Researching herbicide tolerance in winter wheat breeding with Melinda Zubrod

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Drew Lyon: Hello, welcome to the WSU Wheat Beat podcast. I’m your host, Drew Lyon, and I want to thank you for joining me as we explore the world of small grains production and research at Washington State University. In each episode, I speak with researchers from WSU and the USDA-ARS to provide you with insights into the latest research on wheat and barley production.

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My guest today is Melinda Zubrod. Melinda is a second year Ph.D. student working with Dr. Arron Carter in winter wheat breeding at WSU. Melinda received her bachelor’s degree in agronomy from Iowa State University in May of 2023 and moved to the Palouse to start her Ph.D. in plant breeding. Melinda’s dissertation focuses on utilizing high-throughput phenotyping to identify herbicide tolerant lines in the WSU Winter Wheat Program.

Hello, Melinda.

Melinda Zubrod: Hi, Drew.

Drew Lyon: So, what is high-throughput phenotyping and how is it applicable on the field scale?

Melinda Zubrod: So, in plant breeding, a phenotype results from the interaction between genotype and environment. And a phenotype is something measurable like plant height, grain yield–things like that. Ways that we can apply high-throughput phenotyping on the field scale are just measure all of those variables and try and use that data for breeders to make selections.

The kind of data that I work with is spatial or imagery data. There are multiple ways that we can collect that. First of all is with satellites. That’s not really good for our plot-level analysis, just because the resolution isn’t very good. There’s also a lot of handheld sensors out there that you can use to take images and get different phenotypic data from your plots, but that’s still pretty labor intensive and time consuming because somebody still has to walk the field and use those sensors, and a lot more things can go wrong.

And in my opinion, and many others, drone-mounted different kinds of sensors are the most efficient just because they’re time effective and you’re able to collect plot-level data from your breeding trials. And then, there are a lot of different sensors that you can use on a UAV or an unmanned aerial vehicle. So, some popular sensors would be like a thermal sensor, which is used a lot for analyzing drought stress and measuring stomatal conductance of plots.

There’s also red, green, blue imagery, which captures the wavelengths of light that you and I can see–the visible spectrum of light. And then the last type is hyperspectral and multi-spectral sensing, which measure wavelengths of light reflected from the Earth. Hyperspectral sensors measure multiple narrow bands of light and that can range anywhere from 20 to thousands of bands of light, narrow bands of light. And then, there’s multispectral sensing, which does pretty much the same thing as our hyperspectral sensing, but it captures fewer broad bands. And in the WSU Winter Wheat Program here, we utilize a multispectral sensor just because it has the bands of interest we’re interested in for winter wheat analysis and because it’s the most cost effective and efficient strategy that we’re able to utilize. So that’s what we use.

Drew Lyon: Okay. So, do you actually do the drone flying or is somebody else doing that?

Melinda Zubrod: Yeah. So, I am certified FAA drone pilot. And so, we have lots of different locations across the state. So, my job during the summer is to go out and fly the drone over our plots and then analyze that data and get it back to my boss for him to use in different genomic prediction models. And then, like with my dissertation, in particular, looking at herbicide injury.

Drew Lyon: Okay. Well, congratulations–that’s a nice, fun thing to do, but it also does require a little extra study on your part, doesn’t it?

So, we’ve talked about herbicide resistance a fair bit on this program. Can you describe how wheat cropping systems in the PNW led to this herbicide tolerance or resistance problem in weeds? And how as breeders, you’re working to overcome this herbicide tolerance?

Melinda Zubrod: Right. So, in the past like 50-ish years, a lot of really diverse crop rotations have been simplified. Here in eastern Washington, we have two main cropping systems we use. In the drier land regions, winter wheat fallow rotation is utilized, just to leave enough moisture in the ground for that following wheat year. And in a lot of those systems, chem fallow is used. And with only a single crop, the herbicide modes of action—a herbicide mode of action, in case somebody doesn’t know, is just how the herbicide is metabolized and kills the plant. And so, utilizing the same types of herbicide modes of action has led to resistant weed populations.

And then, here in higher rainfall regions such as Pullman, we utilize like a spring crop–like barley or a spring wheat–winter wheat, and then a pulse crop rotation and the same set of agronomic conditions being applied to a weed population over time–just like herbicide modes of actions, different tillage, the same crop–that is leading those weed populations to be more resistant or tolerant to those herbicides we apply and making it harder for growers to control weeds in their field–especially in that winter wheat fallow rotation where there’s not many options for rotational crops.

