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2011 late-season report on Tennessee weed control

Overview of control of glyphosate-resistant pigweeds in Tennessee cotton and soybeans. Control programs discussed. 

Now that we are in August it is a good time to look back on how well we managed glyphosate-resistant (GR) Palmer amaranth in our cotton and soybeans in Tennessee.

While driving across Tennessee it is clear that this year’s Palmer amaranth weed control in cotton was, for the most part, good. The definition of ‘good’ in the GR Palmer era is not the same as back when glyphosate controlled Palmer and every field was pristine. The definition of ‘good’ now means that though most every field does have some Palmer in it, the infestation is at a low enough level to not be a serious threat to yield.

For more, see Steckel.

There are a number of reasons for this overall good weed control. First, this year, Tennessee cotton growers have moved almost completely from a glyphosate-based weed control system to an Ignite-based system. Chris Main, University of Tennessee cotton specialist, estimates that well over 80 percent of the cotton planted in the state was a WideStrike or LibertyLink cotton variety. We both concur that almost all that cotton received at least one Ignite (many got two) applications over the top to control Palmer amaranth. The ability to apply a herbicide over the top that can control Palmer that has escaped a residual herbicide has been critical to the consistency of our weed control successes.

Second, almost all our cotton acres get one -- and in many cases, two -- herbicides applied for residual pigweed control prior to cotton emergence. These pre-applied herbicides buy growers more time by delaying the first serious Palmer amaranth flush. This greatly increases the chances that a grower can spray Ignite on more of his acres while the Palmer is still small enough to control.

Third, almost all our acres had either Dual Magnum or Warrant herbicide applied over the top early post-emergence. These residual herbicides often provide enough residual control of Palmer to reduce the number of Ignite post applications needed.

Finally, most cotton growers have moved back to using hooded sprayers to clean up the row middles and provide a residual component for late Palmer control.

All of this sounds complex, expensive and time consuming. It is that. However, the system outlined above has proven to be successful this year. I believe it will be (with some variations) the weed control system we employ in our cotton for the next several years.

Of course this system has had drawbacks. The biggest has been that most of the acres that received a post application of Ignite were in WideStrike cotton. Widestrike cotton does not have the Ignite tolerance a true LibertyLink variety has. As a result, every time Ignite is sprayed on Widestrike cotton it injures it to some extent. Typically, the visual injury ranges between 15 to 25 percent. In most cases, the Ignite injury to the cotton has not been serious enough to reduce yield. However, in situations where the post Ignite application is tankmixed with another herbicide and/or in some cases insecticide we have observed some yield loss. This is particularly true if the cotton is already under stress. Most growers are willing to live with that potential yield loss as it is more than offset by the yield reduction that is assured with Palmer amaranth competition.

The second drawback has been the development of glyphosate-resistant goosegrass. Ignite will often struggle to control goosegrass that is more than two tillers. This can partially be offset by increasing the Ignite rate, but this option is limited on Widestrike cotton due to injury concerns.

Most growers have followed up with glyphosate to control escaped grass weeds in WideStrike cotton but now that goosegrass has become resistant to glyphosate this option is no longer viable in some fields. The results of our research this year would indicate that the graminicides (Select Max, Fusilade, etc.) that were widely used before the Roundup Ready crop era will have to be used again to control goosegrass going forward.

In general, the 2011 GR Palmer amaranth control in our soybeans has been worse than what we saw in 2010. I know of more 2011 soybean fields in Tennessee lost to GR Palmer pigweed than in the last several years combined. What I mean by ‘lost’ is that Flexstar was applied too late to the Palmer and the soybean field had to be replanted in a number of cases. In other cases repeated PPO herbicides were applied to try to rescue the soybeans without a lot of success.

Why did we have more problems with Palmer in our soybeans than our cotton?  I believe the main reason is that we have three times the soybean acres to control Palmer amaranth on than we do cotton. It is difficult enough to cover all the cotton acres timely in Tennessee let alone the 1.6 million soybean acres.

Moreover, from a post emergence standpoint we are relying on PPO herbicides (like Flexstar, Rhythm, Cobra, Ultra Blazer, etc.) which will not consistently control a Palmer pigweed much over 3 inches tall. Also, if the Palmer amaranth recovers from the first PPO herbicide application, there is no chance sequential PPO herbicides will control the Palmer.

In the Ignite-based system that was widely used in Tennessee cotton, a sequential Ignite application on escaped Palmer amaranth in the 4- to 7-inch range has rendered good control provided that the follow up application is no later than seven to 10 days after the first. Judging by the GR Palmer weed control results this summer in both crops, I think we very well may see a significant shift to LibertyLink soybeans in 2012. 

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