Herbicide-resistant pigweeds are a rapidly-expanding threat to row-crop agriculture. Researchers are hard at work trying to find the best way to regain control of burgeoning resistant populations.

At the recent PigPosium — a forum on resistant pigweeds held in Forrest City, Ark., and co-sponsored by The University of Arkansas Division of Agriculture and Farm Press — attendees heard that wick applicators are one of the control methods being studied.

“We’re here for a very serious reason,” said Eric Prostko, University of Georgia professor and Extension weed specialist, to a packed house. Pointing to a picture of a row-crop field overtaken by pigweeds, Prostko said, “This summarizes things.”

According to Prostko, among Palmer amaranth control tactics: starting clean, tillage, extreme cover crops, residual herbicides, irrigation, post-harvest (corn), hand-weeding, and non-selective applicators (NSAs).

The list above summarizes much of the ongoing resistant weed research in the Southeast and Mid-South. “There may be some variations based on some things we’re doing in Georgia a bit differently. We’re one or two years ahead of you in our fight against this problem.

“At the very end, I list hand-weeding and non-selective applicators. I’ll refer to the applicators as non-selective because there are more than just wick applicators available. Regardless, we want growers to do everything possible before they get down to those last two options. Those are certainly something we’re not used to doing.”

NSA technology isn’t new, said Prostko, as an alarming picture of a trailer full of huge, freshly-cut pigweeds appeared on the screen. “It’s been around 30 years, although we’ve never really used it that much. This illustrates why (NSAs are now in the mix). This is where we are in Georgia, now. … But this isn’t sustainable in the long run. In some cases, our growers are spending a lot of money hand-weeding. So, they’re interested in any option other than this — including NSAs.”

Types

Over the last two years, Prostko and colleagues have been “actively researching in the area of non-selective use. I thought there were a lot of things going on, and we didn’t have good science behind some of the recommendations being made. That’s how I got into it.

“So, we’ve looked at different applicators and I’ll share what we’ve found.”

• A traditional rope wick and gravity flow.

“This is one of the earliest designs and can be effective under certain situations. But things have come a long way since.”

• WickMaster Ropewick

“Another applicator we’ve looked at is the WickMaster Ropewick. This was made in Georgia and it’s a pressurized rope wick. Behind the apparatus there’s a small electrical pump that percolates the solution from the bottom to the top to keep the wicks more uniformly moist.”

• GrassWorks WeedWiper

“We probably have the most data on this particular implement. It’s a ‘carpet roller.’ Basically (it employs) a carpet-type material that turns in the opposite direction the tractor is traveling.”

• TopCrop Super Sponge Weed Wiper

This NSA is made by a company in Oregon and utilizes a sponge-like material to hold the herbicide solution.

“It was shipped in a box — the company wanted to make an implement that is easily shippable.

“We pulled it out of the box and kind of chuckled. It just didn’t look as stout as we might need.”

After testing the applicator, researchers found that “surprisingly, it turned out to be a very effective applicator. So, don’t always” go with your first impression.

• LMC-Cross Wick Bar

The LMC-Cross Wick Bar, made in Albany, Ga., “makes use of a pressurized system. The frame that holds the wick is sealed and it’s filled with air, which can be regulated. That keeps the wicks saturated to the point they need to be.”

Efficacy

How effective are the NSAs?

A 2010 trial compared the WeedWiper to the TopCrop Sponge. “The pigweed was 66 inches tall, on average. We were using a 50 percent solution of Gramoxone Inteon — that’s what we’re focusing on in Georgia, right now.”

Another study looked at the WeedWiper and LMC-Cross Wick — again using a 50 percent solution of Gromoxone Inteon.

“This fall, we were able to get a later trial in. The pigweeds were a bit smaller at a 34-inch average — although 34 inches is still pretty tall. Both implements were pretty effective in managing those larger populations.

“It’s interesting because we know that Gramoxone isn’t a translocated material, generally. But we’re seeing movement down in the lower portions of the plant. And we’ve gotten almost complete control in some cases.”

Summary of Palmer Amaranth control (percentage) with non-selective applicators using Gramoxone Inteon

Summarizing the data, so far, “I believe we have eight to 10 field trials. We’re certainly on a learning curve trying to conduct research with these type of implements. They’re all pretty much custom-made.”

Prostko and colleagues have collected the most data on the WeedWiper, which provides over 90 percent control.

