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Efficacy of Residual Herbicides and their Influence on Soybean Yields

Published on Friday, April 07, 2017

As planting season begins, this is a great time to evaluate the effectiveness of your pre-emergence (PRE) herbicide program. When evaluating a program, take a look at your weed pressure, type(s) of weeds, crop injury, and/or your reduction in stand. No matter what post-emergent (POST) herbicide platform you select (conventional, LibertyLink®, Roundup Ready® or Xtend®), its success starts with your PRE herbicide.

An effective PRE will reduce the number of weeds present for your POST application, ultimately influencing the short and long-term success of your POST herbicide program. Any weed escapes from the PRE or POST application will increase your risk of developing resistant biotypes. PRE applications offer you the ability to apply more modes of action (MOA) and different active ingredients that may not be available as POST options. Keep in mind, many of the PRE herbicides available may have the same active ingredients, just in different mixtures (Figure 1). For a program to be successful, utilize different herbicide MOAs as well as different active ingredients to reduce the development of resistant biotypes to develop.


Figure 1. Herbicide groups and their active ingredients

Over the past two years, Beck’s Practical Farm Research (PFR)® has looked at multiple PRE herbicide programs at the Southern Illinois (SIL) site. In 2016, herbicide ratings were taken 28 days after treatment (DAT) to evaluate the efficacy of many of the common herbicides used in SIL (Figure 2).

 

 


Figure 2. Visual control ratings 28 DAT

 

 

In the plot for this study, the main weed pressures were waterhemp, yellow foxtail, and morning glory. For small seeded broadleafs such as waterhemp, all treatments resulted in over 90 percent control of waterhemp 28 DAT (Figure 3).

 

 

To target the larger seeded broadleaf weeds such as morning glory, Pendant™ 3.3 EC (Group 3) and Zidua® (Group 15) resulted in less than 80 percent control 28 DAT. All other herbicides that contained more than one MOA resulted in greater than 88 percent control of morning glory 28 DAT. All treatments except Pendant 3.3 EC resulted in greater than 90 percent control of yellow foxtail 28 DAT. The use of multiple MOAs broadens the weed control spectrum of the PRE herbicide application. This also allows us to control weeds that may have developed resistance to a certain herbicide active ingredients or MOAs. For example, the waterhemp at Beck’s SIL PFR site was believed to be resistant to Group 2 (ALS) herbicides. However, the products containing a Group 2 as well as other herbicide MOAs such as Authority® XL, Authority® Maxx, Fierce® XLT, Sonic® or Trivence® resulted in more than 95 percent control 28 DAT.

 

As the season progresses, PRE herbicides become less effective, allowing for a new emergence of weeds. The active ingredients in the herbicide and the amount of those ingredients can influence how fast a PRE herbicide breaks. The products that are the safest for the crop are typically the first to break. For example, Group 15 herbicides such as Zidua are very safe, even in wet cool conditions. By 39 DAT however, the waterhemp control offered by Zidua fell to 89 percent (Figure 8), as Zidua struggles with larger seeded broadleafs. Likewise, the control of morning glory offered by Zidua also fell from 76 percent to 63 percent by 39 DAT.  These Group 15 herbicides also have excellent grass control and Zidua provided 85 percent control of yellow foxtail by 39 DAT.

Group 3 herbicides such as Pendant 3.3 EC are also believed to safer for crops, however in 2016 Pendant 3.3 EC resulted in 78 percent control of waterhemp, 65 percent control of yellow foxtail and 50 percent control of morning glory at 39 DAT. The herbicides with two MOAs offered greater weed control compared to a single MOA. Herbicides with a Group 2 and Group 14, such as Authority XL, Authority Maxx and Sonic, resulted in more than 88 percent control of waterhemp, 87 percent control of yellow foxtail, and 91 percent control of morning glory. Group 2 herbicides such as chlorimuron and cloransulam provide excellent control of morning glory, therefore products like Authority XL, Authority Maxx (chlorimuron + sulfentrazone) and Sonic (cloransulam + sulfentrazone) provided greater control when compared to Group 14 and Group 15 herbicides such as Prefix® and Warrant® Ultra.

Group 14 and Group 15 herbicides struggled to provide control of morning glory, only averaging 73 percent control by 39 DAT. However, these same products (Prefix and Warrant) resulted in almost 97 percent control of waterhemp by 39 DAT. Warrant Ultra is fomesafen (Group 14) and acetochlor (Group 15) whereas Prefix is fomesafen and metolachlor (Group 15). Though they have two different Group 15 active ingredients, they both struggled to control the larger seeded broadleaf weeds such as morning glory. Thanks to the Group 15 herbicide, however, they were able to increase the control of yellow foxtail compared to Group 2 and Group 14 herbicides such as Authority XL, Authority Maxx and Sonic.

