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Corn+Soybean Digest

Resounding Resistance

“It's kind of sickening.” Jarrett Nehring frowns when making that response. Who wouldn't, when a 25-bu. drop in soybean yields flashes on the yield monitor in parts of a field walloped by weed resistance?

Glyphosate-resistant weeds aren't as prominent in northern states as down south, where there are numerous marestail (or horseweed), pigweed and other weeds that Roundup and other glyphosates can't control. And they're not going away.

Mike Owen, Iowa State University (ISU) agronomist, notes that while glyphosate-resistant common waterhemp was discovered 10 years ago in Iowa, the economic importance of such resistant weed biotypes hasn't been of major consequence. “But it's clear that the evolution of glyphosate-resistant weeds is increasing at a notable rate,” he says.

It has already swept into southern Illinois, where Nehring and his father Gerald farm at Murphysboro. In fact, their soybean and corn fields have become a virtual research smorgasbord for the agronomy department at Southern Illinois University (SIU), says Bryan Young, SIU agronomist and weed scientist.

“Our SIU research plot (with marestail growing several weeks after bean harvest) is also a good example of typical fields in southern Illinois,” says Young.

Nehring says initial glyphosate resistance was discovered on their farm about four years ago. “We farm highly erodible soil and are about 100% no-till,” he says. “That's why we used lots of Roundup.”

The Glyphosate Mode of action did a job on cantankerous weeds through its over-the-top applications. But too much of a good thing led to resistance problems that only got worse.

“Marestail is our biggest problem,” says Nehring, who also manages the SIU agronomy research farm at nearby Carbondale. “But we also have problems with giant ragweed. And we're hearing about waterhemp showing up elsewhere.”

Jim Klein, SIU soybean breeder, farms in the Flora area farther north of Carbondale. But not far enough north to escape a rash of resistance.

“We've seen some marestail,” he says. “We spray (glyphosate) three times and we're not killing it.”

Young says a survey he took from about 60 chemical retailers in late November showed that over 50% of them considered “marestail their biggest problem.” And close to 90% of them claimed weed control was their biggest nuisance when compared to insect or disease problems. Young says growers should use glyphosate and two other modes of action that have activity against marestail. If possible, they should include 2,4-D and one ALS inhibiting herbicide. Or, move away from glyphosate in the burndown and use paraquat plus metribuzin or 2,4-D.

“There are probably five or six products that can be used,” he says. “Our area doesn't have any documented ALS-resistant marestail like some other geographies. But others should be cautious.

“The additional modes of action will prolong the effectiveness of the herbicides for the future,” he says.

Nehring's weed control program involves both glyphosate and alternative herbicide and tillage — an added cost in a no-till operation.

Nehring is looking at two or three different chemical modes of action for soybeans. In one, glyphosate is applied at its normal rate, along with 1 pt./acre of 2,4-D, then in-season Roundup and Firstrate herbicide applications.

In another trial on his farm and at the SIU research field, there are preplant applications of Gramoxone at a rate of 3 pt./acre and Sencor at 4 oz./acre, followed by in-season Roundup treatments.

These herbicide combinations are showing solid control in most cases until the beans develop a canopy. But weed resistance problems are still there if early season rainfall — as seen in 2008 — delays spraying.

“Marestail was already a foot tall before we could spray,” says Nehring. “After we finally treated the field, it looked like we had killed it. But when the beans were about a foot tall, it started popping up again. When we harvested, the yield monitor in the combine showed a 25-bu. reduction in yields when we hit the marestail patch.”

Klein says his family farm in Urbana also used a Roundup/ 2,4-D pre-emergence treatment and added Valor to it. “That combination gave us good residual and cleaned up everything that's out there.”

For corn, Nehring is seeing better marestail control by using a combination of Roundup, 2,4-D and Atrazine. “We'll probably be facing resistance problems for a long time,” he says. “Hopefully we can determine which modes of action work best.”

Klein and Nehring see more fall herbicide applications to knock down growth. “It seems like marestail germinates nearly all year,” he says. “We saw it growing 6 in. tall during harvest. We made fall applications (of Roundup and 2,4-D in 2008) and will likely need to in 2009.”

ISU's Owen says that since cultivating non-controlled weeds is a last resort for most growers, a fall herbicide program is likely the best alternative. He suggests applying residual herbicides before or immediately after planting and to assure that herbicides have activity on weeds that are a problem.

“The use of early preplant (EPP) or pre-emergence (PRE) herbicides is intended to delay the development of weeds, allowing soybeans to become more competitive and providing a better opportunity for a timely postemergence herbicide application,” says Owen.

“There are significant benefits of an EPP or PRE herbicide application, including better time management in the spring when time availability during planting is critically short,” he says.

There is also a longer window of opportunity to make postemergence treatments, better environmental stewardship and improved profitability, he adds.

Owen says that just because a field is clean of weeds at the end of the season doesn't necessarily mean that the weed-management program was successful.

“Glyphosate-based programs can control larger weeds and can be applied later in the season, thus providing excellent weed control when viewed in September,” he says. “However, if weeds caused reductions in soybean yield potential in May and June, profitability was forfeited.

“The objective of using herbicides is to protect soybean yield, not to control weeds,” says Owen. “Furthermore, the use of EPP and/or PRE herbicides also will serve to reduce selection pressure on the weed community thus slowing the evolution of herbicide-resistant weeds or weed population shifts.”

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