What’s up with weeds

The weeds were popping right along with the crops after a soggy first half of the 2010 growing season at last month’s South Dakota State University (SDSU) plant science farm tour in Brookings, South Dakota.

Fortunately, though, SDSU scientists showed lots of good technology to keep those weeds in check. Here are some weed management ideas from the tour.


Clean Beans


These clean soybeans make it pretty clear Roundup Ready technology works, as Mike Moechnig, South Dakota State University Extension weed specialist points out. That’s especially true in this case, if you look a bit closer at the signs.

Do not try this at home

Yes, the 20 gallons per acre preemergence rate (and another 10 gallon per acre one) of Roundup WeatherMax are the actual rates in these plots! In comparison, Roundup WeatherMax’s label states the combined annual total of preplant, in-crop and preharvest applications must not exceed 5.3 quarts per acre. 

Glyphosate-induced manganese deficiencies?

So why the high rates? Well, there’s a belief by some researchers that a buildup of glyphosate over time in soils can trigger manganese deficiencies. The idea behind the high glyphosate rates was to see if any manganese deficiencies would result.

“So far, so good,” says Moechnig. “There is no sign of any deficiencies. We will examine yields this fall to see if there was any difference.”

High concentration heightens drift potential

A side finding of this study is how drift potential rises with concentrated formulations. This plot just north of the glyphosate-micronutrient study showed curtailed growth due to glyphosate.

“It was a calm day when it was applied,” says Moechnig. “The rate of Roundup used was like using a firehose to put out a candle.”

Moechnig notes much of the newer chemistry is highly concentrated, with rates of just several ounces per acre.

 “When you become more concentrated (with chemical), you have more chances of injury,” he says. “There are more potent droplets floating around. We have to be careful during application.”

Pre is the word

Take a look at the corn on the left, then on the right. Both are weed-free and have the  “bloody green” glow of healthy corn plants.

So why are the plants on the left much higher than the ones on the right? The taller plants received a preemergence herbicide treatment, the shorter ones didn’t. The early preemergence herbicide fended off yield-robbing early weed competition.

“Without pres, corn can take an early yield hit,” says Moechnig.

The SDSU scientists will take yield checks at harvest to see if any yield loss results. Data compiled by Iowa State University shows early weed growth from VE (emergence) to V2 (two-leaf stage) can slice .5 bushels daily over a 16-day period (8 bushels per acre total). A residual preemergence herbicide can protect fledging seedlings from this yield damage.

Tough sell

Still, spending an additional $10 to $15 per acre on a preemergence herbicide is a tough sell, considering the price of glyphosate has dropped like a rock in the past year. Still, more farmers are weighing the extra cost against yield losses keyed by early weed competition.

“It’s been a good year for pres,” Moechnig says.

Glyphosate-resistant weeds

Glyphosate-resistant weeds, such as common ragweed, have surfaced in South Dakota. “Roundup (glyphosate) is still tops (for weed control), but we are getting more glyphosate-resistant weeds,” says Moechnig.

The good news is there are some attractive alternative programs to the Roundup Ready system. Still, they face an uphill battle.

 “Last winter, we made a push to diversify (weed control) programs, but it is a tough sell,” says Moechnig. “Glyphosate, even with resistant weeds, is still the most consistent program out there.”

Moechnig notes the LibertyLink system featuring Ignite herbicide requires more management. “It is not as consistent as Roundup Ready, but with pres and tank-mix partners, it does well,” he says. 

Atrazine still keys corn weed control

Atrazine is the “baling wire and duct tape” of weed control in corn. This decades-old herbicide can still be found in many corn weed control packages.

In corn, for example, the old standby atrazine is a good tank-mix partner on corn fields plagued with wild buckwheat like in this photo.  Wild buckwheat is a problem in many South Dakota fields.

Atrazine lockdown

One drawback of atrazine, though, is it locks you into corn at planting. That’s not a serious hurdle in bluebird spring weather that allows timely corn planting. Adverse weather that pushes back corn planting, though, is another story.

If all else fails, you could always plant soybeans on those corn acres. However, that’s not an option if you have atrazine down.

“If you have to change planting plans from corn to soybeans, you are stuck,” says Moechnig.

Kixor products

Enter the Kixor stable of products from BASF that are new in 2010. BASF included soybeans under the OpTill and Sharpen product label, and corn under the Sharpen and Integrity product label.

One plus these products have is there’s less applied than atrazine.

Integrity, for example requires just roughly one-fourth of the application rate compared to the older, preemergence atrazine-based products, says Moechnig.

Another perk-no soybean lockout if weather nixes corn planting.

“With a 2 ounce rate of Sharpen as a pre, it may be possible to plant soybeans if your corn gets hailed out,” he adds.

The downside? Cost. Recommended rates of Kixor products used for these purposes at $10 to $15 per acre dwarf those of atrazine, says Moechnig. 

Crop safety test

Just to see if all this held true, SDSU scientists went with a high off-label rate of Sharpen at 6 ounces per acre. (Don’t do this at home—this off-label rate is for research purposes only.)

“We put the 6 ounce rate down two weeks ahead of planting,” says Moechnig. “We wanted to see if we could sill plant soybeans after it was too late for corn when Sharpen was down. So far, it’s not affecting the beans. The key is at Brookings, we have heavy soils with high organic matter. But with coarse soils, it could be a different story.”

Moechnig stresses that it is important to follow label guidelines regarding rotation restrictions, but this trial demonstrated the potential for crop flexibility when Sharpen or Integrity is applied.

Fierce herbicide coming

KIH-485 probably doesn’t ring a bell with you. It’s been around a long time, though, having been researched and researched and researched some more during 2000s without ever hitting the market.

This will likely change in 2011. That’s when federal registration of Fierce herbicide is anticipated. Valent U.S.A. Corporation, Kumiai Chemical Industry Company, and Ihara Chemical Industry Corporation plan to launch this preemergence residual premix of flumioxazin (the active ingredient in Valor herbicide) and pyroxasulfone (KIH-485). Company officials say Fierce will provide residual control of weeds including glyphosate-, ALS- and triazine-resistant Palmer amaranth, waterhemp, common lambsquarters, velvetleaf, dandelion, marestail, annual nightshades, pigweed species and annual grasses.



Read more about

Tip of the Day

Agronomy Tip: Manage Increased Weed Pressure

A water hemp plant in a farmer's hand. Preventive planting in 2019 has led to increased weed pressure.

Crop Talk