Parts of southern Wisconsin have received less than 0.25 inches of rainfall since crops were planted in early May. As a result, many growers are asking the same questions: What is happening to my PRE herbicides? Are they still there? And do I need to adjust my weed management plans?
I was on the phone with a farmer this morning discussing this exact situation. He told me that nearly 40 years of farming experience has taught him to always have a Plan A, B, C, and D. In a year like this, that advice certainly applies.
Are My PRE Herbicides Still There?
The good news is that unless the topsoil has physically moved due to wind erosion, most PRE herbicides are likely still sitting on or near the soil surface waiting for rainfall.
Many growers are familiar with the rule of thumb that approximately 0.5 inch of rainfall is needed to activate residual herbicides. Under the conditions we are experiencing this year, however, activation is about more than simply moving the herbicide into the soil profile.
We need enough moisture to wet the upper few inches of soil where weed seeds are germinating. The goal is not only to move the herbicide into the soil but also to create an environment where germinating weeds can absorb the herbicide. In many of the driest areas of southern Wisconsin, a rainfall event greater than 1 inch may be needed to adequately recharge the upper soil profile. The reality is that crops in these areas desperately need that amount of rainfall as well.
A common question is whether herbicides are degrading while they sit on the soil surface. The answer depends on the herbicide, but many of the PRE residual herbicides commonly used in Wisconsin corn and soybean production are relatively stable and can still provide meaningful control once adequate rainfall occurs. To persist under hot, dry, and sunny conditions, herbicides benefit from having low vapor pressure, which reduces losses due to volatilization, and high photostability, which reduces breakdown from sunlight exposure. Fortunately, most PRE herbicides used in Wisconsin corn and soybean production have relatively low volatility and moderate to high photostability, allowing them to remain effective while awaiting activation by rainfall.
Don’t Wait on Waterhemp
The bigger concern right now is not necessarily herbicide degradation but the weeds that are escaping while we wait for rain.
Fields should be scouted frequently, especially where difficult-to-control species such as waterhemp are emerging. If weeds are present and growing, do not wait indefinitely for rainfall to activate residual herbicides.
Timely POST applications remain critical. Waterhemp can grow rapidly under warm conditions, and every day of delay can make control more difficult and reduce the effectiveness of POST herbicides.
What About “Reach-Back” Activity?
Some herbicides, such as mesotrione, provide what is often referred to as “reach-back” activity.
In situations where rainfall is delayed but eventually occurs, these herbicides can sometimes control weeds that have recently emerged and are still very small, generally less than 0.5 to 1 inch tall. However, once weeds have emerged, established, and begun actively growing beyond that stage, the likelihood of residual activation providing adequate reach-back control decreases substantially.
In other words, if waterhemp has already taken advantage of the dry conditions and gained size, don’t count on a delayed rain event alone to solve the problem.
Should I Add More Residual Herbicide to My POST Application?
This is another common question this season.
If crops are still relatively small (for example, corn or soybean less than V3), difficult-to-control weeds with extended emergence window remain a concern, and meaningful rainfall is forecast, including an additional residual herbicide with the POST application may provide value.
Layering residual herbicides can help extend control later into the season and reduce the risk of additional flushes once rainfall eventually returns. In these situations, adding residual may not be a bad investment, particularly if forecasted precipitation materializes.
As always, consider crop stage, herbicide labels, rotational restrictions, and total seasonal use rates when making these decisions.
Simple, right? Unfortunately not. Weed management decisions during a season like this rarely come with perfect answers. They require balancing current field conditions, weed size, weather forecasts, herbicide stewardship, and a healthy dose of experience and flexibility.
Final Thoughts
This season serves as another reminder that successful weed management requires flexibility. Herbicide programs are designed based on expected weather patterns, but sometimes Mother Nature has different plans.
The fields that are managed most successfully this year will likely be those where growers remain proactive, continue scouting, and are willing to adapt their weed management plans as conditions change.
As my farmer friend reminded me this morning, years like this are exactly why we need Plans A, B, C, and D.
