The early weeds in corn are too small to hurt anything, right?

Wrong. Very wrong, according to Corn Belt weed scientists.

“Early season weed control is vital to both future yields and profitability, because early weed flushes compete intensely with corn for both nitrogen (N) and water,” says Jeff Gunsolus, University of Minnesota Extension weed scientist. “Dense weeds can also shade soils and make them cooler so that corn grows more slowly.”

Okay, but exactly when does early season weed control need to be done before it’s too late to stop yield loss?

“Assuming you have started with a clean field, the most competitive weeds in corn will be about 3-4 in. high when corn reaches the V3-V4 growth stage,” says Gunsolus. “If you don’t remove those 3-4-in. weeds promptly, you’ll be losing about 3 bu./acre for every day you delay. Our studies over three years show corn lost between 12-13 bu./acre within the first week and 27-29 bu./acre within the second week if weeds were allowed to remain in the field after they reached 4 in. in height.”

Such a big yield loss early in the season could mean the difference between making or losing money, says Lowell Sandell, University of Nebraska Extension weed scientist. “Depending on soil moisture and fertility levels, waiting to control weeds until corn reaches the V3-V4 growth stage can push you over the economic threshold for profitability,” Sandell says. “At about 4-6-in.-tall corn and weeds, that’s when you typically pass the breakeven mark and start losing money to lost yields from weed pressure after factoring in the cost of the herbicide application.”

Especially in corn, profitable weed control is all about timing, agrees Aaron Hager, University of Illinois Extension weed scientist, but that’s not everything a farmer needs to keep in mind to ensure successful weed control, he adds. Hager, Sandell and Gunsolus, provide the following five tips to help guide farmers towards more profitable corn weed management:

Start clean. A clean field at planting is essential for starting the corn crop off right, says Hager. “This can be achieved by using tillage, herbicides or some combination of the two,” he says.

Sandell recommends using a burndown with residual chemistry that is targeted to the specific weed spectrum for each field. “Use of a soil-residual herbicide will help to both start the crop off clean and to manage the field for any potential glyphosate-resistant weeds, such as waterhemp and giant ragweed, or to reduce the potential development of these and other herbicide-resistant weed biotypes,” he says.