Weed control has been a real issue for many corn and soybean fields this fall, especially for soybean fields. In extreme situations, applying a paraquat-containing herbicide as a preharvest aid is necessary to prevent issues with the harvestability of the soybean crop. Growers facing less than desirable weed control at harvest should consider whether they did everything possible to manage the “controllable” herbicide factors. Alternatively, they should reflect on whether adverse environmental conditions or other “out of our control” factors impacted the application.

A few examples of “controllable” herbicide items to check are effective tank mix partners to control targeted weeds, proper adjuvant selection, and nozzle selection. Whereas temperature and humidity are considered “out of our control”. Were we able recognize these conditions and make appropriate changes to spray application to maximize herbicide effectiveness?

While reflecting on 2024, below are a few items to consider when making seed and crop protection decisions for 2025.

Some soybean herbicide trait platforms advertise a 2.7 bu/A advantage over other trait platforms. Remember, if weeds are not controlled, that bu/A advantage quickly disappears. Research from many universities in the U.S. and Canada has documented over many years that if post-herbicide applications do not effectively control weeds, yield reductions may be as high as 20 bu/A. The graph below illustrates the importance of starting free from weeds in soybeans. This research was from Michigan State University.

Figure 1. Soybean yield due to delayed burndown herbicide applications. Burndown applications were made seven days prior to planting at unifoliate to V1 (1-trifoliate) soybean and V3 (3-trifoliate) soybean. (Credit – Michigan State University

Another item to consider is a soybean variety’s field tolerance of herbicides or herbicide sensitivity ratings for Group 14 or PPO, metribuzin, as well as the STS trait that improves tolerances to ALS or Group 2 herbicides. If soybean varieties are known to have high field tolerance to any of the previously mentioned herbicides or groups, we can leverage higher labeled herbicide application rates to minimize the risk of negative soybean response.

Extension weed science specialists from the University of Illinois Urbana-Champaign have suggested rates of 15 oz/A of metribuzin 75DF may be necessary in pre-emerge residual applications to control waterhemp effectively. However, crop responses to metribuzin are a genuine concern at these suggested rates in times of cool, wet weather or even high soil pH. Knowing the soybean variety’s field tolerances to herbicides will be vital in avoiding this concern that could result in replant scenarios given the right environmental conditions.

My last concern is the effectiveness of pre-emerge herbicide residuals. Often, when I receive complaints that residuals did not seem to be effective, I first check to see if they were incorporated with tillage and find out what the rainfall pattern was like for approximately ten days after application. If residuals were incorporated, I ask what the tillage depth might have been. Generally, if incorporation was greater than two inches, the herbicide could have been diluted to an ineffective rate because of too much soil mixing. If tillage is likely not a factor, then too little (not enough for activation) or too much rainfall (leached out of the zone where seeds germinate) could have occurred. Both of these items refer to the water solubility of a herbicide. Generally, a good pre-emerge herbicide plan should include a residual herbicide with a high water solubility and a lower solubility residual herbicide should be included with a post-pass because generally less rainfall occurs later in the season. Remember that the only type of herbicide that truly needs to be incorporated is the “yellows” or Group 3, such as those older formulations of pendimethalin or trifluralin that were significantly degraded when exposed to direct sunlight.