Authority XL, Authority Maxx and Sonic together averaged 87 percent control of yellow foxtail 39 DAT. Prefix and Warrant Ultra (Group 14 and Group 15) averaged 97 percent control of yellow foxtail 39 DAT. When it came to waterhemp control, both the Group 2 + Group 14 and the Group 14 + Group 15 herbicides were very effective and resulted in more than 90 percent control.

Herbicides with three MOAs such as Trivence and Fierce XLT resulted in the greatest control across multiple weed species. Across all three weed species present, the control provided by these two products never fell below 90 percent by 39 DAT. Most importantly, their control of waterhemp never fell below 99.5 percent by 39 DAT.


Figure 8. Visual ratings 39 DAT

During cool, wet conditions, stand lost and stunting can occur. This is especially true for Group 14 (PPO) herbicides. In this study, stand counts were conducted for each herbicide and compared back to the control at 28 DAT (Figure 9). Out of all the herbicides tested, Zidua was the safest but it also began to break by 39 DAT. Pendant 3.3 EC negatively impacted the stand and resulted in a 12,000 plant loss compared to the control. This, however, was not as substantial as some of the other herbicides that were tested.


Figure 9. Reduction in soybean stands compared to the control at 28 DAT

Though Group 14 herbicides may cause PPO injury in cool, wet conditions, products containing fomesafen (one of the actives in Warrant Ultra and Prefix) can cause less injury than other PPO such as flumioxazin and sulfentrazone. Warrant Ultra (Group 15-acetochlor) and Prefix (Group 15-metolachlor) also contain Group 15 herbicides and, just like Zidua, are relatively safe to the crop. Therefore, Warrant Ultra and Prefix were the only two MOAs that did not negatively impact the soybean stand. Warrant Ultra and Prefix also resulted in more than 90 percent control of waterhemp and yellow foxtail by 39 DAT. When it came to larger seeded broadleaf weeds such as morning glory, Warrant Ultra provided 75 percent control and Prefix provided 71 percent control. Group 2 and Group 14 herbicides resulted in the largest reductions in stand with Authority XL causing a 72,600 reduction, Authority Maxx leading to an 82,764 reduction, and Sonic resulting in a 66,792 reduction in stand compared to the control. However, their control of morning glory and waterhemp averaged 91 percent by 39 DAT. Fierce XLT resulted in a 50,820 reduction and Trivence resulted in a 65,340 stand reduction compared to the control. These same products also resulted in the greatest (and longest) control of waterhemp, yellow foxtail, and morning glory. A rescue application of Flexstar® and Roundup PowerMAX® was done at 39 DAT and resulted in a clean field. All treatments that had a PRE applied resulted in more than 97 percent control of all three weeds 40 DAT following the rescue application.

It’s important to note that some of these herbicides may negatively impact your soybean stand. The question is, how will that reduction in stand influence your yield? The Group 2  and Group 14 herbicides such as Authority Maxx, Authority XL and Sonic resulted in the largest reduction in stand (Figure 8). These same products, however, only resulted in a 3.7 Bu./A., 1.2 Bu./A. and 7.5 Bu./A. reduction in yield compared to the control, respectively (Figure 10).


Figure 10. 2016 PRE herbicide yield data

Authority Maxx caused a 82,764 reduction in stand but remained within 4 Bu./A. of the untreated. Zidua yielded the highest out of all the herbicides and was within a 0.5 Bu./A. of the control. None of the herbicides tested resulted in a yield loss of more than 7.5 Bu./A. compared to the control.

This spring is the perfect opportunity to evaluate your PRE herbicide program. Though your yield monitor may report your bottom line, when it comes to herbicides, it’s only a small part of the story. The best time to evaluate your PRE herbicide program is right before your POST application is made. Remember, multiple MOAs will help you control a broader range of weeds and will also help reduce the chance of resistance. While some herbicides may negatively impact stands, our data proves that soybeans can compensate for this reduction in stand. An effective PRE will reduce your number of weed escapes, ultimately increasing the efficacy of your POST program.

 

 

 

 

 

 

 

 

 

Practical Farm Research (PFR)® is a registered trademark of Beck’s Superior Hybrids, Inc. LibertyLink® is a registered trademark of Bayer. Roundup Ready® and Roundup Ready 2 Xtend® are registered trademarks of Monsanto Technology, LLC. Pendant™ is a trademark of Winfield Solutions, LLC. Zidua® is a registered trademark of BASF. Authority® brands are registered trademarks of FMC Corporation. Sonic® is a registered trademark of The Dow Chemical Company. Prefix® is a registered trademark of Syngenta Group Company. Warrant® is a registered trademark of Monsanto Technology, LLC. Fierce® is a registered trademark of Valent U.S.A. Corporation. Trivence® is a registered trademark of DuPont Pioneer.

 

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Author: Joe Bolte

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