Eliminating Weed Competition

Weeding Out the Competition
Weed management isn’t as simple as it once was. You could make one pass over the field with a herbicide application and maybe make a second pass later in the season to catch any stragglers and hold weeds back until canopy. While herbicide resistance seems to get most of the attention as one of the biggest weed management challenges, it’s just one factor that is making weeds tougher to control.

Consider that a single velvetleaf seed can remain viable in soil for up to 50 years and the likes of waterhemp or Palmer amaranth can produce hundreds of thousands of seeds — per plant. Small weeds can no longer be overlooked under the assumption that they might not affect the overall crop.

Yields Take a Hit

Take, for example, a 2009 to 2011 study conducted by University of Minnesota Extension. Once weed height reaches 3 to 4 inches — usually when corn is around the V3-V4 growth stage — the crop loses nearly 12 to 13 bu/A in the first week and 27 to 29 bu/A within the second week.

Weeds also hit soybean yields hard. According to a 2012 study conducted by North Carolina State University and the University of Georgia, soybean yields were reduced by as much as 68 percent due to Palmer amaranth interference at a density of one plant per square foot. Likewise, they saw waterhemp reduce yields by as much as 44 percent in 30-inch rows.

Five Factors

These yield reductions are due largely to the crop competing with weeds over five factors, explained in detail by the University of Illinois Extension:
  1. Light: This is especially critical when weeds grow taller than the crop.
  2. Moisture: After removing a dense population of weeds, the amount of water a crop can access is an essential factor in how well the crop recovers.
  3. Nutrients: Particularly in coarse-textured soils and soils with low fertility, weeds can exhibit a “luxury consumption” of certain nutrients, such as nitrogen.
  4. Space: Dense infestations of weeds lead to allelopathy — the suppression of plant growth due to release of natural plant-derived substances — which contributes to yield loss due to additional competition.
  5. Environmental: More than any other factors, soil and air temperature and soil moisture and rainfall before, during and after initiation of competition affect weed emergence and growth, herbicide effectiveness, the competitive interaction between crop and weed, and the ability of the crop to recover from early weed competition once weeds have been removed.


One of the most effective ways to control weeds is to stop them before they have a chance to emerge. Products such as Corvus® herbicide can be applied before your corn is in the ground and any weeds have had a chance to emerge. It can even be applied through the V2 corn growth stage, when the weeds that might emerge are still posing minimal competition. Corvus has two effective modes of action for weed control and reactivates throughout the season with just a half-inch of rain.

Corvus can be tankmixed with other herbicides, such as dicamba, 2,4-D and amide chemistries. Individually, these herbicides can cause a response in corn. When tankmixed with Corvus, however, the proprietary safener that is part of the Corvus formulation helps safen the use of the other herbicide on corn.

Depending on the weed pressure and spectrum, a second pass might be needed to keep competition at a minimum through the corn canopy — particularly if waterhemp or Palmer amaranth is in your field. If you planted corn with the LibertyLink® trait, you can make a postemergence application with Liberty® herbicide — either on its own or tankmixed with Laudis® herbicide or Capreno® herbicide.  Laudis and Capreno also can be tankmixed with glyphosate, if you are spraying glyphosate-tolerant corn.

Corn & Soybean Digest outlines five practices for maximizing weed control in corn.

AgProfessional magazine and the University of Nebraska review burndown and weed competition in corn and soybeans.
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