Weed management is more than just selecting a post applied herbicide trait in your soybeans and waiting to spray when weeds emerge. Over the last 10 to 15 years, it seems like many producers aren’t sure if they will have a weed pressure problem and so are reluctant to get aggressive with a foundation effective herbicide. Instead, it’s become more of wait and see, then react with post herbicide.

We apply nitrogen every year ahead of corn because we know a corn crop will need nitrogen. In fact, many producers have implemented strategies to maximize season-long supply by splitting application and units across multiple applications throughout the season. In some cases, soybean weed management needs to adopt a similar mentality.

We know that weeds such as waterhemp will typically start to emerge by May 1, give or take a few days, and will germinate through fall if opportunities are presented.  So, when developing a weed management system, a producer must understand all weed survival opportunities and ask “how effective is the selected program to meet the farm’s short- and long-term goals?”

Short-term Goal:

  • To minimize weed impact on a growing soybean crop
  • Not to use an in-crop rescue treatment against waterhemp
  • Spray all emerged weeds prior to 4” of growth

Long-term Goal:

  • Increase soybean production by 10-15%
  • Reduce weed resistance
  • Become less reliant on short term fixes or reactive herbicide applications

Waterhemp is considered a pest because it’s an organism that inflicts damage onto another organism such as corn and soybeans. The inflicted damage isn’t attacking leaves or rooting structures like disease and insects do, but waterhemp populations are competing against the soybean plant for soil moisture, nutrients, space, and nightlight. All of these factors directly impact the soybeans’ ability to properly grow, develop, retain maximum pod development, and ultimately the amount of energy that is converted in the harvestable yield.

Integrated Pest Management (IPM) is a series of pest control strategies that are developed and implemented to reduce crop and economic damage.

Mechanical Control

  • Tillage: Is a good option to bury waterhemp seed, however this method will also bring up other seed species from the seed bank, so be prepared.

Biological Control

  • Cover Crops: Can help prevent marestail and waterhemp from emerging. Concepts such as planting “green” have become more popular in recent years as mechanical support methods have been developed. This includes crimping tools that can be mounted on the tractor or between row units on the planter.
  • Strip-Till: Has evolved in some areas by tilling the strip where the crop will be grown (6-8”) and the space in between is where the selected cover crop is growing or had been grown.

Cultural Control

  • Changing Seeding Rates to allow plants to branch out more and develop a strong canopy
  • Changing Row Configuration, less than 30”
  • Planting Earlier, to allow crop a jump start in growth development and overall size
  • Selecting Soybean Genetics, that offer a medium bush to bushy canopy

Chemical Control

  • This category is the most common and is becoming the most challenging due to growing resistance and application restriction. The easiest way to chemically control waterhemp is to NEVER allow it to grow and develop a growing point above the soil surface. Once growing points are established and leaves are growing, photosynthesis kicks in and the weed seedling becomes self-sufficient and ready to grow aggressively.

Tips & Ticks:

A residual herbicide that has an Active Ingredient (AI) with strong effective activity needs to be applied, specifically toward waterhemp. Many pre-mixed herbicides promote that they have multiple modes of action and on the surface can provide a false sense of security. How many of those multiple modes of action are still effective against waterhemp (resistance)?

  • Use the proper volume (gallons per acre) that it takes to maximize the AI and the application investment.
  • Use proper adjuvants & water conditioners.
  • Use compatible tank mix partners (other herbicides, fungicides, insecticide, nutrients)
  • Follow label weed height: With more growing points and more time to allow photosynthetic processes, a weed will become stronger and harder to have good control and minimize resistance.
  • Be aware of the cash crop growth stages: Do not apply herbicides when major physiological events are occurring.  A soybean plant still needs to metabolize the herbicides that are applied on it, and non-burner herbicides still need to be used with caution.

At the end of the day, IPM does not mean we will WIN the battle over waterhemp by selecting and using a post applied only herbicide. The battle against waterhemp will be won through several effective options that are strategically planned and managed. The best tool is the tool you never have to use. It’s good to have the option, but develop a weed management system that allows for flexibility, effectiveness and profitability with the option to use the “Big-Guns” when needed. But do not always lead with them or rely on them 100 percent, otherwise mother nature will find a way to bench the best tools. Remember, when using chemical controls only, biology will find a way to beat biology.

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About the Author: Todd Steinacher

Steinacher is an ISA CCA Soy Envoy alum and currently supports ISA on agronomic content as well as serving as an Illinois CCA board member. He was recently awarded the 2020 IL CCA of the Year & the 2021 International CCA of the Year. He has over 15 years agronomic experience, currently working with AgriGold and GROWMARK previously. Steinacher has an associate degree from Lincoln Land Community College, a B.S. in agronomy and business from Western Illinois University and a master’s degree in crop science from the University of Illinois at Urbana-Champaign.