Soybean and cotton growers are likely to get some new weed control technology over the next 2-3 years and using it properly is going to be critical to both its early success and its longevity, says North Carolina State University Weed Specialist Alan York.

As early as this year soybean growers may have access to Xtend, a combination of dicamba and glyphosate for use on transgenic plants with tolerance to the two herbicides.

Enlist, a combination of glylphosate and a new choline formulation of 2,4-D for use on plants with double stacked weed protection technology from these popular herbicides will likely follow in the next couple of years.

What products ultimately come out won’t change the need by growers to use these products correctly.

It also won’t change the fact that 2,4-D and dicamba have been around a long time and while these chemistries will add  something to weed control in cotton and soybeans, they do not represent a new silver bullet for guaranteed control.

Several future combinations of herbicides will likely contain glufosinate, the active ingredient in Liberty herbicide.

When to use these herbicides may be a critical factor on how well they work, York says.

For several years Ignite (glufosinate), now sold as Liberty, has had on its label precautions against spraying two hours before sunset or two hours after sunrise.

“I didn’t think too much about it. I figured the label was primarily for the Midwest, where velvetleaf is a leading weed problem. It droops its leaves at night and with leaves drooped, coverage is not good,” he says.

“With all the new combinations of herbicides coming down the road in the next few years, we decided to take a look at spraying glufosinate too early or too late. Not all the new combinations of herbicides will contain glufosinate, but several will,” he adds.

Began before sunrise

“We started spraying before sunrise — I mean it was dark when we got to the field. We sprayed glufosinate every hour until sunset, again it was dark when we left the field.

“The results were as surprising as most any I’ve seen in all my years of doing weed studies,” York says.

The first glufosinate application the North Carolina State research team applied didn’t look much different than the check plot and the next hour wasn’t much better.

By two hours after sunrise, control was close to 100 percent. They came back and sprayed the same plots with the same materials 14 days later, but waited until two hours after sunrise to start.

The second application masked the loss of efficacy from the first application, but it was still there, York says

Control didn’t vary much until the researchers got to within two hours of sunset. The results weren’t so dramatic the hour before sunset, but there were differences in control. “When we got closer to sunset, control dropped off dramatically,” York says.

“We were only looking at control of Palmer amaranth, so it’s hard to know whether the time of day of application will have a similar impact on other herbicides.

“It sure piqued our interest in looking at application time impact on other herbicides and on other weeds and grasses,” he adds.

Though grass wasn’t officially part of the study, there was some grass pressure in the North Carolina State test plots.

York says it appears there was less dramatic, but some reduction in control on grasses in these tests when glufosinate was sprayed early and late in the day.

The researchers ruled out humidity related impacts and dew as causes for reduced control early and late with glufosinate.

York says the loss of control is related to light, but exactly what the mechanism for causing the light-related loss of efficacy is not certain.

Time of application isn’t the only concern growers will have with some of the new transgenic crops and subsequently their resistance to long-used herbicides like 2,4-D and dicamba.

Drift always a concern

With the diversity of crops grown in the Upper Southeast, drift is always going to be a concern, and with 2,4-D and dicamba in the mix, old horror stories of drift damage to non-target crops should be on the mind of growers.

Next season dicamba and glyphosate resistant soybeans may be available to growers, and along with the new technology will come Xtend — a combination of glyphosate and dicamba.

For growers who remember using Banvel, the concern over vapor pressure and vapor drift to non-target crops is still around.

Then came Clarity, which had significantly lower vapor pressure and much less threat from drift. “Xtend will have a lower vapor pressure than Clarity, so we’re not too concerned about vapor drift,” York says.

A bit farther down the road, cotton and soybean growers may have access to seed with triple stacked herbicide tolerance to glyphosate, glufosinate and dicamba. And in the same time period, they may have Enlist/Duo, which adds 2,4-D to the mix.

2,4D ethers have long been linked to vapor drift and non-target crop damage. “The new formulations will have significantly less vapor pressure than these older materials, and if applied properly, should have little risk of drift and damage to non-target crops, the North Carolina State Specialist says.

York says that growers should approach this new technology with the upmost of care.

“Glyphosate tolerant crops and using the herbicide to kill a broad spectrum of weeds and grasses was the best weed management technology we will ever have, and we blew it, by over-using it, York says.

“Glufosinate, or Liberty herbicides used on LibertyLink crops is good technology and the only real new technology available to growers for weed management.”

York says that many of the new double and triple stacked cotton and soybean varieties will have glufosinate tolerance in the mix. He urges growers to avoid the same mistakes made so frequently with glyphosate tolerant crops.

Despite the cautions against overuse and misuse, York says long-term studies indicate this new technology can significantly lower the seed bank of Palmer amaranth.

“None of these combinations of herbicides is going to be a new silver bullet for weed control, but they can be good tools, if we use them wisely,” he says.

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