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Corn+Soybean Digest

Poast Protected Corn Ends Grass Troubles

One of the oldest herbicide-resistant technologies still may be the least-expensive solution to tough grass problems in corn.

Sethoxydim-resistant (SR) corn, also known as Poast Protected corn, is resistant to the active ingredient in BASF's Poast and Poast Plus herbicides. One of the first postemergence grass herbicides for soybeans, Poast is deadly on a long list of grasses and even handles tough ones like woolly cupgrass, shattercane, wild proso millet, jointgrass, johnsongrass, wirestem muhly and quackgrass.

If these or other grasses cause your worst weed-control headaches, SR corn may well be part of your solution, say weed scientists.

When hybrids resistant to Liberty and now Roundup herbicides became available, there was some question about whether SR hybrids would silently fade away.

At least six seed companies sold a dozen or so SR hybrids for 1998. Most are in the 102- to 110-day relative-maturity range, but Dekalb has a 99-day SR hybrid and one rated at 118 days.

While some companies have dropped their SR corn, the same companies that sold seed for 1998 are expected to have seed available in 1999. New hybrids from elite genetic parents are currently in research. So, while the selection may be small, SR hybrids should yield with better conventional hybrids.

"It shouldn't be too difficult for Midwest corn growers to obtain a variety that would work in a grass-infested cornfield," says Harry Leffler, Dekalb agronomist.

Dale Sorensen, Dekalb's director of agronomic strategy, says his company is not giving up on SR corn.

"When you look at each of the herbicide technologies, there is a specific situation where each herbicide resistance may have a fit," he says. "The fit for SR corn is where grasses are the major weed problem."

Sorensen says Poast is especially strong on grasses that reproduce from rhizomes because it translocates rapidly from the leaves to the roots.

Dekalb introduced two new SR hybrids for the Southeast this past spring. Both were welcomed by growers in the Carolinas, Georgia, Alabama and northern Florida, where Texas panicum has become nearly impossible to control.

Bob Youmans, Furman, SC, had been battling Texas panicum, johnsongrass and a bit of common bermudagrass for several years.

"One application of Poast on Poast Protected corn was enough to control the panicum," Youmans tells. "It takes a couple of applications on the perennials because of the rhizomes, but it's still less expensive than other chemicals."

He planted in April and early season weed control looked good. He isn't looking forward to harvesttime, though.

"To make any of the herbicide- resistant corn systems work, you must have crop canopy to hold back later-season weed growth," Youmans states. "The weather here was so bad that even corn that was near canopy by mid- June was burned back by heat and drought. Any rain we get from now on is likely to grow weeds, too."

George and Nancy Krom grow corn and soybeans in heavy muck (high organic matter) soils near Rochester, IN. They've been growing Poast Protected corn on those soils for four years.

"Most soil-applied herbicides get tied up in the soil organic matter, so it's difficult to get prolonged control," George explains. "The wide application window for Poast in SR corn lets us wait until shortly before the corn canopies to apply the herbicide."

The Kroms, who also have tried other herbicide-resistant corns, find that the SR corn system is the least expensive.

This year, they used 1.5 pints/acre of Poast Plus at a cost of about $10.50 an acre. They applied it with crop oil and ammonium sulfate, which added another $1.50 to their per-acre cost.

Finally, to get some residual control, especially of broadleaf weeds, they added a pint of atrazine, which cost about $1.64 per acre. Total for the SR corn program, then, was $13.64/acre.

That compares to $14/acre for weed control in their Roundup Ready corn. They paid $12.50 an acre for a quart of Roundup plus $1.50 for crop oil and ammonium sulfate.

"We could use atrazine with the Roundup, too, without the costs getting out of line," says George Krom.

Both programs contrast with the roughly $25 an acre they spent for Liberty on Liberty Link corn in 1997. Even with the lower price for Liberty this year, cost of herbicide and ammonium sulfate was projected at roughly $20/acre.

The Kroms and Youmans list crop safety as another reason they like Poast Protected corn.

"Poast has no effect on the SR corn, unlike other popular grass herbicides," says Krom. "And if we used a product like Accent, we couldn't wait as long to apply it as we do with Poast."

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