Here in eastern Washington, the most common herbicide mode of action is a Group 2 herbicide. And that herbicide is used in the Clearfield wheat production system, as well as in a lot of pulse crops, which is a popular rotational crop. And so, that has caused a lot of issues with weed resistance and tolerance to those Group 2 herbicides.

And so, as breeders and in the WSU Winter Wheat Breeding Program, we are working on incorporating resistance or tolerance to different herbicides into commercial wheat varieties to just provide producers with other options to control those resistant weed populations in their fields.

Drew Lyon: Okay. So, allowing them to perhaps bring in a different herbicide mechanism of action into the program that maybe they haven’t used before to… Okay. I’m kind of curious, as a wheat breeder, how have you found the study of herbicides to be a little bit different area than a lot of genetics students. Have you been taking courses or just doing a lot of book reading or what’s the approach there?

Melinda Zubrod: Definitely a lot of book reading. I did get my undergraduate degree in agronomy, so kind of just like a broad overview of agricultural systems. So, I have taken a couple weed science classes. I am taking more, and yeah, just a lot of literature review and learning, especially like how the modes of action in herbicides I’m working with, in particular, affect and kill the plant and then how I can use the drone to like quantify those injury symptoms for different herbicides.

Drew Lyon: Okay. So, Group 1, the ACCase inhibitors, Group 2, the ALS inhibitors, and Group 4, the auxin imitators, tend to be the herbicides that are used a lot in small grains. So, what herbicides are you exploring in the breeding program for breeding purposes?

Melinda Zubrod: Right. So, the two herbicides that–this is actually objectives or chapters one and two of my dissertation–is incorporating Group 1 herbicides Quizalofop resistance in winter wheat, which is the CoAxium wheat production system. There are already CoAxium tolerant lines on the market, but I’m just working to make the breeding of those more efficient because usually breeders will walk the field and take herbicide injury ratings based on a healthy check of the same variety. And they’ll just walk through and say, “Oh, that looks 80% injured; that looks 30% injured.” So, I’m just trying to find a baseline for that quantification of injury that can be used across like multiple breeders and multiple environments and things like that.

And then we’re also looking into using Group 5 herbicides, which is metribuzin tolerance. And so, Group 5 herbicides inhibit photosystem 2 at site A. So, our main injury symptoms there are like chlorosis or yellowing of the leaves, and as you may imagine that’s pretty easy to pick up with our drone-mounted camera. And so, there are lots of different vegetative indices that we are using to be able to quantify that injury and kind of make a baseline for not only breeders in our program but breeders everywhere.

Drew Lyon: So, you’re using drone systems to measure this. How do you go about making sure that your drone is measuring what you want it to measure and figuring out what it is that you actually are measuring using the drone? Because I assume that’s important to your work.

Melinda Zubrod: So, I’m running a lot of correlations and regressions with yield and also that visual injury score given by the breeder. And what I found when I was kind of writing up my metribuzin study is that there is a very weak correlation between the visual injury score the breeder gives and final yield.

And so, I’m trying to use the drone and those wavelengths that we can’t see to see if there are–is it any like permanent injury to the plant that the breeder can’t see, like even when it looks like it has come out of herbicide injury or things like that. We can use a vegetative index to quantify that injury and get a better correlation with our final yield than just the breeder walking the field.

Drew Lyon: Okay. So, a breeder’s probably just looking at that chlorosis/necrosis-type thing and you think your drone might be able to pick up some other symptoms that the human eye or the subjective evaluation isn’t. Okay. Do you feel like you’ve made some progress there?

Melinda Zubrod: Yeah. So, we are seeing stronger correlations. Two of like my indices of interest are NDVI, which is the normalized difference vegetative index. And that’s just a measure of general vegetative greenness. And then also NDRU, which is a measure of chlorophyl content in the crop canopy. So, as you can imagine, like inhibiting photosynthesis, we are able to pick up those injury symptoms with the drone and see stronger correlations with final yield than [with] initial breeder visual injury score.

I’m also running a project that I’m very excited about this summer. Dr. Ian Burke is on my committee and he is hosting some sort of, like, little weed conference here and so there are going to be a lot of weed scientists here, and we are going to have them walk the field multiple times. There’s going to be 20 to 30 weed scientists. We’re going to have them walk the field twice and give ratings of the plots. And these are all scientists who have experience rating different sorts of injury and should be familiar with the process. And then I’m going to also fly the drone over those plots and just observe the variation in those human ratings and see which one has a better correlation with yield. And that’s a really exciting study that I’m doing this summer.

