After all you do to choose the right hybrid, wait for prime planting conditions, provide plenty of nutrients and place the seed in the soil just so, it’s heartbreaking to think of all the damage weeds, insects and disease can do to your yields. And it’s shocking to realize how quickly that heartbreak can break the bank.

Early-season weed competition has been documented to reduce corn yields by up to 40 bushels per acre in South Dakota, according to agronomy and weeds field specialist Mark Rosenberg at South Dakota State University. The greatest damage is right at the beginning of the season, he adds. Just one foxtail per foot of row can steal 1% of corn yield per day — per day! — starting one week after corn and weeds emerge.

Daily yield loss

Research in Iowa, Minnesota and Wisconsin illustrates that 3- to 4-inch-tall weeds in corn at the V3 to V4 growth stage cause losses of an average of 3 bushels per acre per day until the end of June. That adds up dramatically, notes University of Minnesota Extension weed scientist Jeff Gunsolus.

“Our studies over three years show corn lost from 12 to 13 bushels per acre within the first week and 27 to 29 bushels per acre within the second week if weeds were allowed to remain in the field after they reached 4 inches in height,” Gunsolus reports.

All that early-season loss shows the importance of an integrated weed control program that starts with a soil-applied, residual herbicide, notes Aaron Hager, Extension weed scientist at the University of Illinois. A split application of early preplant herbicide followed by either a preemergence or postemergence spray could provide more consistent weed control than a single, early preplant application, he adds.

Multiple modes of action and multiple application timings also help address herbicide-resistant weeds.

Early weeds steal N, roots

“Corn is much more sensitive to early-season weed competition than soybeans,” Rosenberg says.

For one thing, early-season weeds can consume 30 to 45 pounds of N per acre between planting time and the end of June, notes Gunsolus.

“You don’t get that N back during the season after your weeds are controlled,” he adds.

Corn also appears to react to weeds by pouring more energy into shoot growth at the expense of root growth. The result is plants of widely varying heights competing with each other for light — the very phenomenon that makes uneven emergence such a yield-buster. That compromised root mass could also end up being inadequate to seek out moisture if the summer turns dry.