Early-Season Weed Control and Options with Product Shortages

By Christy Sprague and Erin Burns

Early-season weed control is the first step in protecting crop yields and maximizing economic returns. Not controlling early-season weeds can interfere with planting, compete with emerging crop seedlings for water, nutrients and light, and can physically interfere with crop emergence. Uncontrolled winter annuals can also harbor destructive insects and act as hosts for soybean cyst nematode. Additionally, weeds are generally harder to control as they get larger in size and when winter annual weeds start to flower. But most importantly, all of these factors can lead to reduced crop yields.

For example, several years ago we conducted research over six locations that examined removing early-season weeds at least seven days prior to planting, at planting, and at several different soybean growth stages in no-till soybean. From this research, soybean yield was 6.7 bushel per acre lower when weeds were removed when soybean was at the unifoliate stage compared with controlling weeds prior to or at planting. With current soybean prices ($16.00 per bushel) this loss in yield could be as much as $107 per acre loss in economic returns at the end of the season. Early-season weeds can be controlled with either a burndown herbicide application or tillage to start the growing season with clean fields.

However, this year farmers are facing unprecedented herbicide shortages and prices that may alter some of their early-season weed control strategies. Two of the main herbicide active ingredients in short supply this year are glyphosate (Roundup, others) and glufosinate (Liberty, others). While there are other herbicide active ingredients that will likely be limited in supply, shortages of glyphosate and glufosinate will pose some major weed management challenges. To help minimize these effects, weed scientists from Michigan, Indiana, Kentucky, Ohio and Pennsylvania had a brainstorming session on how to best overcome some of these shortages. Below are some considerations and alternative burndown herbicide programs for early-season weed control from this session.

Resources to search for alternative herbicide products. The Michigan State University Extension “Weed Control Guide for Field Crops” (E0434) is one of your most important tools in searching for alternatives to glyphosate and other herbicide shortages. Additionally, industry technical guides and herbicides labels can be used to search for alternatives.

Spring tillage as an alternative to a herbicide burndown. Tillage needs to be done when weeds are small and soil conditions are conducive. Waiting to till large weeds can lead to survivors that will be harder to control with a postemergence (POST) herbicide. Vertical tillage tools are not effective for weed removal.

Burndown applications with glyphosate. With glyphosate shortages, famers maybe limited to only one glyphosate application for a crop. Under certain circumstances, particularly where cover crops are present, the best use of glyphosate may be in the burndown application. However, adding a herbicide like Sharpen (saflufenacil) to glyphosate can enhance cereal rye and annual grass termination while controlling horseweed (marestail). Additional information on cover crop termination strategies can be found in the Cover Crop Termination factsheet from MSU Extension.

Burndown applications without glyphosate. If glyphosate is omitted from the burndown, grasses become a bigger issue than broadleaf weeds. Alternative options for grass control include Gramoxone, rimsulfuron (corn only), and the ACCase herbicides clethodim (wait seven days prior to corn) and quizalofop (soybean only). ACCase herbicides need 60 F days and are best when applied alone. Reduced rates of glyphosate (0.38-0.56 lb ae/A) will control most annual grasses. Tables 1 and 2 provide example alternative burndown herbicide programs that deemphasize the use of glyphosate in soybean and corn, respectively.

Two to three “burner or foliar” herbicides in the burndown treatment. When using glyphosate in a burndown we typically include two to three herbicides (i.e., 2,4-D, Sharpen or dicamba) with foliar activity in that burndown application. This should also happen when glyphosate is not in the mix. There are several soil-applied herbicides in both corn and soybean that have some foliar activity that can help. Check out Table 2Q in the MSU Weed Control Guide for Field Crops to learn more about the activity of PRE herbicides in soybean.

Effective soil-applied (PRE) residual herbicide programs are necessary. Using a good soil-applied program will reduce the number of weeds that will need to be controlled by a POST herbicide program. Focusing PRE herbicide selection on herbicide-resistant and hard to control broadleaf weeds will be important. There are several POST options available for grass control in crop.

Have a good POST herbicide program in place including choosing a soybean technology that gives you more than one option for glyphosate- and multiple-resistant weed control. In soybean, the Group 1 herbicides are useful for POST grass control. Planting Enlist E3 or XtendFlex soybean provides additional options for controlling broadleaf and resistant weeds that do not solely rely on glufosinate (Liberty). In corn, there are several effective POST herbicide options for both grass and broadleaf weed control. Remember, it will be important to include a good PRE residual herbicide in the burndown program to help with the overall weed management plan.

Where is your limited glyphosate or glufosinate supplies most beneficial? It is important to consider where you will get the most benefit to the limited supplies of these herbicides that you may be able to secure. This may include using glyphosate in a different crop (i.e., sugarbeet) or applying reduced labeled rates for easier to control weeds (i.e., grasses).

* Adjuvants are needed; be aware of 2,4-D restrictions.

Newer soybean traits provide additional burndown weed control options. If a grower plants Enlist E3 soybean, higher rates of 2,4-D (Enlist One or Enlist Duo) can be applied without the seven-day waiting period prior to planting soybean. Additionally, these higher rates can also help with horseweed and perennial broadleaf weed control. Roundup Ready 2 Xtend or XtendFlex soybean allow growers to use registered dicamba products in their burndown program. Dicamba can help improve control of perennial broadleaf weeds, legume cover crops, and can provide some residual control of horseweed. More information on the restrictions and precautions for 2,4-D use in Enlist E3 soybean and dicamba use in RR2 Xtend or XtendFlex soybean can be found in the MSU Weed Control Guide and on the herbicide label.

Adjuvants are needed.