Given the weather conditions many parts of the country are experiencing, farmers are likely more concerned about getting their corn and soybeans planted at this point than implementing their weed-control program. But wherever possible, don’t let weeds get a foothold in either crop, and especially corn. While soybeans have some ability to recover from limited early-season weed pressure, that’s not the case with corn.

Research commissioned by Syngenta and conducted by Clarence Swanton, University of Guelph, Ontario, Canada, indicates the presence of any weeds can impact a corn crop’s yield potential as soon as the crop emerges from the ground.

Swanton’s theory, published in a Syngenta technical bulletin, is that a “change in light quality caused by weeds emerging at the same time as the corn plant is the true inhibitor of healthy root development.”

What occurs is a negative chain reaction. As corn plants emerge, they detect light that’s reflected off the green tissue of any weeds that are present. The crop reacts to the weeds by going into growth overdrive to compete for nutrients. In the process, corn plants shift available nutrients away from their root system to shoot mass development. That shift results in taller, leafier plants with the potential for poor nutrient uptake due to the corn crop’s compromised root system.

Swanton notes in the 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."

The overall result is that corn development across a field becomes uneven with little chance for the picket-fence stands that everyone wants, and yield potential ultimately takes a hit.

That’s not all. In the presence of weeds, Swanton’s research also indicates that corn leaves will orient parallel to the row, which contributes 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.

Syngenta tested Swanton’s theory by placing green carpet between rows of planted corn 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," Syngenta reports.

What to do now? If time and weather conditions allow, farmers’ best bets are still to use some combination of preplant or preemergence herbicides that provide residual control, possibly cultivation if cultural practices allow it, and the use of postemergence herbicide applications.

In cases where weather compromises farmers’ ability to get early weed-control measures in place, the need for scouting fields is more critical. Field observations once corn is 2” to 4” tall (the earlier the better) will provide retailers and farmers with a good sense of the weed pressure and need for postemergence herbicides.

Aaron Hager, University of Illinois weed scientist offers a recommendation that farmers and applicators might not think about in the rush to address weeds.

“Before you apply, scout corn fields to accurately determine the crop’s growth stage,” Hager says, in a University of Illinois news release. “Adverse environmental conditions can result in corn plants that are physiologically older than their height suggests, so assess the plant’s developmental stage by evaluating leaf/collar number in addition to plant height.”

Look for the maximum corn stage listed on the respective herbicide label and do not apply the product if corn exceeds the labeled stage, he adds. If tankmixing two or more products, follow the most restrictive corn growth stage listed on any of the tankmix component labels.

He adds that as you scout, record the types of broadleaves and grass weeds that are present in each field to plan future weed-control strategies.