“The GravityFLow wasn’t effective in one trial. The WickMaster was also less effective. The Top CropWiper was up there with 99 percent control. The LMC-Cross wickbar gave us about 85 percent control up to 30 days after treatment. Again, (those results) are from using a 50-50 solution of Gramoxone Inteon and water with the exception of trial four. You can see from these numbers there’s certainly potential for these implements to be utilized.”

Herbicides other than Gramoxone have been used with the NSAs.

“We have tried Ignite and Cobra in some of these systems. Neither one, in my opinion, is effective in NSAs. At least that’s true at this point — we may change our minds after more study. But I don’t feel comfortable telling anyone that Ignite or Cobra is effective in an NSA.”

Cost

How much do the rigs cost?

Prostko said the numbers may vary a bit, but says the following prices are close:

• 20-foot Weed Wiper + gauge wheels + 40 gallon tank = $6,800

• 20-foot Wickmaster = $5,500

• 6-Row (18-foot) LMC-Cross Wick Bar = $6,225

• 15-foot front-mount, pump-fed, TopCrop Super Sponge Weed Wiper = $1,995

“The TopCrop Super Sponge Weed Wiper has the lowest price tag. But I have some concerns about its ability to withstand some of the larger pigweeds we see. I have to wonder how that implement will stand up when dealing with large plants.”

Benefits

So what are the benefits of using an NSA?

First, particularly in soybeans, is improved harvest efficiency.

Second, NSAs allow improved fungicide application. “I spend most of my time working on peanuts. Not having those pigweeds when applying a fungicide is certainly beneficial.

“But probably the most important thing we’d do with these applicators is managing the weed seed. We tell Georgia growers that they must think about the weed seed bank. This is where I think the greatest effect of the NSAs comes in.”

Is an NSA cheaper than hand-weeding? “We have growers in Georgia spending upwards of $100 per acre to hand-weed. That isn’t sustainable.

“From what I’ve been told by some of the growers who are using these rigs, they use, roughly, a quart of solution per acre. If half of that is Gramoxone Inteon, that’s only $4 in product cost.”

Considerations

NSAs work best in peanuts, said Prostko, because the crop only gets 12 to 18 inches tall. That allows a good height differential between the crop and weed.

Recently Gramoxone Inteon received a 24-C label for peanuts in Florida, Georgia and South Carolina for the management of Palmer amaranth and beggarweed.

For the Mid-South, the big question is “how will the NSAs work in soybeans and cotton? My biggest concern is that soybeans and cotton are much taller than peanuts. You need a height differential (between the crop and weed) to get a maximum application to the weed and minimal application to the crop.”

And that brings up crop response. When using an NSA, “you must worry about drippage.”

Also: what products are legal to use in soybeans or cotton with an NSA? “Right now, I don’t know anything that’s legal other than glyphosate.”

Tips

All the NSAs aren’t equal, said Prostko.

“Some are more effective. Some are more expensive, as well.”

Farmers need to ensure at least a 50 percent wipe on a plant for an NSA to be effective. “That means that if you have a 50-inch-tall pigweed, that applicator needs to be set at 25 inches, or less.”

Remember, “you’re driving a tractor, not a star ship. As much as we’d like to be Captain Kirk, the faster you go the less effective the applicator will be. In most of our research we’re driving 3 or 4 miles per hour — maybe 5 miles per hour. Slower is better.”

If you use a wiper, it needs to be on the plant before seed is formed. Based on newer data, viable seed can be produced as early as two weeks after pollination — target applications for that time period.

“If you already have seed on a plant, you’re wasting your time with NSAs because it is likely the Gramoxone won’t affect seed germination.”

Read the operator manual, advised Prostko.” There are some good things in there, problems you can avoid.”

Maintaining stewardship “is extremely important. Again, we’re dealing with a (product) concentration that’s much higher than normal. If you’re using Gramoxone in a burndown prior to planting, that’s 2 pints per acre in 10, or 15, gallons of water. That’s roughly a 2- to 3-percent solution of Gramoxone. With the NSAs, you’re looking at a 50 percent solution. So, you must be more cautious on how you mix, where you mix, how the mix is transported, etc.”

Learning experience

Prostko related a recent NSA learning experience and the importance of minimizing drips.

“Because we didn’t have anything labeled, I hadn’t done anything on a large scale in growers’ fields. Once we got the label for peanuts, though, I decided to take a rig into a local grower’s field to see how it would work.

“The good news is most of the problems have been addressed by the manufacturers and are fixable. But if you’re using one of these, they’re likely to drip. You need to work with the machine — it’s part art and part science — because it takes a little experience to figure out how best to set up the machine and see how it works.”