As more and more corn is planted using some form of conservation tillage, it becomes increasingly imperative that weeds are controlled early.
“Just resist the urge to plant into a weedy area,” says Ken Smith, Arkansas Extension weed scientist. “Growers need to get a field clean before putting seed in. The temptation is there to plant and then run over the field with glyphosate or some other product. The thinking is, if a little atrazine is put out at the same time, it'll provide residual benefits and everything will be wonderful.”
That's okay in theory, but often not so in practice. “I've cautioned growers repeatedly on not getting behind the weeds. If we miss broadleafs at burndown — grasses too, but broadleafs in particular — then they'll put us in a real bind once the corn gets up and starts growing a bit.”
Arkansas research has shown that the first three weeks of a corn (or grain sorghum) plant's life is when weed competition is most harmful. Corn suffers a lot more early than it does at tassel, says Smith. That's not terribly surprising since the corn plants aren't large and don't have the root system to help overcome competition.
“So don't get behind. Get the fields clean, get a pre-emerge down and — assuming he's planted a conventional corn — a grower is probably looking at something like Dual, Lasso, Outlook and maybe an atrazine pre-mix. That'll give the crop a good start.”
In Arkansas, Growers usually come back with another shot of atrazine to get a total of around 2 pounds, says Smith. After that, it's normally “special circumstance” weed control.
“If a crop has nutsedge, Permit is probably the answer. If you've got some broadleafs that break through the atrazine, you'll look at 2-4,D or Callisto. Callisto came onto the market last year and has done a really good job on broadleafs. It's much safer for a crop than other products. For example, 2,4-D can lead to brittle stems while Callisto doesn't do that.”
In Louisiana, if growers plant corn early, it helps control weeds, says Roy Vidrine a weed scientist at the Dean Lee Research Station in Alexandria.
“One thing I've suggested, since atrazine now has a big target on it, is to not only use the new transgenic technologies, but also the traditional products available to us in the early season,” says Vidrine. “That way we can save as much of the atrazine as possible for the end. By harvest time, there will be far fewer stalks of corn wrapped in morningglories.
“We've been using Callisto since it was a numbered compound. It also works well when a quarter-pound of atrazine is added. That combo really shines as a broad-spectrum weed control. We also see use of Dual and Permit.”
The challenge Arkansas growers have isn't in handling weeds through lay-by, “but instead we're having troubles with late-season germination of morningglories,” Smith says. “Atrazine does fine on morningglories until late season when it starts to break down. When they emerge, morningglories run up corn stalks and cause us all kinds of problems at harvest. They may not really reduce yield, but they do reduce harvest efficiency.
“So, in the end, we may have less in the bin because we lost so much corn during a morningglory-infested harvest.”
Smith has also seen growers put Callisto on 30-inch corn late in the season. The product provides some insurance — some residual — into harvest.
“We're also seeing farmers going to 30-inch rows to get more canopy closure and to keep out late-season morningglories,” says Smith.
In his area, Vidrine says there's a lot of crop rotation going on. Into the mix goes cotton, corn, soybeans and — in some cases — sugarcane.
“With that kind of rotation going on, it's difficult for us to switch to narrower rows. Unless you switch over all crops to a narrower row, it would be too much trouble. The research does suggest that the closer the row spacing, the greater yield potentials, though. So it isn't a dead issue around here.”
Roundup and Bt
“Both Roundup and Bt corn is available. Bt corn has worked fantastic, according to entomologists,” says Smith who estimates around half the state's corn is in Bt varieties.
Not so popular are Roundup Ready corn varieties. Such varieties continue to lag in yield compared to the conventionals.
“It's just like every other Roundup Ready crop that's come out — yield drags at the front end. After a few years, the yields tend to catch up. Hopefully that happens with Roundup Ready corn. I'd guess there's less than 10 percent of the Arkansas crop in Roundup Ready corn,” says Smith.
Tracking research data
Smith gets numerous calls on weedy fields every year. He says it's amazing how what he sees in growers' fields tracks his research data. “It's almost like looking a mirror.
“Last year, in south Arkansas particularly, when it was time to put on the second shot of atrazine, we couldn't get in. The rains were coming down with few breaks in the clouds. As a result, we had corn that really had problems with grasses and broadleafs both.
“In such circumstances, our favorite thing to use is Accent. We were in 6-inch corn with 3-inch grass. That ended up hurting the end result. That's why early-season weed control is so important.”
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