Are We Already Selecting for Dicamba Resistance?

Dr. Greg Kruger, Associate Professor at the University of Nebraska, oversees wind-tunnel testing of adjuvants and nozzles used with the new dicamba-resistant technologies. CropLife spoke with him about issues that have come to light since the 2017 dicamba roll-out, including takes on drift that some might deem controversial.

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Are We Already Selecting for Dicamba Resistance?

“Any time we take a spray and atomize it through a hydraulic nozzle, there’s going to be off-target movement. The difference is the dose response to dicamba on soybeans.” – Dr. Greg Kruger

CropLife: Have you been heavily involved in dicamba training to prepare for this season?

Yes – it’s a recurring theme (laughs). Applicators are recertifying, and also want their dicamba recertification (in order to fulfill the revised label requirements.) We had 225 applicators today at my training in York, which is a big crowd for Nebraska.

CropLife: Back in 2016 when we spoke, there was a dilemma about the need to get every tank-mixed combination of dicamba and adjuvants tested. It seemed like a tall order. Where are we at with that today?

It’s a little bit tricky to answer, but getting to the bottom of it, we got through the initial round of testing and got a lot of products approved with dicamba. There are probably some adjuvants and nozzles that certain companies would have liked to have seen labeled but didn’t pass for one reason or another.

There are still things that haven’t been approved, and we’re continually testing new formulations and other products that people want to see put on the label. It’s going to be a continual evolution as long as we have the dicamba product out there. There are a lot of options out there for guys for tank-mixing adjuvants.

One of the interesting things that we knew a bit about two years ago that’s become a lot more apparent, is that lot of adjuvants and other tank-mix partners require the use of specific drift reduction agent in addition to an adjuvant.

CropLife: So there is an issue with DRAs used in the dicamba tank mix?

A lot of our tank mixes require drift retardants. If we stack a drift retardant on top of ultra-coarse sprays such as a TTI nozzle, we’re pushing that droplet size really high. The TTI nozzle, for example, produces an ultra-coarse spray with 650-micron average droplet size on up. You start coupling that with a DRA, we push that droplet size even larger. Now instead of a 650-micron droplet size we might be 750 or 800. As we do that, we start reducing the coverage.

Also, the thing we’re seeing that as we do that, we’re reducing uniformity of that spray pattern. We’re seeing what we call “pattern claps.” Most of our nozzles are set up to have 110-degree fans. If we don’t get enough pressure on some of these highly viscous solutions, that fan might drop to something less than 110 degrees. If that happens, then we start to get uneven distribution of that spray across the boom. If the pattern drops to 65 degrees, for example, we get a gap between the two fans where we’re not getting good coverage any longer. So we are delivering a partial or sublethal dose.

My biggest concern going forward is that is going to impact the spray pattern. We’re not going to get nice, uniform distribution of that liquid across the boom. So that leads to incomplete weed control and failures, in some cases. When we get that, we get into the situation where we start selecting for resistant weeds. That is the biggest concern that we have.

CropLife: I was speaking with someone last year who discussed that applicators very often don’t use enough water.

One of the easiest ways to increase coverage is to increase volume. If we think about spray applications, there are only three ways to increase coverage: the first way is to make smaller droplets; we get exponentially more droplets and better coverage. The second way is to increase carrier volume. Say we go from 10 gallons per acre to 15 and then to 20 gallons per acre. As we’re increasing volume we’re increasing coverage. The third way that we have to increase coverage is around using chemistry. We use something that helps droplets spread over the leaf surface better. Beyond that, we don’t really have options to increase coverage.

If we think about the dicamba label and my options for increasing coverage, I can’t make smaller droplets because that puts me off-label. I can use some adjuvants, but likely the products that spread the liquid over the leaf are also going to make droplets smaller. I will have to use a DRA there. The only other option is to increase volume and it really doesn’t cost me anything.

Most of the time, increasing carrier volume is the easiest thing we can do to increase coverage and subsequently the control we get out of that application. However, to say it is best would be difficult because we don’t know the trade-off between what we gain in efficacy versus the impact from the change in droplet size from using some of the approved adjuvants.

CropLife: Have you found there are other improvements that should be made that aren’t in place right now?

Education is the first place. Maybe that’s my academic institution bias. I think the more that we can do to educate people on what happens with the application, and potential ramifications, the more cognizant people will become of what we’re doing and the implications of their decisions.

CropLife: What hasn’t been talked about or reported enough?

We’ve probably become too complacent in saying (the drift issue) is all volatility. I do think volatility is a component of off-target movement of dicamba, but I think walking away or dropping the mic on that really does a disservice to the other things that are causing off-target movement that may be as big or bigger components to the whole equation.

