How long should you allow weeds to grow with corn? If the field only has scattered 4-inch-tall weeds, it may seem pointless. Are you putting yield at risk with weeds only 4 inches tall?

Weed scientists have attempted to answer that question by determining how weeds cost bushels. Some answers may seem obvious, notes Bryan Young, weed scientist with Purdue University. Obviously, weeds compete for water and nutrients.

“We’ve discovered that there is more potential impact on yields than meets the eye,” Young says. “If you review research through the years, you will conclude that you shouldn’t allow weeds to grow with corn at all.”

Nutrient competition and more

Early work indicated that the curve on a graph for corn yield loss takes off as weed height goes from 2 to 4 inches and higher, Young says. In 2000, researchers at the University of Nebraska looked at how nitrogen fertilization and timing of weed control in corn impacted yield loss.

You’ve likely heard old-timers say that if you apply nitrogen, you fertilize weeds, too. Surprisingly, in the Nebraska study, yield loss as a result of weeds growing with corn was noticeably higher when no nitrogen was applied. The higher the N rate, the less the effect, although it was above 5% at all N rates if weeds grew with corn until the V5 stage.

“Fewer macronutrients go into corn plants if they’re competing with weeds,” Young says. Once weeds die, nutrients within weeds return to the soil, but not right away. The greater the weed height, the longer it takes to return. Calcium returns slower than other nutrients.

In 2007 and 2008, Purdue, Ohio State University and Southern Illinois University studied the impact on corn yield when glyphosate applications were delayed.

“The return on investment for even a strong preemergence herbicide program can be reduced when the post treatment is delayed,” Young recalls. “Even when the weed density was reduced by using a stronger preemergence herbicide, delaying the glyphosate application on sparse weeds still reduced corn yield.”

Let there be light!

A study at the University of Guelph in Canada proved that corn and weeds compete for more than nutrients and water in the soil. Researchers found that young corn plants grown with sod intercepted less sunlight. Fewer leaves were perpendicular to the corn row.

They discovered that just the presence of other plants affected the quality of light reflected to corn. More specifically, it affected the ratio of red to far-red light that the corn plants received.

“Add light as a factor besides just soil resource competition,” Young says. The light effect means not only less light captured during the season due to fewer leaves between rows, but also less root growth compared to other parts of the plant.

“If you think 2 to 4 bushels per acre are important, then avoid weeds growing with corn early in the season,” Young concludes. “If you can see weeds, your corn can see them, too.”