Drew Lyon: Yeah, that sounds like fun. That’s the GROW group out of the eastern part of the US. Getting Rid of Weeds, I think, is what GROW stands for. And, yeah, that sounds like a lot of fun. I’ll be interested to see how that that turns out.

With metribuzin tolerance, are you making progress or are you finding some cultivars or lines that show differential tolerance to metribuzin?

Melinda Zubrod: Right. So, winter wheat is tolerant to metribuzin, not resistant. So, with some different herbicides you can incorporate genes or like use different genomic tools to see if they’re herbicide resistant. But, with metribuzin it’s just like a natural varietal tolerance to metribuzin. And so, it’s a little harder to differentiate and we don’t have the genes tracked down for that yet.

But we definitely are making some progress with that and there are some lines coming through the pipeline at the WSU Winter Wheat Breeding Program that will be available–I don’t know for sure when–for growers to utilize.

Drew Lyon: Okay. Yeah, you can use metribuzin at lower rates in wheat already but sometimes you get by with it and sometimes you can see injury. So, it’d be nice to have something that has a little more tolerance so you could be a little more confident that you’re not going to get that injury.

Well, good. What are some of the broader impacts of your research?

Melinda Zubrod: So, incorporating those herbicides will give producers or farmers more options to use in their cropping rotations and help to negate some of those herbicide tolerant–like winter annual grasses we’re seeing that are being a really big problem for wheat producers here in the Pacific Northwest. So, that will increase like economic value of the wheat and just the wheat cropping system overall.

And then, the other objectives of my dissertation will help scientists and people using spectral data–will help them be more confident in what they’re seeing. And the last objective of my dissertation is testing different calibration panels. So, with the spectral reflectance, we have calibration panels with known reflectance values that we use as a baseline in the image processing pipeline to calibrate the data. So, I’m running a study using different sets of calibration panels and different sensors–all that measure the same wavelengths. But we’ve seen some data and seen different literature that says that those don’t necessarily always line up. So, there’s lots of like federal regulations around drones and image–in like they malfunction–and it’s moving forward so quickly that a drone and a sensor you bought five years ago is kind of outdated, just hardware- and software-wise.

So, in these different studies that, you know, can take –you need to collect four years of data–just finding ways, seeing if switching sensors or calibration panels, is viable or if you need to be more careful. So, yeah, a broader impact would be just making scientists or people who use spectral data more confident in their data or maybe warn them a little bit that it might not be the same and that they may need to make some adjustments.

Drew Lyon: Okay. The drone technology, the imaging technology is all changing so quickly. And, yeah, keeping up with that can be difficult.

So, it sounds like really important work. As a weed scientist, I’m really excited to see this kind of work because we are, in small grains, really limited in the modes of action we can use and bringing some different modes of action would really allow us to maintain the herbicides we have now longer than we’ve been able to because we’re using such a small variety of herbicide.

So, I’m going to be watching what you’re doing, and I’m sure others will be interested in seeing that as well. So, stay in touch. And, if somebody wants to learn about your research, is there somewhere they can go to find out about it? [Does] Dr. Carter have a web page or something?

Melinda Zubrod: Dr. Carter does have a web page, but I haven’t gotten any of my papers published yet. There should be one coming out soon. I’ve published a couple of abstracts online in the Dryland Research Lind Field Day pamphlet.

Drew Lyon: Very good. We’ll get some of those into our show notes so people can find them if they’re interested. Thank you very much, Melinda. Enjoyed having you on today as my guest.

Melinda Zubrod: Thank you.

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Thanks for joining us and listening to the WSU Wheat Beat podcast. If you like what you hear don’t forget to subscribe and leave a review on iTunes or your favorite podcast app. If you have questions or topics you’d like to hear on future episodes, please email me at drew.lyon — that’s lyon@wsu.edu — (drew.lyon@wsu.edu). You can find us online at smallgrains.wsu.edu and on Facebook and Twitter [X] @WSUSmallGrains. The WSU Wheat Beat podcast is a production of CAHNRS Communications and the College of Agricultural, Human, and Natural Resource Sciences at Washington State University.

I’m Drew Lyon, we’ll see you next time.

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The views, thoughts, and opinions expressed by guests of this podcast are their own and does not imply Washington State University’s endorsement.