CropLife: Is it because it’s easier to pin the blame on a single cause?

I think you hit the nail on the head. I think part of it is because it’s complicated. It’s easier to say, “It’s volatility.” Let’s be honest, if it’s volatility, the only person that can fix it is the companies. If I start thinking of tank contamination or physical particle drift as a big component of what’s occurring, then suddenly it means the applicator changes the ways he does things. Part of it where to go, in terms of who’s at fault.

It’s unfortunate, because I think things like tank contamination are a much better explanation for the patterns we’re seeing in the field. Things like particle drift are good explanations. We just have to make sure that when we’re diagnosing problems, we’re not just looking for the easy way out.

CropLife: Are there any innovations that have caught your eye that will help decrease volatility or other factors related to off-target movement?

In my mind, as important as coming out with a new gadget, new tool, or chemical to manage particle drift or volatility, we have to think about putting the system all together. There are a lot of tools out there – spray hoods and advanced sprayers coming onto the market like the Weed Seeker.

As important as having something like that, we need to understand that we’ve got to get that sprayer set up right. We’ve got to get the right boom height, the right nozzle, and the right adjuvants in the tank, and really put all the pieces together. I tell everybody that minimizing off-target movement of dicamba is kind of like baking vs. cooking. When I cook, I sprinkle a little bit of less and a little of that. When I bake, I measure out every ingredient. If I forget one ingredient, what I’m trying to bake isn’t going to come out right. The same thing’s true with dicamba: for the recipe to come out the same every time, I must follow all of the directions exactly. I cannot just sprinkle in the components that I want to follow.

In a lot of cases where we’re seeing off-target movement, there is a piece missing.

CropLife: What piece is most often missing?

The biggest one we see violated is the buffer requirement.

I think we have a lot really good applicators out there that follow it, but we need 100% compliance. It can’t be 99% or 95%.

CropLife: Last season was supposed to be the bellwether, and drift complaints were still relatively high – in some states, they were even higher than in 2017. Yet EPA renewed the label with only some added restrictions. What did you make of that?

I hear what everyone is saying, but I’ve seen very little evidence of how much yield loss there was on the complaints on soybeans. Most of the complaints I hear come out of soybeans. I hear more of the complaints in the summertime, and for whatever reason we don’t hear much about it when the combines start rolling.

CropLife: It’s true that we haven’t heard comprehensive reports of yield loss from drift damage.

I hear about different states and their soybean yield contest winners getting drifted on. How can they be the highest soybeans grown in the state if it’s doing that much damage? You’re getting into an area, though, where I don’t think I or anybody else has the answers to these questions.

I know I’ve got a very different message than a lot of my colleagues, but I see the other side of it. I see the guys that are using dicamba to control weeds that would otherwise see significantly reduced yields. I’m not saying that I think it’s all right we have off-target movement, but I also think we have to be pragmatic about how we approach it. We have to look at it from a risk-benefit standpoint. If we are seeing cupped soybeans, but we’re not getting yield loss, I struggle figuring out what the risk really is.

CropLife: If they don’t use dicamba, perhaps they don’t have as many options and would suffer worse yield loss than from drift.

In my mind, the much bigger issue is all the other crops out there that are being reported damaged by dicamba: peach trees, cypress trees, fruits and vegetables, tobacco, and things like that. These are a much better barometer of how we’re doing than soybeans. It seems like everything coming from academics and media is all soybean, and my gut reaction is soybean is just a distraction from what the true problem is or isn’t.

We really need to focus on the other environmental impacts from the application. Some of those questions we don’t have any data on. What are the long-term implications of dicamba on trees for example? Those seems like more pertinent questions.

I think we have to be really careful in the message we put out there. Any time we take a spray and atomize it through a hydraulic nozzle, there’s going to be off-target movement. The difference is the dose response to dicamba on soybeans. I think in a lot of ways dicamba is showing what a lot of our other chemistries do – we’re just not getting high enough dose of those other chemistries to cause a visual response.

I’m reluctant to say that just because we see it we need to do something about it. We need to truly evaluate: When we say “damage,” what are we talking about? When we say “injury,” what are what are we truly talking about? Are we talking about a visual response in the plant, or are we talking about seeing something that’s causing yield loss? I don’t think those two things are exactly same.

Maybe we do need to get rid of the technology; maybe we can’t control it. But before we go there, let’s make sure we get all of the facts straight. If we just walk away from it, we could set a precedent for every chemistry going forward, and we don’t want to do that.

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[…] mix InterLock with that. The challenge with the dicamba system is that the nozzle we’re using increases the size of those droplets from approximately 650 to 750 to 900 microns. The recent University of Nebraska article talks about […]

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