Knock Out Early Weeds

December 6, 2013 06:46 PM
Knock Out Early Weeds

Protect your corn crop and prevent yield loss

Floats like a butterfly and stings like a bee was heavyweight champion boxer Muhammad Ali’s signature statement during his storied career. The memorable phrase also describes how weed seed can move into your fields, take root and compete with your corn crop in early spring. What you might not realize, though, is just how early "early" is.

Weed scientists have told farmers for years that small weeds, even in limited quantities, can damage corn yields. For instance, research shows that just three 2"- to 4"-tall waterhemp plants every 100' row of 5"-tall corn will eliminate at least 2 bu. of grain. The same is true for two morningglory, two velvetleaf or one-and-a-half cocklebur plants.

"Corn is much more sensitive to early season weed competition than soybeans, which is why we often recommend using pre-emergence herbicides in corn to prevent yield loss and minimize selection for glyphosate-resistant weeds," says Mark Rosenberg, South Dakota State University agronomy and weeds field specialist.


Now, there’s another reason to apply pre-emergence herbicides. Research commissioned by Syngenta and conducted by Clarence Swanton, University of Guelph, Ontario, Canada, indicates that weeds can impact a crop’s yield potential as soon as the corn plant emerges from the ground.

The theory is that as corn plants emerge, they can detect light reflected off the green tissue of weeds that are present. In a weedy environment, the corn plants will then go into growth overdrive to compete with the weeds, often at the expense of yield.

Swanton notes in a Syngenta technical bulletin that "the presence of weeds made the corn grow 17% taller, produce 45% more leaf area and 40% more dry leaf weight, producing a smaller root system—10% to 15% less than in the weed-free environment."

What occurs is that the corn plants shift their available nutrients away from their root system to shoot mass development, explains Mark Lawson, a Syngenta agronomist based in Indiana. "The latter results in taller, leafier plants with the potential for poor nutrient uptake due to their compromised root systems," he notes.

At the same time, uneven corn development ripples across the field as a result of the sudden, rapid plant growth. In the end, yields take a hit.

Champion your crop. In the presence of weeds, Swanton’s research also indicates corn leaves will orient parallel to the row, which can contribute to an open canopy. In a weed-free environment, corn leaves orient perpendicular to the row, which results in a more rapid canopy closure and, therefore, improved weed suppression.

This past summer, Lawson wit­nessed the corn-leaf growth phenomenon in a demonstration plot he developed. But instead of allowing weeds to take root, he placed green carpet between the rows to determine if Swanton’s light-reflection theory worked, and it did.

"There wasn’t any weed pressure, but the crop responded to the green carpet as if weeds were present," Lawson reports.

Upon evaluation, he found the corn plant’s root mass in the test plot crop was significantly smaller than the root mass of plants in the check plot."We have to move a lot of nutrients through the root system because when it’s compromised the corn crop is vulnerable, especially in unfavorable growing conditions," Lawson says.

Rosenberg says the most effective and consistent weed control is achieved by applying herbicides at pre-emergence or before weeds are 4" tall. 

You can e-mail Rhonda Brooks at

For more information on weed issues, from resistance to control measures to stewardship, visit

Weed Corn chart


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