With the proliferation of herbicide-resistant weeds in crop fields, Larry Steckel has some not-so-good news for Mid-South farmers: “Nothing about weed control is going to be as easy as it once was. Even with the new technologies in the pipeline, weed control is going to be much more complex.”

“The good old days of weed control with two or three shots of Roundup over the top are gone and are never coming back,” he said at the annual conference of the Mississippi Agricultural Consultants Association at Mississippi State University. “We’re just running herbicides into the ground one after another.”

The Mid-South “has the distinction of having the most glyphosate-resistant weed species anywhere,” says Steckel, who is associate professor/row crop weed specialist, University of Tennessee Department of Plant Sciences, Jackson. “Mostly, the problems are with marestail and Palmer pigweed, but there are others.

Giant ragweed, in Tennessee and Arkansas, is becoming a huge problem. It’s very competitive, and once it’s growing in a field, it’s hard to deal with. Up and down the Mississippi River, there’s a lot of concern about common water hemp. Italian ryegrass is increasingly a problem.

“We don’t have glyphosate-resistant johnsongrass in Tennessee yet, but it’s a problem in Louisiana, has been documented in Mississippi, and there are some spots in Arkansas. At the Southern Weed Science Society meeting, Louisiana researchers confirmed they have johnsongrass that is resistant to both Select and Fusilade.“I don’t think glyphosate-resistant johnsongrass will spread like marestail or Palmer pigweed have, but we need to keep an eye out for it and do all we can to limit its spread.”

An increasing problem in Tennessee, Steckel says, is two glyphosate-resistant weeds in the same field — pigweed and marestail. “Generally, we’re seeing marestail following pigweed, and sometimes those are followed by giant ragweed, making a three-way problem. This makes control much more complex.

“I had a lot of calls last year asking what to do with fields that have both Palmer pigweed and marestail. I told them to forget the marestail and go after the pigweed, because it’s a lot more competitive. It’s the one we’ve got to try and control. It’s going to be the main weed we’ve got to deal with going forward. Just one pigweed per 3 meters of row can reduce soybean yield by 17 percent. If you have a greater number of pigweeds, yield losses can mount significantly.”

Pigweed, a native of the desert Southwest, “is an unbelievably competitive plant,” Steckel says. “From an agronomic standpoint, you can’t help but admire it — its taproot can go 5 feet deep, it can tolerate very high temperataures, and it’s amazingly drought- and stress-tolerant. Work at the University of California-Davis back in the 1980s showed it had the highest photosynthetic rate of any plant they had measured.

Pigweed: "An amazingly resilient plant"

“Our county agent in Crockett County sprayed some 8 to 10-inch Palmer pigweeds with glyphosate and nothing happened. He sprayed them with Ignite, which will control pigweeds if they’re less than 6 inches tall. Ignite burned them, but they came back. He sent in a chopping crew — but if you leave any root stump at all, it’ll send out auxiliary buds that will re-grow. If you pull them up and stack them at the end of the row or the edge of the field, more often than not they’ll re-root. It’s an unbelievably resilient plant.”Resistant pigweed is “on the move,” he notes. “We’re seeing basically a three-year progression from just a few plants to major infestation. Three years ago, you could hardly find one anywhere in middle Tennessee. In 2009, we found them in a few fields. Last year, we had them a big way.

“A single 5-foot plant can produce 500,000 or more seeds, which are easily spread by wind, animals, equipment. You may have just a few plants in a field this year, and if you run a combine or a picker through them, you’re spreading seeds everywhere, and next year you’ll have pigweeds galore.

“In Tennessee this past year, growers generally did a better job of managing Palmer pigweed in cotton than in soybeans. Of course, we had a lot more soybean acres, and it’s harder to get around timely with postemerge applications in soybeans. Also, with 38-inch rows, you can run hooded sprayers through the field, it’s easier to chop out weeds in cotton than in soybeans, and in cotton many growers have moved to an Ignite system.

“With Flexstar in soybeans, the cutoff for pigweeds is 3 to 4 inches, whereas for Ignite in cotton its 6 inches, 7 if you’re lucky.”

A key component in controlling pigweed, Steckel says, is residual herbicides, including Authority, Fierce (a new experimental material), Prefix, Optill, Prowl H20, Valor XLT, KIH485 (“which we were hoping would get labeled this year, but didn’t, so we’re looking for it in 2012”), and Valor SX.

The amount of control is important, he notes. “If you get less than 90 percent control, pigweed is going to get away from you. It can grow from 0.21 to 0.18 centimeters per growing degree day. If you go out in a middle Tennessee field mid-June, when daytime temperatures will hit the mid-90s and the nighttime low is about 70, if you have 2-inch Palmer pigweeds early morning, they can grow 2.5 inches that day.

