Hit ’em High, Hit ’em Low

It’s only natural that the most promising research in the battle against glyphosate resistance is coming out of the Georgia cotton fields.

Macon County, GA is, after all, considered to be ground zero for glyphosate resistance in Palmer amaranth. The resistant strain was first reported there in 2004. Since that time, growers in nearly 40 different Georgia counties have been forced to deal with the costly pest on a year-round basis.

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“I’ve told everyone, my biggest challenge was to get my growers to accept the severity of the issue,” says Stanley Culpepper, Georgia Extension weed scientist. A noted voice in the battle against glyphosate resistance, Culpepper says he feels like Georgia cotton farmers are beginning to attack the resistant weed problem with the right attitude.

“Now resistant Palmer is continuing to spread in my state, but this is the best year we’ve had with growers fighting back. People have done all types of things to battle resistance,” Culpepper says.

The glyphosate-resistant Palmer “war” – as Culpepper refers to it – is a year-round fight, though. And as growers bring in the last of their crop, Culpepper is making his rounds this off-season to spread the news of some exciting new research. The message this go-round is simple: be proactive about glyphosate-resistant Palmer amaranth, and you can start in the fall months.

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Running for Cover

For years, Culpepper and others from his school of thought have been in favor of a cover crop in the fall months to keep soil erosion at a minimum. While experts in the area rely on herbicides throughout most of the year to combat resistance, that method is ineffective in the fall months against the primary resistant target, Palmer amaranth.

Because pigweed typically won’t emerge until April, fall applications of herbicides would only control secondary pests and would also keep the ground bare. Culpepper’s answer to the problem is to plant a cover crop in the fall with as much above-ground biomass as possible for spring plantings. Biomass, he explains, means actual “cover” – or parts of the plant that will block the sun’s nutrients from reaching the hibernating pigweed seeds buried just beneath the surface. Culpepper is adamant that attacking pigweed seeds is the key to turning things around in Georgia.

“The more biomass you have, the more mulching effect you have, the greater effect you will have in reducing Palmer amaranth emergence. So, for example, if we were going to compare a weed that will come up to your waste versus a rye cover crop that will get seven feet tall, the rye cover crop is clearly more effective because it gives you more of a mulching component,” Culpepper says.

While he hasn’t explored all available cover crop options, Culpepper says cover crop rye has proven more effective than wheat or oats, due to the amount of cover and residue rye can provide.

“As I was planting my cover crop for 2010, I still had residue from my 2009 rye cover crop,” says Culpepper. “And that’s really what you need if you’re going to try and get a benefit in controlling Palmer amaranth and other weeds by using mulch.”

The value in a cover crop is that it keeps sunlight from reaching pigweed seeds, which are often buried within one inch of the soil surface. While a Palmer seed has the ability to live for years, Culpepper’s research has shown that 50% of seeds will die within 18 months if they have not emerged.

The idea, then, is to get a cover crop in the ground as soon as possible following harvest so that cover crop can provide a maximum amount of residue by spring.

Going Deep

But cover crops aren’t the only weapons available to attack pigweed seed banks in the fall. Culpepper’s research suggests that deep tillage can be equally effective in knocking out pigweed seed. Because most of the germinating seed is located within an inch of the soil surface, inverting the land can go a long way towards burying its chances of survival.

Nearly 40% of resistant Palmer amaranth seeds that will emerge to challenge growers is buried at just half an inch deep. At a depth of four inches, however, virtually none of those seeds are able to germinate, meaning quite literally that none of them will ever see the light of day. And while a Palmer seed has the ability to live for years, research shows that 50% of seeds will die within 18 months if they are placed at a depth of at least two inches beneath the soil surface.

“The challenges we have in deep-turning is you have to be able to invert the land. You can’t just slide the land from side to side because all you’re doing is moving those seeds up in the top from side to side. You truly have to invert that top layer to a depth of four inches or deeper,” says Culpepper.

The benefits of deep-turning are two-fold. The process significantly reduces the Palmer amaranth population that a grower will fight in the following crop (often by 40% to 60%), but it also shuffles countless pigweed seeds deeper into the soil profile, in effect killing future populations before they ever have a chance. Again, Culpepper stresses that the longer those seeds stay in the ground, the greater the chance that they will die before they ever emerge.

Of course, deep tillage does have its drawbacks. No-till proponents will say it causes its own set of problems, and many growers are all-too familiar with the cost of such a project. Still, Culpepper argues, desperate times call for desperate measures.

“People have got to understand we’ve got to survive. And we’re not looking at deep turning every year, but hopefully one time every five years. Or maybe one time and then we get really good and aggressive and we don’t have to do it again,” says Culpepper.

One-Two Punch

What are most promising for Culpepper are the results he’s seen when he combines these two techniques. After only one year of research, the weed scientist saw 98% of control on resistant Palmer amaranth in a Roundup Ready system where he deep turned the land in the fall and immediately planted a rye cover crop on his plot. He notes that these tactics are one component in a systemic approach to this battle that includes residual herbicides and crop rotation, among other tactics. Still, the results are proomising.

“It’s the first time since 2004 (when resistant pigweed was first discovered) that we’ve had better than 90% control on a Roundup Ready program on dryland production, which is where our growers are struggling the greatest.

“But that’s huge for me, I’m telling you. So we’ll see if it’s repeatable and I’m not saying it is, but that one year’s data was very exciting.”

 

 

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