Robbing the Seed Bank
April 19, 2010 
Ken Smith isn’t the only weed scientist in the Cotton Belt to identify the soil seed bank as a key battleground in the war against glyphosate resistant Palmer amaranth.
He might be the first, however, to frame his message with words like “sustainability” and “simple math.” The University of Arkansas Extension Weed Specialist wants to make sure growers recognize the new measure of success against the resistance epidemic.
“We have taught you ― and you have thought ― that the way to develop a threshold for weed tolerance is when it starts reducing yield. That may be the wrong threshold to look at,” Smith says. “The right threshold may be the soil seed bank. That’s the number of weed seeds that are in the furrow slice or top few inches of soil.
“If we are adding more seed to our soil seed bank than we are taking out, we are not in a sustainable agriculture. And that’s where we have been in the last few years. We have to turn that around.”
The statistics back up Smith’s claim that management practices need to change in Arkansas. Last year weed experts estimated that 750,000 agronomic acres were infested with glyphosate resistant Palmer pigweed. Of the state’s 250,000 cotton acres, approximately half had resistant pigweed.
The Plan
In some of the more extreme cases in the Southeast, weed scientists have advocated deep tillage as an option to bury pigweed seeds deeper below the surface. That option simply isn’t applicable in the Mid-South, where growers need to plant into a seed bed.
“And so when we bed back up, and bring those bedders through there, we bring some of those seeds back up and we lose that benefit of deep tillage. So we’re looking for other ways of managing that seed bank,” says Smith.
The solution, he says, is a zero tolerance method. Growers should start clean with a residual herbicide, and be careful to choose one with the residual activity to last until a second application can be made.
“Then they will have an opportunity to scout from that point on. And if (the residual activity) does break, then get right back in there with a post-direct application. There are two or three good herbicides they can use, like Caporol, MSMA, Valor, or Reflex. Probably all of those will fit into that post-direct application,” says Smith.
Applications of glyphosate still have a fit in this system. In a situation where a grower has made an initial application of Dual or another metolachlor, and a weed like morninglory or teaweed breaks through, then a glyphosate application may be in order. The key, as always is to diversify the herbicide program.
After a solid layby program, growers using this system would probably only see a few escapes later in the season. That’s when Smith’s zero tolerance method calls for even further action.
Foot Soldiers
“If we have those few escapes per acre, they’re probably at a level that’s not going to reduce our yields. But if we want to take that extra step, go in and hand-weed those escapes. I thought we would never, ever say go back to hand weeding, but we will hand weed much of our cotton,” Smith says.
“If we can take that one extra step to take out those seed producers, and get those out of there, then I think that’s a real step toward reducing that soil seed bank.”
While the idea of reverting back to hand-weeding may seem drastic to many growers, Smith believes the “hoe-crew” system should be easier to operate this year than in year’s past.
“I know that there are reasons why we don’t want to hand weed. People say it’s expensive, and it’s hard to find the crews. But in reality, the crew thing seems to be much more available now than maybe two or three years ago. Because some of these crews have realized that this is a pretty reliable thing,” he explains.
“In most situations it’s walking through fields with the hoe on your shoulder, and chopping one here and walking and chopping one there. Its not walking and ‘chop, chop, chop, chop, chop’.”
Many experts across the Belt agree with Smith that it will take that kind of dedication to defeat the spread of glyphosate resistance.
“This issue is not going to go away. It’s just going to take a little different approach to farming than we’ve had in the past,” Smith says.
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