Residual herbicides, weed resistance, modes of action, sites of action - pretty soon your head is spinning if you’re sorting through weed control information. When the rubber meets the road; when you’re looking at a field and deciding the best herbicide option, what goes through your mind? What decisions do you make?
Here’s what goes through Bill Johnson’s mind. We sent him photos of various fields and various scenarios, and asked him for his best recommendations. Johnson is a Purdue University Extension weed control specialist. His recommendations are suggestions, and are not meant to be all inclusive of all possible products on the market.
Perhaps more interesting than his recommendations are the questions he asked about each field before suggesting weed control options. They included:
• What crop is intended for this field?
• Where is the field located?
• Is the whole field like this picture, or is this the worst spot?
• Will tillage be done before planting?
• How early will the field likely be planted?
If perennial weeds were prevalent and the field was scheduled for soybeans, Johnson actually suggested rotating to corn first to work on those weeds. There are generally more options in corn, especially for perennial broadleaf weeds like Canada thistle and dandelion.
Location of the field in the state can matter because it can help nail down which weeds to expect. One key to a successful program is knowing what weeds are in your fields, and if they’re herbicide-resistant or not.
If tillage is in the system, it may open up other options than if there is no tillage, Johnson says. For one thing, if there is no tillage scheduled for spring but there was fall tillage, then you may need to decide if a burndown is needed or not. If there will be tillage you may decide between applying the herbicide during tillage or pre-emerge after tillage and planting.
Timing of planting relates to timing of herbicide application. That can relate to how long a residual herbicide applied at planting may last. It could determine whether a residual goes on early, or if a residual application is planned for later to extend the window of control.
Check out each picture and see how Johnson sizes it up in terms of what herbicide options he would consider. What follows on the next three pages are three images and the questions and options that Johnson explored.
Label restrictions an issue - >>>
Think about label restrictions
This field features a ditch along one end of the field. It’s a field prepared with a combination of vertical tillage in the fall, followed by a field cultivator pass before planting in the spring. It’s in central Indiana, and it’s going to corn. Giant ragweed and other typical broadleaves are common in the area.
“Atrazine or some herbicide containing atrazine would typically be a soil-applied herbicide for a field going to corn with conventional tillage,” Johnson says. “You need to think about the creek, however. Atrazine and products with atrazine in them have set-back restrictions from waterways on their labels. These restrictions should be followed.”
In this particular field, you might be able to spray a non-atrazine herbicide on the end rows if you still wanted to use atrazine in the field, Johnson says. Refer to the herbicide label for actual setbacks.
“Something like Resicore, Accuron or Verdict might be an alternative for this field,” he says. “You would need to check the label, but these products do not contain atrazine, so I wouldn’t anticipate strenuous set-backs. Verdict would also be a good fit. Other products would also be alternatives.“
Make choices carefully - >>>
Typical spring growth
Mostly what you see in this stalk field in early May is chickweed. It’s in central Indiana. Obviously, it’s going to be soybeans, and will be no-tilled. Pretend the sprayer isn’t already there. What would Johnson have recommended to put into the tank?
“If you’re planting right away, it takes 2-4-D out because there are required time intervals before planting soybeans, even at lower rates,” Johnson says.
One burndown option would be glyphosate, Sharpen and Sencor, he says. Another choice could be Gramoxone with Sharpen and Sencor. It depends on how quick you want to knock it down, and what the actual weed spectrum is in the field as to which one to choose, he says.
Sharpen and Sencor have some residual activity as well if rates are high enough. Johnson says you may want to come back later with another shot of residual. If pigweeds are a primary target, a Valor or Auhtority-type product would be a good choice. If giant ragweed is an issue, he would consider a herbicide which contains Scepter, First Rate or Classic, as long as you’re sure the ragweed isn’t ALS-resistant.
If ragweed is an issue and is ALS-resistant, Johnson would look at something like Valor as your soil-applied herbicide. “You’re not going to get 100% control of giant ragweed with any soil-applied herbicide, but you should thin it out,” he says.
Even if giant ragweed is the biggest problem, you may still want to add glyphosate in Roundup Ready beans post-emergence to get other weeds,” he says. If you’re spraying glyphosate, he would consider adding Flex Star if it’s relatively early in the season, or Cobra if you’re spaying later.
Johnson adds a note of caution about Sharpen-based herbicides like Verdict. If you’re applying it with another PPO herbicide or on a light soil with less than 2% organic matter, there is an interval on the label before you can plant soybeans. Refer to the label for details.
Waiting for corn - >>>
Waiting for corn
This field was in soybeans a year ago. Light fall tillage was completed. The field will be tilled again before planting.
“If you’re going to till it, then there’s no need for burndown-there’s not that much there,” Johnson says. “One option is to till, plant and then spray with an atrazine-based product, perhaps all in the same day.”
Another option would be to spray first and then incorporate with shallow tillage, he adds.
Products that would work here include Lexar, Verdict, Harness Xtra, Resicore and Acuron, he observes.
You may want to add a product like Dual, Outlook, Zidua or Surpass to insure grass control if you use low rates, or if you use an atrazine-based herbicice. Be sure to follow label directions for any products you choose, he concludes.