By Leah Ten Napel
Getting a jump on weed control is critical as farmers aim for clean fields all growing season. It’s especially important to think ahead for soybean herbicide applications, as there tends to be shorter application windows compared with corn herbicides.
Putting early-season weed control at the top of the priority list this spring is the best way to set a field up for success. Here’s how to jump-start soybean weed control on your farm:
1. Know your fields. What is necessary for your neighbor’s field may not be what your field requires. Different management practices and weed pressures will require different herbicide application strategies.
No-till fields usually have more winter annual weed species, such as field pennycress or horseweed/marestail. An early burndown application with the addition of a residual product will help achieve a clean field going into planting.
Pay close attention to your soybean seed trait. While this may seem obvious, it is worth double-checking to avoid a big mess in the future! There are a handful of different soybean seed trait packages on the market, and misapplications can kill whole fields. Verify that your herbicide plan matches your seed choice.
Keep in mind any persistent weed issues in your fields. Include multiple effective herbicide sites of actions against your problem weeds. Relying on a single herbicide can lead to herbicide resistance if that is not already present in your field.
2. Have a plan, but stay alert. Once you know your herbicide plan, read the labels to be aware of any restrictions or growth-stage cutoffs. Delayed planting, weeds getting too tall or weather conditions may require a switch to a different herbicide. Sticking to your original plan may result in plant injury or lack of weed control. Being flexible and ready to swap chemicals if needed will increase chances of success throughout the growing season.
One tactic to battle problem fields is using a layered residual herbicide plan. While not every field may need this aggressive approach, fields with a history of late-season weed pressure, especially waterhemp, can benefit from having residuals in both preemergence and postemergence applications.
The timing of a layered residual herbicide plan is laid out in the image above. The timing of the second application is moved up about a week earlier than a typical postemergence application. This timing is important to ensure the initial preemergence chemical still is active, and the field is not susceptible to a period of vulnerability.
3. Check the weather. Spring weather has a major impact on our herbicide programs. Different weather conditions can affect weed-emergence patterns, such as excess amounts of rainfall, cool temperatures or even dry periods. These weather conditions also can impact herbicide effectiveness. Timely scouting is the only way to ensure we are using the correct chemicals for the weeds present and applying them at the correct time.
Cool temperatures can cause concern, especially for burndown applications on weeds or cover crops. Applications made on sunny days with warm daytime and nighttime temperatures (above 40 degrees F) are typically more successful.
While rainfall during the spring is usually welcomed, applications right before expected heavy rainfall can cause concerns of runoff, erosion or leaching of the herbicides applied.
Limited rainfall can reduce herbicide effectiveness or allow weed escapes. It typically takes at least a half-inch of rain to activate the herbicide by moving the herbicide to a depth where seeds are actively germinating. Chemical properties of the herbicide as well as soil moisture before application affect how much rain is required for activation.
A rule of thumb is that an activating rain is needed within five to seven days of application. As with any biological response, though, many factors influence this time interval.
4. Scout, scout and scout. Following all the tips above is just one piece of the puzzle. Scouting throughout the growing season is going to be equally important when it comes to keeping your fields clean this year and beyond. Early in the season, you can alter your applications to fit your field’s needs. Late-season scouting is crucial for weed control in future years.
Follow the Integrated Crop Management News Blog published by Iowa State University Extension and Outreach for timely updates on what is being seen throughout the state when it comes to weed control and other agronomic pests.
Ten Napel is an ISU Extension field agronomist.