Weed control update: Roundup Ready corn, atrazine timing

Corn acreage is on a steady climb in the Mid-South and much of that acreage is being planted with Roundup Ready varieties. How best to manage Roundup Ready corn?

“The system gets good grass and broadleaf control. There has been some concern with morningglory control and resistant horseweed is cropping up. But the system really has a good broad spectrum weed control,” says Tom Mueller, weed scientist with the University of Tennessee at Knoxville.

As far as timing weed control in Roundup Ready corn, studies conducted by Mueller and others have proved surprising. Researchers anticipated a different timing sequence.

“But studies show the timing is the same in soybeans and corn. If you're going with a Roundup-only program, the crop needs two shots — one early in the season and a second about three weeks later. That provides good control,” says Mueller, who spoke at the Delta Center field day in Portageville, Mo.

Regarding glyphosate resistance, the best estimate is that west Tennessee has 400,000 acres of resistant horseweed (or marestail). “If it isn't in Missouri yet, it soon will be.”

Mueller says there's a new formulation of Roundup coming out: Roundup Weathermax. “We've just been cleared to speak on this product, so I'll tell you what I know. Weathermax is a more concentrated load, and that means the use rate will change again. It employs a new potassium salt and has a new surfactant system.”

Atrazine shuffle

Mueller says he's often asked if farmers should put all atrazine down pre, post, or split. There are pluses and drawbacks for each, he says.

“If you put it all down pre, it's certainly easy as it's already packaged. It's good insurance, especially if fields get an early season rain. The other advantage is if you put more pre down, when coming back for post treatments the weeds will be smaller.”

From a control perspective on certain weeds, atrazine is better pre. For example, if you try to kill crabgrass with atrazine post, it won't work. But as a pre on crabgrass, atrazine does well.

“If you do put atrazine out post, you have flexibility. If there's a crop failure on bottom ground that floods, you don't want atrazine out early because you're likely going to be planting soybeans. By holding it away from a pre application, you can go with late season soybeans without fear.”

Also, the new products Callisto and Steadfast tend to work better in conjunction with atrazine — another reason to use hold atrazine for post, says Mueller.

“Our findings, after several studies, suggest a split is the best way to go. Keep some atrazine available for some post spraying.”

There are a couple of new corn herbicides available. A new chemistry — Callisto — comes from Syngenta. Mueller says he's been surprised at how this has good activity on broadleaf signalgrass.

“It does miss foxtail and crabgrass, so it isn't brilliant on post-emergence grass control on corn. For this area, we want to use it post emergence with a low rate — maybe a quarter pound — of atrazine.”

Basis Gold is a product Mueller has been happy with “for many years. It's a great product, but DuPont is moving away from Basis Gold. So they're changing to Steadfast, which is a combination of Accent and Matrix. Steadfast has a double load of Accent and that means it'll be better on Johnsongrass and you can cut the rate more. Put some atrazine in a mix with Steadfast and you'll get good control.”

e-mail: [email protected].

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