• Early burndowns + residual applications will no longer be effective against problematic weeds like ragweed and waterhemp after 3-4 weeks after activation.
• Even if the field has received one burndown herbicide application early, it will likely need an additional burndown if tillage isn’t used.
• For soybeans, another residual herbicide at planting will help reduce weed pressure postemergence.

“I applied a burndown with a preemergence residual 4 weeks ago, and it has been wet since.” This has been a common theme across much of the Midwest. After a rainy spring and not being able to get back in the field for weeks, the question arises around whether that residual that was applied with the burndown will be there for the crop once it gets planted. Many are now wondering if it was a waste to apply that residual herbicide early? First off, the residual is not a waste of money if it was activated and controlled weeds. If it had not been applied then the field would have been weedy, potentially costing more to control the weeds prior to planting. With the emergence patterns of weeds like ragweed and waterhemp, those residuals most likely provided great value even though they were not utilized by the crop. Whether that residual will be available for the crop once planted depends on a few things: 1) How long ago was the residual applied and how wet has it been? If it was applied 4 weeks prior to planting (or longer) and the application was followed by wet weather then not much residual, if any, will be available to keep the crop weed free. However, if the residual was applied 2 weeks prior to planting then likely some residual weed control will be available for the crop. Keep in mind that the postemergence spray will need to be applied sooner when following preplant applications than when following a true preemergent application.

Now, what about acres that had a burndown application with the intent to conventional till at planting, but weather didn’t allow a timely planting? Weed germination is likely in clean fields and so you now have weeds that need to be tilled under prior to planting. In this case tillage aids are recommended. Tillage aids are typically contact herbicides that will burn leaf tissue and desiccate weeds within a few days. This application will allow the tillage pass to be more uniform with less clumps from green weeds that “balled up”. Additionally, it promotes soil aeration, better seed-to-soil contact and you will have fewer tillage escapes which become harder to control. If a residual herbicide application was done earlier in the season, 4 weeks prior to planting or longer, it is recommended that another residual be applied at planting or as close to planting as possible. This second residual will help maintain the crop weed free although depending on the application date it may further restrict your recropping intervals for next season.

The current weather encourages rapid weed germination and growth. Keep in mind that it is easier to maintain a weed free field, if it is planted weed free. If you have questions regarding your residual choice and what to apply, contact your local FS Crop Specialist, and remember to always read and follow product labels. 

Prevented Planting Acres: Weed Management Options

We are in the middle of June and a fair number of acres have not been planted yet due to the persistent rainfall events and saturated soils. Some growers may opt out of planting a crop and take the prevented planted option through their crop insurance. In terms of weed management, the concern is that bare ground will soon become a field of problematic weeds that if left unmanaged will be the cause of future headaches. In these situations, the ultimate goal is to not let the weeds grow and produce seed, as this will become problematic for years to come. A few options are suitable on those acres; chemical and mechanical weed control. Chemical products like Glyphosate and 2,4-D are effective on grasses and broadleaf weeds, not to mention very economical as well. Keep in mind that multiple applications may be needed until the first killing frost occurs. Chemical controls are good tools to use especially in those acres where tillage is not an option and fields are more prone to erosion. Mechanical weed control options include tillage and mowing. Tillage (disk, field cultivator, but not a vertical tillage tool) will remove weeds that have emerged and prevent them from going to seed as long as they are removed before they get too large. Mowing, when done as close to the soil surface as possible, will cut off the growing point. If you mow too high, species like waterhemp and grasses will re-grow. All these options (chemical and mechanical) can be used independently or in combination to help manage weeds on those prevented planted acres this season. If you have questions about weed control on those acres contact your local FS Crop Specialist for recommendations. 

Other information:  Weed Management on Prevented Planting Acres- U of Illinois