There are many good options for weed control in corn. That's good news for producers in the South where corn acreage is expected to boom in 2007.

“Weed control in corn is actually easier than in other crops,” said Jason Meier at the recent Multi-County Feed Grains Meeting in Brinkley, Ark. “It's very good to rotate into corn for resistance management, especially out of cotton. We are concerned about glyphosate-resistant pigweed coming into the state and planting corn is a good way to deal with that.”

It's estimated that in 2007 about 75 percent of Arkansas corn will be in Roundup Ready varieties. That percentage would be even higher if seed were available.

“I hope those planning on a corn crop have seed already,” said Meier, a graduate student working with Ken Smith, Arkansas Extension weed specialist. “From what I've been told, there's none left.”

Roundup Ready corn is “very convenient and allows for a total post program, like we see in cotton. It also allows for some late-season applications on 20-inch to 30-inch corn.”

Roundup Ready corn also protects against glyphosate drift. “In years past, we've had a lot of problems with drift on conventional corn.

“With it, you're allowed up to three glyphosate applications up to 1 quart per acre. You can do that up to 30 inches in height or V-8.”

Atrazine

When growers think of corn it's often in conjunction with atrazine, one of the best and most common herbicides for the crop. But there is a 12-inch cutoff for atrazine, said Meier.

“We do have Callisto, developed to replace atrazine. It isn't quite as effective but still does a good job. We've also got Steadfast — a combination of Resolve and Accent — that can be applied up to 24 inches.”

Lexar is “another good herbicide. It's a three-way mix: atrazine, Dual and Callisto. Steadfast ATZ also looks good. But both contain atrazine, so be aware of that 12-inch cutoff.”

Common weeds in Arkansas corn include broadleaf signalgrass, barnyardgrass, some crabgrass, pigweed, morningglories and teaweed.

In recent years, “we've had questions about Roundup Ready soybeans and cotton in our corn. If you're using atrazine, that isn't a big problem.

“Nutsedge can also be trouble in some fields. Permit is probably the first-choice solution.

“Johnsongrass is easier to control in corn than in grain sorghum.”

Some of the common herbicides for corn include atrazine, Dual Magnum, Roundup, Accent, Resolve, and Callisto.

“We also have Lexar and Lumax, which are similar and both contain an atrazine/Callisto/Dual mix at different ratios. There's also Steadfast, Steadfast ATZ and Sequence (a Dual/glyphosate pre-mix) — all very good herbicides.”

Among the most common questions asked of Meier and colleagues: should corn herbicide programs go all post, all pre, or a combination of the two? There are advantages and disadvantages to each.

“In tests, we looked at different programs with different herbicides. What we saw 43 days after application — on pigweed, morningglory, and barnyardgrass — is Sequence all pre at 2.5 pints per acre provided 70 to 80 percent control of the three weed species.”

Using Lexar at 2.8 pounds, Touchdown Total at 24 ounces at V-2 or early post or separated using Lexar pre followed by Touchdown at early post, “there isn't much difference. And we're getting excellent control, above 90 percent, on all three weed species. That's very impressive.”

In further studies, “126 days after treatment we were getting very good barnyardgrass control with all programs — 90 to 95 percent control. The pigweed control is dropping a bit to 60 to 70 percent with Sequence.”

The Lexar — due to atrazine, said Meier — provides better control.

“When it's put out pre followed by Touchdown at early post, pigweed is controlled well.”

There are advantages, he says to splitting applications.

“We were also looking at all early post herbicides, at V-2. The study included Impact, a newer herbicide, plus atrazine at 1.5 pounds, Steadfast plus atrazine, Option plus atrazine, and Roundup plus atrazine. All provide similar control 111 days out. Atrazine makes everything look better. But the glyphosate/atrazine at early post still provides good control cheaply.”

Risks

There are some risks involved if a producer chooses to go all pre or all post.

“Most growers have bougt their corn seed, and there's little, if any, left. So if you plant and put atrazine out pre and then lose the stand, there could be trouble. What will you go back with? You can't go back with soybeans or cotton. The atrazine means the only other choice is grain sorghum.

“However, if you wait until you have a sufficient stand before applying atrazine, weeds already will be emerged. If the early post coincides with rain, you could be delayed getting into the field for a week to 10 days. In that situation, you'll be behind the eight ball. If you lose your corn stand, your replant options are limited.”

Leaf configuration

Meier also mentioned some interesting research on corn leaf configurations.

“Look at the arrangement of these plants with little weed pressure,” he said while showing a slide of young corn plants with leaves lined up diagonally from each other. “The study suggested the first leaf of corn comes out randomly. The second leaf, though, positions itself in a manner to intercept more light.”

In a greenhouse test, corn was grown in Styrofoam cups. In one long line of cups the corn was left by itself. In another test strip the corn cups were placed against turfgrass that was even with the cup lips.

“What they found was the first corn leaf was random. The second leaf positioned itself away from the turfgrass and the corn plants on either side. The conclusion was the plant compensated for light interception at that early stage.”

Meier doesn't know if the early leaf placement could have a yield impact. However, “the next time you're in the field, check and see this happening. A cleaner field early may be to our advantage.”

Morningglories

Another problem weed scientists continue to address is late-season morningglory control.

“Early on, morningglories are small and won't do. However, when the corn starts drying and sunlight hits the morningglory, that little plant will take off. By the time harvest rolls around, there can be severe problems with harvest efficiency.

“Morningglories hurt yield through harvest. Combines will get into this weed and pull down stalks 20 feet in front.”

Aim can be used pre-harvest and works well. But it's “a hassle” to apply.

Recommendations for late-season morningglory control?

“You can use a residual, atrazine, Dual pre or early post and then a late post up to 24 inches. If you have a history of late-season morningglory problems, go back with glyphosate and Callisto or glyphosate and Steadfast and morningglories shouldn't be a problem at harvest. That program does a good job of knocking little morningglories out.”