Mark VanGessel, Extension Weed Specialist; firstname.lastname@example.org
Antagonism between herbicides occurs when more than one herbicide is included in the spray tank and one herbicide reduces the effectiveness of another herbicide. This is not something new, we have been dealing with this issue for a long time. There have been reports of 2,4-D and dicamba interfering with the grass control from glyphosate. Today, rating one of my studies, I observed this occurring. Glyphosate applied with dicamba or tankmixed with fomesafen at a rate of 1.5 pts/A did not control fall panicum as well as glyphosate by itself. However, I did not see any antagonism with glyphosate plus fomesafen at 1 pt/A (low rate). I have had over 20 trials with Xtend soybeans over the past three years and this is the first time I have observed this. So, while there is a potential for antagonisms to occur, environment and other factors also determine if antagonism will occur. And it is not something that will occur every time an auxin herbicide and glyphosate are tankmixed together.
The only way to ensure antagonism does not occur is to apply these herbicides separately; which is not practical in most situations. To reduce the likelihood of antagonism, apply when weeds are actively growing, before weeds get too large, use proper rate of glyphosate, and use the appropriate recommended adjuvants. Antagonism can also occur if auxin herbicides are tankmixed with ACC-ase herbicides such as Select or Poast. So, I am not sure if including an ACC-ase herbicide is a viable option to overcome the auxin and glyphosate antagonism. There are a number of programs around the country are researching this topic, so I hope to have better recommendations for next season.
Photo 1. Glyphosate plus dicamba applied at V-2 stage
Photo 2. Glyphosate plus low rate of fomesafen