Dicamba Mixing Order for Xtend Soybeans (05/27/21)
Recently, my colleagues in the southern US have reported some issues on the commercial level when spray applicators are mixing dicamba formulations approved for use in Xtend soybean, glyphosate, and Sentris (a Volatility Reducing Agent/pH Modifier). The University of Arkansas recently wrote an article about this issue and described excessive build up of foam and pressure in inductor tanks when mixing these products together. They were able to replicate this issue in jar tests as well (Article found here: https://arkansascrops.uada.edu/posts/weeds/sentris-glyphosate-tank-mix-compatibility-problems.aspx)
It appears that a reaction between a herbicide with low pH (glyphosate) and the VRA with a high pH (Sentris) is causing production of CO2 that leads to foaming and increased pressure in an enclosed tank (think of your favorite childhood baking soda + vinegar volcano experiment). The adjuvant load in many glyphosate products is likely adding to the amount of foaming observed as well. I decided to try and replicate the jar tests using both tap water and RO water in my lab. The mixtures I tested did indeed create plenty of foam, and some pressure increase, but I was not able to recreate the magnitude of problem that has been reported. I will point out that the level of foam created should be some cause for concern during the mixing process. Most commercial drift reducing agents (DRAs) that are approved for use with dicamba (and in fact are required for dicamba + glyphosate tank-mixes) will help reduce foam, and a defoamer should also be used if needed.
I think the important points for us to consider are to revisit the mixing order of the products. First and foremost, keep in mind that we never want pure products to come in contact with each other. So always make sure to start with plenty of water, allow a product to go into solution before adding the next product, and make sure to flush with plenty of water if using and inductor tank. Keeping our childhood volcano experiment in mind, we are never told to start with half a container of water, then add the vinegar, then the baking soda….it wouldn’t be as cool! We get a much better reaction by adding the baking soda directly to vinegar since they are pure products coming into contact with each other, which allows the chemical reaction to occur quickly without a lot of water to buffer any reaction.
Next, I would follow the mixing order recommended by BASF or Bayer (depending on which products you use). Both companies recommend to start with ½ to ¾ of a tank of water, then add the VRA and any defoamer as the next step. The Sentris label does a nice job laying out the tank-mix order for all products you might add in these mixes (http://www.cdms.net/ldat/ldHR8000.pdf). Hopefully by taking our time and following the correct mixing order, we will not experience the issues with excessive foaming and pressure buildup that some have seen in the south. Just another new reason to try our best to be patient and follow the steps when mixing during the heart of spray season.
Extension Weed Specialist