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5 Things Missouri Weed Scientists Have Learned About Dicamba

Even dicamba formulations pegged as low in volatility for dicamba-tolerant crops still can be volatile.

Kevin Bradley and Mandy Bish, University of Missouri (MU) Extension weed scientists, have studied the use of dicamba in dicamba-tolerant crop systems over the past several years. Here, they discuss their observations and the work of MU weed science graduate students in researching dicamba in dicamba-tolerant weed management systems over the past few years. 

As we prepare for another year with the Xtend soybean and cotton system, we thought it would help to summarize some of the most important things we've learned about dicamba.  This has been a result of the research our outstanding graduate students have conducted over the past several seasons.

1. Dicamba can be detected in the air following treatment. 

This isn’t really any grand "new" finding. We’ve seen this trend in (MU) graduate student Shea Farrell’s research for the past two seasons. Shea is now finished with all his experiments, and his results clearly show dicamba concentrations can be detected in the air following on-label applications of the approved dicamba products: Xtendimax, Engenia, and Fexapan).

In other words, there is volatility of these herbicides. The real questions the industry should be asking are: 

  • How much volatility results in off-target injury? 
  • Can we do anything to minimize it? 

Shea’s results have also shown that highest dicamba air concentrations occur within the first eight hours or so after application. He has also found that higher dicamba air concentrations will occur if dicamba is applied on dicamba-tolerant crops in the evening during stable conditions that favor temperature inversions vs. daytime applications made in unstable, noninversion conditions. However, Shea detected dicamba in the air for as many as 72 hours after the initial application in the 2017 and 2018 season. 

2. Adding glyphosate (Roundup PowerMax, etc.) matters. So does spray tank pH.

In the past year, the results from several university weed scientists show adding glyphosate to an approved dicamba product will lower the spray solution’s pH. If the spray tank pH becomes too low, it is possible that dicamba can dissociate to the acid form that’s the most volatile dicamba. 

This is the reason for the statements that were added to the 2019 labels such as: "Knowing the pH of your spray mixture and making the appropriate adjustments to avoid a low-pH spray mixture (e.g., pH less than 5) can reduce the potential for volatilization to occur."

3. Soil pH matters.

We have another graduate student in our program, Eric Oseland, who is investigating the effect of soil pH on dicamba volatility. Eric conducted two experiments last year and will be doing the same again in 2019.

Thus far, Eric’s research shows the lower the soil pH, the more volatility there is with the approved dicamba formulations. While it may be that dicamba volatility from the soil surface may not be as significant of a contributor as volatility from leaf surfaces, we believe it is still an important piece of the puzzle that should be recognized.

4. Temperature inversions are common.

Last week we published an article, What Have we Learned From Four Years of Studying Temperature Inversions? That article summarizes our research on temperature inversions and how it relates to dicamba applications. 

5. Burndown applications of dicamba products can still cause problems.

This is the time of year when farmers apply spring burndown applications.

We’ve been on record in numerous articles and presentations over the past several seasons as saying that these herbicides are less likely to move off-target and cause injury when they are applied as a burndown in April and May.

It’s still true, especially when it comes to the likelihood of injuring your neighbor’s non-Xtend soybeans. However, less likely doesn’t mean not possible.

In 2018, we received more calls and complaints in April and May about off-target dicamba movement to specialty crops and trees than at any other time during the season. It is also probably worth mentioning that bud break and leaf unfolding typically occurs during April in many tree species. Therefore, these species can be especially sensitive to off-target movement of dicamba at that time. 

MU graduate student Brian Dintelmann's research on the effects of driftable fractions of dicamba on common tree fruit, nut, and ornamental species also shows that there are certain species that are especially sensitive to off-target movement of this herbicide. 

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