent on the rates applied; cutting rates is not a recommended strategy when attempting to manage troublesome weeds like waterhemp.

It’s important to note that under cool and wet conditions during crop emergence, PRE-emergence herbicides containing metribuzin (Group 5), PPO-inhibitors (Group 14; e.g.: flumioxazin, saflufenacil, sulfentrazone) and VLCFA-inhibitors (Group 15; e.g.: acetochlor, dimetenamid-P, pyroxasulfone and S-metolachlor) may cause some early-season crop injury, particularly in lighter soils with low OM and/or higher pH. Our preliminary research in Nebraska has demonstrated that early-season crop injury caused by metribuzin (group 5) and sulfentrazone (group 14) did not lead to yield reduction (soybeans are indeed very resilient). Moreover, the benefit of an early-season weed-free field outweighs the concerns of early-season crop injury (assuming a herbicide is applied according to the label and no significant stand reduction is observed).

To visualize the various types of injury potentially caused by different SOA herbicides, check our Herbicide Site of Action Key for Crop Injury Symptoms

For assistance with herbicide selection:

Always read, understand and follow the pesticide label.