Crop & Pest Report

Accessibility


| Share

Glyphosate + 2,4-D Compatibility – Part 2 (6/29/17)

Question: I have been following the delay on Enlist registration from the inference that there is ‘synergy’ when glyphosate and 2,4-D are mixed together. Is there any truth to this potential antagonism?

Glyphosate + 2,4-D Compatibility – Part 2

Question: I have been following the delay on Enlist registration from the inference that there is ‘synergy’ when glyphosate and 2,4-D are mixed together. Is there any truth to this potential antagonism?

Answer: Mixtures of 2,4-D and glyphosate (in other formulations) have been used successfully for weed control in various systems for many years. Glyphosate + 2,4-D is still the standard herbicide mixture for fallow weed control in many dryland areas.

NDSU weed scientists have observed for many years the antagonism between different salts of glyphosate and 2,4-D. This illustrates a principle that many people may not be aware of. When glyphosate-ipa and 2,4-D-dma (dimethyl amine) are mixed, there is often grass antagonism. The different salts induce the reduced grass control similar to when hard water cations bind with glyphosate and weak acid herbicides. It is assumed that Monsanto knew this many years ago and developed LandMaster BW where both the glyphosate and 2,4-D contained the same salt (glyphosate-ipa + 2,4-D-ipa (isopropylamine)) which resolved the grass antagonism because the ipa salts were the same. The only commercial source of 2,4-D-ipa was in LandMaster BW. Many growers thought the activity would be the same if they mixed Roundup and 2,4-D independently to make their own LandMaster BW, but growers did not realize the salt formulation made a difference in control.

Antidotal evidence: In 2016 I attended a meeting where many weed scientists across the country discussed the over-use of PPO herbicides and development of glyphosate + PPO resistant weeds in soybeans. Some academicians across the country reported something very interesting – they have observed grass antagonism from RU Xtend [glyphosate-mea (monoethanol amine) + dicamba-dga (diglycolamine)]! They also observed that these dual salt herbicides coupled with the extremely large droplets from TTI increased the antagonism or reduced weed control. Their use of the term ‘antagonism’ is partly correct but the recommended TTI nozzles produce large droplets that may miss some small grass plants and result in lower weed control.

An historical example of this is the long-used product Fallow Master which contains glyphosate + dicamba, the same active ingredients as in RU Xtend. The difference was RU Xtend is formulated as glyphosate-ipa + dicamba-acid. The acid form of dicamba does not produce physical incompatibility with the ipa salt from glyphosate, which resolves the salt antagonism problem. However, the dicamba-acid exacerbates the volatility problem and is probably why it was not developed for the dicamba resistant soybean technology. AMS may lower the spray solution pH, forming dicamba acid which has high volatility and is why AMS is restricted. Spray acidifier adjuvants are not allowed with RU Xtend as low pH (below 5.5) will negate the mysterious Vapor-Grip technology and create more volatile dicamba-acid molecules.

I was surprised to hear of the synergism between glyphosate and 2,4-D and was not surprised that there was little published data to support it. The possible antagonism from the differential salts of glyphosate-dma + 2,4-D-choline seem more accurate based on historical data described above and observations.

Rich Zollinger

Extension Weed Specialist

Creative Commons License
Feel free to use and share this content, but please do so under the conditions of our Creative Commons license and our Rules for Use. Thanks.