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Managing Weeds in a Roundup Ready, LibertyLink® and XtendiMax™ Platform

Published on Wednesday, January 03, 2018

When it comes to weed control, a post-emergence (POST) herbicide is one of the keys to a successful herbicide program. Today, there are different options to choose from such as LibertyLink®, Roundup Ready® and Roundup Ready 2 Xtend®. Each platform will have strengths when it comes to weed control.

Over the past three years, the team at Beck’s Southern IL Practical Farm Research (PFR)® site has been evaluating the Roundup Ready and LibertyLink platforms. In 2017, the Roundup Ready 2 Xtend platform was added and the study. In each platform, the treatments included: (1) a pre-emergence (PRE) herbicide with a POST herbicide, (2) a PRE with a POST herbicide plus an in-season residual, and (3) a two pass POST program (Table 1).


*Anthem® Maxx cannot be tank mixed with XtendiMax®, so it was applied in a separate application. For an in-season residual, use Warrant® with XtendiMax.
**For research purpose only and must be applied before R1.

Visual ratings were taken throughout the year at the Southern IL PFR site. In this study, Authority® Assist was used as the PRE and Anthem® Maxx was used as the in-season residual. Anthem Maxx contains Zidua®, which results in an in-season residual barrier that reduces new flushes of small seeded broad leaf weeds such as waterhemp and grasses. The purpose of this study was not to promote a single platform of weed control, but rather to determine the best management practices within each platform. Ultimately, we know that a PRE and POST herbicide program need to work together for season-long weed control.

SIL PFR Results

At the Southern IL PFR site, we observed that all Liberty® treatments applied at the V3 growth stage 22 days after treatment (DAT) were very effective and resulted in 99% control of both waterhemp and fall panicum (Figure 1).

Since Liberty was so effective, we did not see an increase in efficacy with the use of Authority Assist or Anthem Maxx. The fall panicum pressure at the time of application was minor. In environments with heavy grass pressure, consider the addition of clethodim to increase efficacy. In the Roundup Ready system, the use of Authority Assist followed by Roundup® PowerMAX® increased waterhemp control by 35% when compared to Roundup alone at 22 DAT. When Anthem Maxx was tank mixed with Roundup PowerMAX, the waterhemp control increased by 59% over applications of Roundup applied alone at 22 DAT.

PRE herbicides such as Authority Assist last, on average, for up to four weeks. However, there will still not be canopy closure four weeks into the season. Canopy closure is important because it limits the amount of light that reaches the soil surface and reduces emergence of new weeds. An in-season residual will result in another residual layer, reducing the emergence of new weeds. In this study, Anthem Maxx, which has both residual and POST activity, was used as the in-season residual. Anthem Maxx contains Zidua, (Group 15) as well as Cadet™ (Group 14). Group 15 herbicides result in residual activity that is very effective on grass and small seeded broadleaf weeds such as waterhemp. This will cause a barrier that reduces emergence for an additional four weeks until canopy coverage hopefully occurs.

When it comes to grass control, Roundup PowerMAX is a systemic product that is translocated throughout the plant which makes it very effective on grasses. In the Roundup Ready platform, all treatments resulted in greater than 98% control of fall panicum 22 DAT. In the Roundup Ready 2 Xtend platform, all treatments resulted in more than 89% control of waterhemp 22 DAT. The use of Authority Assist followed by XtendiMax increased waterhemp efficacy by 2% versus applications of XtendiMax alone. When Anthem Maxx was made in a separate application, waterhemp control increased by 10% compared to applications of XtendiMax alone. It’s important to remember that Anthem Maxx cannot be tank mixed with XtendiMax, so another Group 15 herbicide such as Warrant should be used. When spraying XtendiMax, make sure to first review appropriate tank mixes. Click here for more information. Though XtendiMax will not have activity on grasses, Roundup PowerMAX can be tank mixed with XtendiMax, which has proven very effective on grasses such as fall panicum.

By 28 days after making the R1 applications, all of the Liberty treatments resulted in greater than 99% control of both waterhemp and fall panicum (Figure 4).  

The Liberty application made at the V3 growth stage effectively controlled the weeds that had emerged prior to the application, and the R1 application controlled the weeds that emerged after the V3 application. Once the soybeans reached R1, the larger soybean plants reduced the light exposure on the canopy floor, which decreased the emergence of new weeds. Also, shortly after the V3 applications, moisture became very limited which reduced germination of weeds. The two-pass POST system was successful in the LibertyLink systems but must be avoided to ensure these systems remain effective. 