“Twenty-four hours later, they can be 4.5 inches tall, too big for Flexstar to control. In 36 hours, can be too big for Ignite to control. At that point, you don’t have enough postemerge spray power to control these weeds. Two days can make all the difference in the world.

Timely preemerge, postemerge applications important

What does work? A preemerge application that gets enough rainfall (or irrigation) for activation, followed by a timely postemerge application, Steckel says.

“You may or may not have to come back with a second postemerge application, depending on how quickly your beans grow and shade the ground. A high rate of Envive followed by Flexstar on 3-inch Palmer pigweed works well. Boundary, a premix of Dual and Sencor, followed by Flexstar, also works well.

“What I am worried about is that this weed will begin developing resistance to the herbicides we’re plugging holes with now, like Flexstar and Ignite. If that happens, we’re sunk. I don’t think growers realize just how close to the end of the rope we are on modes of action for this weed. It’s scary.”

For postemerge applications, he says, “Flexstar is my pick of the litter for 1-inch to 4-inch pigweeds. Cobra and Blazer don’t have the residual that Flexstar does, and 2-inch pigweed is about the cutoff for them. With LibertyLink beans, Prefix followed by Ignite on 1-inch to 3-inch pigweeds works well. Envive followed by Ignite works well on weeds up to 6 inches. Warrant is a new herbicide, comparable to Dual Magnum, that is labeled for postemergence in soybeans up to R3 and in cotton post-cotyledon to first flower.”

Another change growers need to make, Steckel says, is in spraying procedures. “We need to get away from AI sprayer tips and booms that are 5-feet above the ground, bouncing up and down, and going through the fields at 18 miles per hour with an 18 mph crosswind; that just is not ideal for effective coverage with contact herbicides like Flexstar, Ignite, or Gramoxone.”

In Tennessee, for marestail control, the primary recommendation is dicamba mixed either with glyphosate or Gramoxone, he says. Kixor, a burndown product that can be applied all the way up to planting.

“There are a host of preemerge materials — Valor-based products and  Dual-based products do a pretty good job of residual for marestail and Palmer pigweed. Dual or other products as an early postemerge can give you overlapping residual when the preemerge is starting to play out.

“Once pigweed hits 2 inches, it can get away from you in a day’s time. I can’t stress strongly enough that it’s critical that you keep an eye on your fields so you’ll know if your preemerge is working and if you’re getting an overlap of residual.

“If you’re in an area that doesn’t have a lot of resistant Palmer pigweed, I’d at least put out a preemerge, if not an early postemerge — $12 or $13 worth of preemerge can be worth $30 to $50 the following year. Anything you can do to delay spread of this weed in your fields will be money ahead.”

A longer window of vulnerability

With Palmer pigweed, Steckel says, “We’ve got to start thinking about weed control as 30 days before we plant the crop to 30 days after we cut the crop — there’s a much longer window of vulnerability now. From the time you cut your corn until the first frost, your fields can build up a heavy load of pigweed seeds.

“One thing I tell growers who have a pigweed problem in soybeans is to rotate to corn; there are a lot of herbicides that will do a good job of controlling pigweeds in that crop.”

Clarity herbicide has done a good job of controlling marestail, he says, “but from walking fields in west Tennessee, I’m wondering if we aren’t beginning to see some tolerance for this product. The 8-ounce rate isn’t as consistent as it was a few years ago. A lot of farmers in west Tennessee are no longer using the 8-ounce rate; 10 or 12 ounces is fairly common now.

“We’ve looked at Sharpen for five or six years, and in research work it has been outstanding on marestail in some years, not so much so in others. Integrity, which was a premix of Sharpen and Outlook, was more consistent. It has been renamed Verdict and will be labeled at a 5 ounce rate on soybeans preplant and preemergence. If you’re going to look at a Sharpen product, this would be the one to go with.”

The LibertyLink technology is being readily adopted by farmers, Steckel says, “but I have concerns about its stewardship and management. The way we’re going, I’m afraid we could see resistance in three to five years.” Down the road, he says, the Dow Herbicide Trait (DHT) can be used with 2,4-D in cotton, and with Fusilade, Assure, and 2,4-D in corn. “The ETA for approval in corn is 2013 and cotton/soybeans 2015. In our research, the crop tolerance is outstanding.

“Glytol LibertyLink is also going to be a useful tool in cotton. Looking at new technologies, I think the dicamba trait and the 2,4-D trait will be useful tools to help bail us out of this glyphosate-resistance quandary.

“But,” Steckel emphasizes, “my take home point about herbicides is this: We must — we must — keep Ignite and Flexstar and the PPOs in play for Palmer pigweed for the next three to five years until we can get to these new technologies.

“If we can manage these two herbicides so pigweed doesn’t develop resistance to them, we may be able to ride herd on this weed long term. If we use rotation, different modes of action, and don’t rely on just one herbicide, I think we have a chance.”