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.

Reduce your risk

Reduce your risk. “A total postemergence program is the most risky weed-control system, because the timing of a postemergence herbicide application is almost completely up to Mother Nature, and no one can control the weather,” points out Hager. “Instead, try using an integrated program with some soil-residual products. Also, farmers could consider a split application of an early preplant treatment followed either by a pre-emergence or a postemergence treatment to provide more consistent weed control than a single, early preplant application.”

A pre-emergence herbicide application will help keep late-emerging weeds small and uniform enough in height to boost odds for success when following up with a postemergence treatment, points out Gunsolus. “Using a pre-emergence herbicide buys you more time to apply your postemergence herbicide for optimal weed control,” he says.

In addition, a pre-emergence herbicide can be an especially good investment with irrigation, which ensures moisture is available at the right time to activate the chemistry, says Sandell. “With dryland corn in Nebraska, using a residual pre-emergence program will still be beneficial in reducing potential problems with glyphosate-resistant weeds, even if a lack of rainfall delays activation past the ideal time for starting corn out in clean fields,” he says.

Pay attention to timing. “Your main management focus should be on controlling those early weeds,” says Gunsolus. “Our research in Minnesota, and more comprehensive research in Wisconsin, shows that at about the V3-V4 stage, if weeds aren’t removed, fields will suffer an average 3 bu./acre/day yield loss up until the end of June. In fact, farmers should plan to have all their weed control completed by the fourth of July.”

Timing is important for both post- and pre-emergent applications, adds Hager. “For pre-emergent herbicide applications, try to time them closer to when you plant, especially if you have a weed spectrum in the field that can emerge later in the season, such as waterhemp.”

Weeds that re-infest after an initial herbicide application can also be very competitive, and Hager recommends being vigilant to control these later weed flushes, if necessary, while they are also still small.

Be careful with reduced rates. Many farmers run reduced herbicide rates of soil-residual herbicides to save costs, says Hager. “However, with reduced rates, you may be setting the product up to fail earlier, depending on weather conditions and weed pressure. Using a full, or a nearly full rate based on soil type often provides an extended period of weed control that you don’t always have with reduced rates.”

Gunsolus agrees. “Especially in the postemergence arena, good early season weed control has a lot to do with proper timing and not skimping on rates,” he says. “Also, when you do postemergence weed control, make sure you don’t go too fast and check to make sure you’re getting good spray coverage on weeds.”

Scout and reassess. After each weed-control practice, farmers should scout fields and evaluate how well their treatment worked and whether or not a remedial treatment might be needed, advises Gunsolus.

Late Weed Control Leads To Wasted N

Want to make sure your corn crop makes the best use of all the nitrogen (N) it can? Then you’d better control weeds early, say experts from Minnesota and Wisconsin.

“Early season yield loss is due largely to weeds that sequester about 30-45 lbs. of N/acre out of the field from planting time up until about the end of June,” says Jeff Gunsolus, University of Minnesota Extension weed scientist. “You don’t get that N back during the season after your weeds are controlled. So, it’s much better to kill the weeds early than to be in the situation where you need to add more N to a field where weeds have sequestered it.”

Early weeds consume a costly amount of N out of the soil, and farmers may be unable to compensate for that nutrient loss by adding additional N later, agrees Carrie Laboski, University of Wisconsin Extension soil scientist. “The bottom line on our two-year study is that N fertilizer is more efficiently used when weeds are controlled pre-emergence or at a 4-in. weed height compared to waiting until weeds are 12 in. tall,” she says. “The economic return on your investment in N fertilizer and herbicide is maximized when weeds are controlled early.”  

January 2011