In the Roundup platform, all treatments resulted in less than 77% control of waterhemp 28 DAT. However, the use of Authority Assist followed by Roundup PowerMAX increased waterhemp control by 57% compared to applications of Roundup alone 28 DAT. The addition of Anthem Maxx increased waterhemp control by 62%, but was still unacceptable 28 DAT.

Though waterhemp control was a struggle, fall panicum control was 100% across all treatments by 28 DAT in the Roundup platform. In the Roundup Ready 2 Xtend platform, all treatments resulted in greater than 90% control of waterhemp 28 DAT. There was a larger response to waterhemp control with the second application of XtendiMax compared to a single application. Just like the Liberty treatments, the first pass of XtendiMax controlled the waterhemp that emerged at the V3 timeframe and the R1 application controlled late escapes. Once again, this is not recommended due to the use of a single mode of action (MOA) and the sight of action (SOA). All XtendiMax treatments resulted in less than 80% control of fall panicum.

By harvest, all treatments in the Liberty platform resulted in more than 98% control of fall panicum and waterhemp (Figure 7A).

In the Roundup platform, all treatments resulted in less than 67% waterhemp control at harvest. The addition of Anthem Maxx increased waterhemp control by 17% and 57% compared to PRE application followed by Roundup PowerMAX and Roundup PowerMAX alone, respectively. Though the in-season residual increased waterhemp control, control was still not acceptable. In the Roundup Ready 2 Xtend platform, all treatments resulted in greater than 90% control of waterhemp at harvest. All Xtend treatments resulted in less than 75% control of fall panicum. 

Our two-year data indicates that the use of residuals in the Liberty platform increased waterhemp control by 5% (Figure 7B). This 5% control increase will limit the number of late escapes as well as allowing the use of other MOA and SOA.Over the last two years, Roundup alone resulted in less than 29% control at harvest. The use of Anthem Maxx increased waterhemp control by 10% and 53% compared to a PRE application followed by Roundup PowerMAX and an application of Roundup PowerMAX alone. In 2017, moisture was limited so the activation of Anthem Maxx may have been reduced. In 2016, however, we had more moisture to activate the product and more moisture to increase weed pressure, thus showing the importance of an in-season residual. In 2017, this study was also placed in an area which had greater waterhemp pressure compared to 2016, proving that in heavier waterhemp pressure more layers of residuals would need to be applied.

This study was planted in early June and did not experience any PPO injury. Therefore, all residual treatments, except those in the Liberty system, increased yield.

The variation in response to residuals in the Liberty treatments was most likely caused by the limited weed pressure. However, it’s important to remember the importance of these residual products. In the Roundup platform, both applications with residuals resulted in an increase in yield compared to Roundup PowerMAX alone. This most likely contributed to the waterhemp pressure we observed. Though the residual may not have resulted in acceptable control in 2017, the residuals still increased weed control. Therefore, the less competition increased the yield compared to Roundup alone treatments. In the Xtend platform, treatments that had residuals resulted in up to a 4.2 Bu./A. yield gain compared to XtendiMax alone. The Southern IL PFR site experienced a large amount of grass pressure in the XtendiMax alone treatments, which negatively impacted the yield. This could have been avoided if Roundup PowerMAX was added to the tank. Averaged across all platforms, Authority Assist and Authority Assist followed by Anthem Maxx increased yield by 1.5 Bu./A. and 0.6 Bu./A. when paired with a POST herbicide compared to POST alone. Though it was only a slight yield gain, there were less weeds present at the time of application with the use of residuals.

Weed control in the Roundup Ready platform varied based upon the weed spectrum. Weeds such as fall panicum could still be effectively controlled with Roundup PowerMAX. Glyphosate-resistant weeds were a struggle, even with residuals, in 2017. This shows that when there is heavier weed pressure, more layers of residual will be needed. In a Roundup Ready system, residuals are critical in preventing the emergence of glyphosate-resistant weeds because POST control options are limited. These weeds can still be managed, but layers of residual as well as moisture to activate these residuals will be necessary. Both the LibertyLink and Roundup Ready 2 Xtend platforms were very successful at controlling broadleaf weeds such as waterhemp, which are known to be glyphosate resistant. In the Xtend platform, tank mixing Roundup PowerMAX was crucial for grass control, even if residuals were used. The two-pass POST system was successful in the Xtend and LibertyLink systems, but must be avoided to ensure these systems remain effective in weed control. Selecting the right PRE herbicide for your weed spectrum can ultimately lower the amount of weeds present at the time of your POST application. Though in-season residuals may not always pay, they are key components to an effective herbicide program.

 

Joe Bolte, CCA | SIL PFR Operator/Herbicide Specialist

 

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Author: Joe Bolte

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