May 4, 2009
If luck holds and the weather allows, most Mid-South corn growers prefer to plant early, get a good stand and then take the crop to harvest. However, too often when that doesn’t work, growers find some of their acreage needs to be replanted.
WHEN CORN replanting is necessary, one hurdle is controlling horseweed as a new stand emerges Ken Smith, Arkansas Extension weed specialist, has some answers.
With that in mind, Ken Smith has lately been asked two major questions. First, farmers want to know how to get rid of partial stands in order to replant. Second, what’s the best approach when corn is coming up in a field where horseweed has already emerged?
“We’ve done some work on getting rid of old corn stands,” says the Arkansas Extension weed specialist. “Several products are labeled for this. One is Valent’s Select Max — at 6 ounces, it does a great job.”
However, after a Select Max application there’s a six-day waiting period before the crop can be replanted.
“That wait doesn’t sit well with some folks. They’ll say, ‘Well, it’s dry enough to get in the field to spray, so it’s dry enough to plant.’ But that six-day period is proper — there can be negative crop response without it.”
Something else Smith and colleagues have tried is paraquat (Gramoxone). Alone, it will take out some 40 or 50 percent of existing corn plants.
“That’s about the best it’ll do, and that isn’t good enough. The reason is paraquat burns so quickly when applied to a plant.”
On young corn, the growing point is still below the soil until the third or fourth leaf. Burning the top off young corn plants — like in an Easter freeze — doesn’t always necessitate a replant. But if one is required, a product that can get to the corn plant’s growing point is needed.
“We’ve found that adding as little as a pint of atrazine or 3 ounces of Sencor to Gramoxone does the trick. That combination jumps the control rate from 50 percent to 99 percent. It works very well.
“Obviously, there is science behind this. The way that paraquat works in the plant is to trap light energy and not allow it to be passed along to the plant during photosynthesis. Eventually, the plant membranes are destroyed, resulting in desiccation.”
There’s no data yet to show this is happening, but Smith says the only logical explanation is paraquat is actually moving in the plant. That isn’t supposed to happen. In most applications of paraquat there is no translocation.
“I’ve demonstrated this by covering part of a leaf with paper and spraying the other part with Gramoxone. The part that was sprayed will dry down and the other part is fine. So, it doesn’t move through the plant in normal applications.
“We think we’ve found a way to allow it to move in the plant. However, regardless of whether that’s the mechanism, or not, the control is dramatic. This works and we’ll recommend farmers try it if they have to replant corn.”
The translocation was also demonstrated in horseweed studies. “I won’t say there’s enough translocation in horseweed that Gramoxone will automatically be the preferred control material. Normally, Gramoxone just isn’t very good on horseweed.”
Whether the new management approach will be adequate to justify Gramoxone on horseweed, “I’m not willing to say. However, I am confident in dramatically improving paraquat efficacy on horseweed.”
Mid-South spring weather hasn’t been conducive for many burndown applications. That has put some corn fields in a precarious weed situation.
“Some put out a fall application of a residual herbicide like Valor, or something similar. The thought was we’d get into corn planting early enough that horseweed wouldn’t be as much trouble. Valor works all winter long and does a really good job. Most of the time it ensures a clean field when corn is ready to be planted.”
However, with this year’s wet weather and delayed corn planting, some of the fields already have horseweed. Other fields were too wet to burn down and winter weeds are abundant.
“Those have been sprayed with Roundup or Gramoxone-plus-whatever. Now, corn is being planted while horseweed has emerged.”
Will atrazine take the horseweed out?
“If they’re still in little rosette stages and germinated late this spring, atrazine at a 2-pound rate will likely take them out. However, if the horseweed is 2 to 6 inches tall at corn planting, my suggestion is to spray it with 8 ounces of Clarity or another dicamba herbicide.”
Many farmers ask Smith about crop safety when using dicamba over-the-top. “There’s plenty of safety if the corn has germinated, imbibed water and sprouted. If I’d just planted corn, I’d wait a few days — maybe a week — before spraying horseweed with dicamba. I wouldn’t spray immediately after planting. Wait until the corn seed germinates.”
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