In the battle against Amaranthus spp. weeds, it is important to fight them early and often. Layering residual herbicides is a tool to aid in this battle.

Pigweed relatives (Amaranthus spp.) like waterhemp and Palmer amaranth are soybean pests that call for new methods to keep them from taking over Illinois fields. CCA Soy Envoy Dawn Kielsmeier did a great job describing these weeds in her April post, “Weeds to Watch Out For.”  Click here to read her post.

These weeds’ resistance to commonly used POST herbicides, rapid growth rate and extraordinary seed production capabilities mean growers need to redesign their control plans to prevent seed banks from building to unmanageable levels.

The first goal of any weed control program is to prevent weeds from producing seed. For the last 20 years glyphosate and the Roundup Ready® trait in corn and soybeans served us well in that regard. Recently, though, the development of glyphosate resistance in certain weed species has made that goal harder to achieve. Pigweed species, especially, have developed resistance to multiple chemistry families.

For this reason agronomists recommend soil-applied residual herbicide products to begin the weed control program before weeds emerge. Starting the growing season clean is an invaluable first step in weed management. A challenge to this practice when combatting pigweeds is that these weeds have a long germination period. In a typical year, Amaranthus relatives begin emerging in late spring, peak around late June or early July, and continue into August (Figure 1).


Figure 1. Waterhemp germination starts early and lasts deep into the growing season

Soil-applied herbicides provide effective control of germinating weeds anywhere from four to six weeks following application. This means that just as waterhemp germination is peaking, the herbicide’s effectiveness is tapering off. An additional difficulty when dealing with these specific weeds is that their stems elongate rapidly, effectively fighting canopy shading (Figure 2). Often, we see apparently clean soybean fields develop “overnight” waterhemp infestations late in the growing season.


Figure 2. Waterhemp plants elongate rapidly, fighting canopy shading.

Layering a residual soil-applied herbicide along with the post-emergence herbicide pass is one way to maintain a barrier to waterhemp and related weeds later in the growing season (Figure 3). The Group 15 herbicides, long chain fatty acid inhibitors, can often extend control of small seeded broadleaves like the pigweeds. By applying a residual treatment in a preplant pass, then adding an additional dose of residual herbicide roughly 30 days after planting, growers can help to maintain a higher level of control later in the growing season.


Figure 3. Concept of layering effective soil residual herbicides to control waterhemp. Dr. Fritz Breitenbach

In two years of research at the University of Minnesota, applying a Group 15 herbicide 30 days after planting provided 20 percent better season-long control of waterhemp in soybeans compared with a preplant application only (Figure 4). In Dr. Fritz Breitenbach’s study, herbicide products used in a tank mix with Flexstar® were Dual II Magnum®, Outlook® and Warrant®.


Figure 4. Percent waterhemp control PRE alone vs PRE/POST residual herbicides. Dr. Fritz Breitenbach

I have been mainly referring to field conditions during a “typical” year. However, with soybean planting still going on in many parts of the state, growers had an opportunity to use tillage to control the first flush of Amaranthus spp. weeds. On the other hand, drown-out spots in fields give weeds an added opportunity to flourish this year. Weed control is rarely 100 percent and it is never easy. Adding a residual product will help manage pigweed populations later in the season and keep the weed seed bank depleted.

CCA Kevin Nelson is a 4R NMS at Northern Partners Cooperative. Kevin works with the Agronomy Sales Team and growers in North Central Illinois. He is a 2017 Illinois Soybean Association CCA Soy Envoy.

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About the Author: Kevin Nelson

Kevin Nelson is a certified crop adviser (CCA) and 4R Nutrient Management Specialist (NMS) serving the ag industry in north-central Illinois. Nelson received his CCA certification in 1994 and is a Senior Agronomist with Prairie Agronomics, his independent consulting firm. Nelson has a strong background in soil fertility and precision agriculture, and he is passionate about providing information and advice to help growers be more profitable and grow better beans.