Residual Herbicides for Soybean Weed Control
With the advent of herbicide-resistant (HR) weeds, especially those resistant to glyphosate (GR), residual herbicides applied before, at, and after planting have become increasingly important as a necessary component of a complete weed control program for soybeans. This has added an additional layer of complexity since available residual herbicides must be chosen on the basis of their mode of action (MOA) and how those MOA’s should be selected to complement later-applied POST herbicides to manage HR weeds that are present.
Soybean varieties that are resistant to glyphosate are now the major ones used in most soybean production systems. However, GR weeds have tilted the playing field so that other weed control measures in addition to glyphosate must be inserted into producers’ weed management programs. These include auxin herbicide-resistant (AR) soybean varieties and the aforementioned residual herbicides that have been available for decades.
Articles from Miss. State Univ. and the Univ. of Tenn. Scientists/Specialists that address the subject of using residual herbicides in an integrated weed control program for soybeans are linked and summarized below.
MSU Info. Sheet 1352, Apr. 2013, Dr. Thomas Eubank. This article titled “Herbicide Programs for Managing Glyphosate- and ALS-resistant Palmer Amaranth in Miss. Soybean” precedes the availability of auxin herbicide-resistant soybean varieties. Thus, it deals only with herbicide programs for Conventional, Roundup Ready, and Liberty Link varieties. Lists of herbicides that can be applied preplant incorporated (PPI), preplant/preemergence (PRE), and postemergence (POST) are given. Major points to consider when using any of the herbicide programs follow.
• Use of residual herbicides at full label rates is imperative in managing HR weeds.
• Efficacious residual herbicides can provide 2-3 weeks of pigweed control.
• Timely POST herbicide applications when weeds are no more than 2-3 in. tall are critical for controlling escaped pigweeds following application of PPI and PRE herbicides.
• Remember that some residual herbicides (e.g., Dual Magnum, Warrant, Zidua) can be applied in-season for additional residual control of pigweed.
MCS blog, Apr. 2015, Dr. Jason Bond. This article titled “Choices of Residual Herbicides in Miss. Soybean” also precedes the availability of auxin herbicide-resistant soybean varieties. Major points from the article follow.
• Most weed management decisions for soybean production are based on managing GR Palmer amaranth.
• Herbicide programs designed to manage GR pigweed should include herbicides with multiple modes of action (MOA) that are effective for controlling this and other HR weeds. This is a critical step in developing such programs.
• A numbering system to designate a herbicide’s MOA allows devising herbicide programs that avoid sequential applications of herbicides with the same MOA.
• Diverse herbicide programs that are designed to control GR Palmer amaranth and other HR weeds begin with the inclusion of residual PRE herbicides.
• A list of the most effective PRE herbicides and herbicide mixtures is provided.
• None of the PRE herbicides provided complete control of GR pigweed, so a POST herbicide application is required to control escapes.
MSU Info. Sheet 2022, Apr. 2015, Drs. Bond, Irby, and Reynolds. This article titled “Programs for Managing Herbicide-Resistant Palmer Amaranth in Miss. Soybean” provides information that can be used for herbicide programs that are designed for overall Palmer amaranth management regardless of which herbicide-resistant weed population(s) may be present. The listed recommendations were developed after confirmation of PPO resistance in Palmer amaranth in Mississippi.
Lists of herbicides that can be applied preplant, preemergence (PRE), and postemergence (POST) are given. Major points to consider when using any of the listed herbicide programs follow.
• Use of residual herbicides at full label rates is imperative in managing HR weeds.
• Efficacious residual herbicides can provide 2-3 weeks of pigweed control.
• Preplant applications of residual herbicides should be made 14-21 days before planting and after beds (if used) are prepared. Do not disturb beds after application.
• All listed herbicides for PRE application contain metribuzin. Since some soybean varieties are susceptible to injury from metribuzin, company information should be consulted for tolerance level of selected varieties. Also click here for ratings of current varieties for sensitivity to metribuzin.
• Spray coverage is critical for the contact herbicides in the POST list, so nozzles that will ensure thorough coverage should be selected.
UTcrops blog, Apr. 2018, Dr. Larry Steckel. This article titled “Soybean Residual Herbicide Consideration” is the most recent. Major points from the article follow.
• Growers must now be aware more than ever of herbicide MOA since they can no longer depend solely on a PPO residual herbicide such as Valor to provide consistent long-lasting control of Palmer amaranth.
• On sites where Palmer amaranth has not yet selected for PPO resistance, PPO residual herbicides will provide 100% control of this weed for 3 to 4 weeks.
• In order to provide complete or near-complete residual control of PPO-resistant as well as PPO-susceptible pigweed, growers are advised to apply a mixture of herbicides with 2 effective MOA’s [e.g. Authority MTZ (PPO inhibitor Group 14) and metribuzin (Group 5)] against pigweed.
The bottom line from all of the above is: 1) a weed control program for soybeans should include both PRE and POST herbicides to control/manage the increasing occurrence of HR weeds, and 2) it is mandatory that the selection of herbicides and herbicide mixtures to use in these programs should pay close attention to their MOA’s so that multiple MOA’s that are effective against both HR-resistant and susceptible targeted weed biotypes are used.
The era of using only POST glyphosate for weed control in soybean has long-since come and gone. Thus, producers are left with devising weed control programs similar to those that preceded HR weeds, with one difference. They now have to contend with HR weeds that will require a more integrated approach that includes consideration of herbicide MOA in the selection and use of both PRE and POST herbicides that are required to achieve acceptable control of problematic weeds in soybeans.
Composed by Larry G. Heatherly, Apr. 2018, larryheatherly@bellsouth.net