Farmers should be looking at cornfields right now to assess weed-control needs, insect threats and causes for any early season difficulties, says Roger Elmore, Iowa State University (ISU) Extension Corn Specialist.

“If you see gaps in stands or uneven plant height, look closely to determine the cause,” he says. “Potential setbacks could be anything from insect infestations, weed pressure, sidewall compaction, poor drainage, improper planting depth, inconsistent fertilizer placement or poor seed quality and lack of cold tolerance in your seed genetics.”

Once farmers find a probable cause for corn-stand irregularities, they can begin to make corrections, notes Elmore. “It’s probably too late to fix most stand problems for this year,” he says, “but you’ll be a lot more likely to find the reason for your problems now than if you wait until harvest and just look at yield variability.”

So far, growing conditions have brought a mix of both good and bad for corn development, adds Elmore. “In Iowa this spring, we planted corn faster than ever in recorded history,” he says. “While we did have some frost and cold damage, some hail and some flooding, all in all, we’ve had a good start to the season and it could be much worse.”

A potential issue with any frost- or hail-damaged corn is that it will appear to be younger than it really is, says Elmore. “If you’re looking to determine the corn’s growth stage based on leaf emergence, and plants have lost leaves from frost or hail, you might get your postemergence herbicide application timing wrong,” he points out. “It’s always important to follow the herbicide label and check the restrictions for growth stage, because there is the potential for crop injury when applying herbicides too late in corn’s development.”

Most corn in Iowa is about at the right stage now for early postemergence applications, says Bob Hartzler, ISU Extension agronomist. He adds that good timing will be important.

“April weather caused weeds to develop quicker than normal in Iowa, and the cold spell in May didn’t slow weeds down much,” he says. “So, control could be a challenge this year.”

Farmers who skipped their normal preemergence herbicide program in the rush to plant should be especially conscientious when trying to control weeds with a postemergence-only program, says Hartzler. He adds that pre-emergent products applied in poorly drained soils in southeast and south-central Iowa this spring may fail to provide full-season control this year due to early season flooding.

However, before applying postemergence herbicides to cornfields this spring, Hartzler advises farmers to make sure the products they select to use are well-suited for the weeds in their fields. He also advises farmers to be attentive to the possibility that fields may be infested with herbicide-resistant weeds.
“If your herbicide doesn’t work well the first time, call your local dealer or herbicide representative,” says Hartzler. “It’s important that they know if herbicide-resistant weeds are spreading in their area to help nearby farmers adjust to this development.”

Farmers should be especially wary if glyphosate applications fail to control weeds as well as they have in the past, he adds. “One of the biggest mistakes you can make is to try to clean up a glyphosate failure with more glyphosate,” says Hartzler. “Instead, switch to a different product for your second application.”

For more information on managing glyphosate-resistant weeds, click here: http://cornandsoybeandigest.com/corn/glyphosate_tolerance/. For more information on corn management issues in Iowa, click here: http://www.extension.iastate.edu/CropNews/.