There are a number of fields of very weedy corn and soybeans across Iowa this spring. In many cases farmers weren't able to apply preemergence herbicides due to rain delays and windy conditions. So they just went ahead and planted the crop without any herbicide applied.

"Also, many growers didn't follow our advice to use an early preplant herbicide application," says Iowa State University Extension weed scientist Mike Owen. An early preplant herbicide treatment provides early season protection against weeds interfering with the crop, and thus helps prevent yield losses.

"The current situation with weedy fields suggests yield potential is being lost rapidly in corn and you'll likely see the same effect in soybeans," says Owen. "The need to get corn fields sprayed will delay the application of soybean herbicide treatments, which in turn aggravates the soybean predicament with weeds."

Spray emerged weeds as soon as you can

Owen recommends that you apply postemergence herbicides to these weedy fields as quickly as possible and consider using a residual herbicide for added weed control and thus provide some yield protection.

Also, don't skimp on the recommended application rate for the postemergence herbicide. And be sure to follow label restrictions for residual herbicides. "Also, scout your fields to make sure you're using the correct herbicides to manage the weeds that are present," adds Owen.

Use a delayed pre-emergence treatment?

Many farmers have been spraying corn the past several days, reports Paul Kassel, ISU Extension field agronomist at Spencer in northwest Iowa. Some are using a delayed pre-emergence treatment of glyphosate mixed with a residual herbicide—applying the treatment to corn once it emerges.

This treatment is usually glyphosate plus Harness or Surpass or generic acetochlor or Outlook or Dual. The herbicide mixture controls the emerged weeds—which usually include some winter annuals--and it extends the time period in which the residual herbicide will control weeds.

Many soybeans are now emerging across Iowa. "Generally, soybean fields this spring in northwest Iowa have a good appearance with better than average emergence and early season vigor," says Kassel. "We haven't seen much of a problem with bean leaf beetle this year in Iowa. The entomologists tell is that the harsh winter and last summer's widespread soybean aphid spraying greatly reduced the over-wintering populations of bean leaf beetles."