May 12, 2022, 11:00 AM

Soybean Pre-Emergence

Weed control in all crops is critical to achieving maximum yield.  In soybeans specifically, there are multiple factors that can negatively affect their growth and development. The two main factors are moisture and nutrient uptake by the weeds.  Although we do have options to kill weeds after they are growing, the list of “effective” chemicals is becoming smaller every year. Compared to other crops, soybeans are more resilient to weed pressure because their yield is determined throughout the entire growing season. However, this also means that keeping the field clean all season long is critical to maximize yield potential.

Factors to Consider

The easiest and fastest way to keep fields clean is to prevent the weeds from growing in the first place.  This can be accomplished in two forms: the first is as a pre-emergent herbicide application, or in other words applying the herbicide before the soybeans emerge and before the weeds emerge. The second form is applying a residual herbicide when the post-emergent herbicide is applied.

Why is weed control so important in soybeans?  Any plant that competes with the soybean plant is considered a weed, so this can be anything from volunteer corn from the previous crop to “weeds” like waterhemp and lambsquarters. 

  • The two main concerns with weeds are water and nutrient usage. Since many weeds grow quickly, their water usage is also very high. Once that water is used by the weed it will not be available to the soybean even if the weed is killed with herbicide. 
  • Weeds cause nutrient depletion. However, unlike water, once the weed is killed the nutrients that were taken up by the weed are released back to the soil. Yet, most are not released for a long time and will not be used by this year’s crop. 
  • If the weeds become too dense or too large, shading will occur. Since all plants need sunlight to grow, this can also negatively impact soybeans.
  • Many times, corn is not thought of as a weed, but volunteer corn in a soybean field can be considered an especially important weed. Although corn does not go to seed, it can serve as a host for diseases and insects the following year.
  • The last factor that should be considered in weed control is future years pressure or what is also referred to as a “weed bank”. Some weeds like lambsquarters can survive in the soil for over 20 years.  Even though the weed pressure might not seem bad in the current year, remember that each plant can produce thousands of seeds, and what might seem like a small problem this year could turn into a big issue next year. Think of it as preventive maintenance on your engine; it’s a lot easier to change the oil 50 times than replace the engine once.

Action Plan

There is not one singular thing that we can do to stop weeds but rather we need to take a layered approach.

  1. Use pre-emergent in combination with post-emergent herbicide applications, the pre-emergent not only does a great job controlling weeds it also gives us larger flexibility in our post-emergent application.
  2. Use residual chemical herbicides in conjunction with post-emergent herbicides. Most of our post-emergent chemicals are only contact killers and do not have much residual effect.  Typically, chemical applied pre-emergents have broken down and are not giving off much protection by the time we apply post-emergent herbicides.
  3. Alternate herbicide modes of action (MOA). Although pre- and post-emergent herbicides work differently, if they are both from the same MOA it increases the risk of herbicide resistance.  If we alternate MOA, we exponentially extend the time before weeds become resistant to herbicides. This is also true between crops.  Therefore, it is recommended you use the same MOA in both corn and soybeans.
  4. Change row spacing. The sooner the soybean crop can canopy the row and shade out the weeds, the easier it is to control weeds. A general rule is that 20-inch rows will canopy 7-10 days sooner than 30-inch rows, and 15-inch rows will close 14-20 days sooner than 30-inch rows.


Although weed control might seem like a never-ending battle, the easiest weed to control is the one that never comes up.  Pre-emergents are not only the best protection, but they are also the easiest because weather like wind and temperature are less of an issue.  When you prevent a weed from growing you don’t have to worry about how much of an impact it will have on your yield. There is less issues with carryover to the next crop with pre-emergents because they have more time to break down before the next crop.