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LibertyLink system must be protected

Cotton growers have the opportunity to use LibertyLink varieties again this year, giving them a new option against weeds, most notably glyphosate-resistant Palmer amaranth. But North Carolina State University weed specialist Alan York says growers must work together to avoid allowing weeds the opportunity to develop resistance to glufosinate, the new herbicide product that LibertyLink varieties can tolerate.

“Looking down the road, we don’t see any new chemistry coming,” York says. “Primarily what we see are more traits being stacked together. Virtually everything we see coming in terms of new traits will be stacked with the LibertyLink trait. So Liberty is going to play an increasing role in the future. We have to protect it. We have to avoid resistance to it because it is going to be important to us.”

The story of resistance

Resistance to a chemical such as glyphosate, or the potential for the resistance to glufosinate, occurs because in the population of billions of weeds in the world there will be some — perhaps only a handful of weeds in the beginning — that won’t be affected by a particular herbicide.

At first these are unnoticeable because there are so few of them. But when the herbicide eventually kills all the other similar weeds in the field, only these few are left unharmed to spread their seeds. And with those seeds, they also spread their trait for resistance.

Multiple modes

The trick then, York says, is to use a number of products that are different: that have different modes of action, but each of which is potentially deadly to the weeds. In the rare instance when one mode of action doesn’t kill a resistant weed, the backup will.

That way, no weeds that are resistant to one mode of action will live to spread that resistance to offspring. York says the number of weeds that would be resistant to a multiplicity of herbicides (or other strategies — including in some cases, even chopping with a hoe) is infinitesimal.

Of course, using different products in order to have the potential of killing most of the weeds several times is relatively expensive. There is a worry that some growers won’t be responsible and do the responsible thing.

On the other hand, with no new chemistries in the wings, the extra expense up-front is worth it if it will prevent the eventual day from arriving when farmers will have no herbicides left able to handle weeds that have developed.

When glyphosate originally came out, it was a super-herbicide. Fields sprayed with it were as clean as a whistle from weeds — or appeared so. But now, resistant weeds have stolen much of its effectiveness. The same thing could happen with glufosinate.

“If we do things in a timely way, we can make a Liberty-only program look pretty good,” York says. “The thing is, we’ve been down that road of using one chemistry before, and it bit us. That is why we don’t want to depend too much on Liberty.”

York and several of his peers at other universities across the South, including Stanley Culpepper, Jeremy Kichler and Lynn Sosnoskie, have developed a strategy with several tactics they believe growers can use to avoid letting weeds develop resistance to glufosinate, the active ingredient in Liberty. See the bulleted points that follow.

Avoid resistance


Use at least five other types of herbicide chemistry in a Liberty-based cotton management program.

Always use residual herbicides at planting and throughout the crop.

Do not make more than two applications of Liberty per year.

Spray Liberty when the biggest pigweed in the field is 3 inches or smaller.

Never use a reduced rate.

Do not apply Liberty within one hour of sunset or sunrise.

Apply in at least 15 GPA (gallons per acre) using a speed, spray tip and pressure that deliver a medium droplet size.

Integrate herbicide programs with hand weeding, tillage and/or heavy rye cover-crop residue.


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GET WITH THE PROGRAM: As these photos from a Wayne County, N.C., test plot from 2009 show, a Liberty program can really be effective compared to an untreated plot. The program treatment on the bottom consisted of a paraquat burndown, a PRE treatment of Prowl and Reflex, a POST 1 application of Liberty plus Dual Magnum, a POST 2 application of Liberty, and finally, a layby treatment of Direx plus MSMA.

Courtesy of Alan York, North Carolina State University

This article published in the April, 2012 edition of CAROLINA-VIRGINIA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.