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Gather 'round, it's #WeedScienceWednesday!
The desires of the tweeple have been made known. We're going to talk about HERBICIDE ECOLOGY! https://twitter.com/WyoWeeds/status/1336467458442477569
I know what you're thinking: "Herbicide Ecology? Dude, herbicide ecology is NOT a thing." But, dear tweeps, you are WRONG.
First, a story.
The year was 2010. Two graduate students and I were out looking at a long-term research study. No treatments were applied yet the year we were wandering around, but different treatments had been applied continuously in the study over the previous 12 years.
The different herbicide treatments over the previous 12 years had selected for different weed communities (as would be expected).
But the results were DRAMATIC.
But the results were DRAMATIC.
You could see perfect lines between plots - on one side would be a monoculture of common lambsquarters, and on the other side would be a diverse mixture of pigweed, nightshade and green foxtail. The next only pigweed. And it was like this through the entire 2.5 acre study.
Completely different weed communities were growing in perfect rectangles corresponding to the historical plot layout. As we walked through the plots looking at how the herbicide treatments had selected for different weed communities, I commented "See? Ecology is COOL!"
And @jzagt responded immediately "Well sure, when there are herbicides involved."
And this, dear readers, was the birth of my use of the term 'herbicide ecology.'
And this, dear readers, was the birth of my use of the term 'herbicide ecology.'
From an ecological perspective, herbicides are a MASSIVE disturbance. And disturbances are selective forces. We can learn a lot about vegetation ecology using herbicides as selective agents.
A lot of really cool data cam from that long-term study we were walking through that day (including my doctoral dissertation). I'm going to show just a couple of the cool things we found related to how weed communities and weed populations adapt to management.
First, a refresher on terminology:
population - individuals of the same species in the same area
community - individuals of different species in the same area
There's a population of redroot pigweed, but a community of weeds (pigweed, nightshade, lambsquarters, etc.)
population - individuals of the same species in the same area
community - individuals of different species in the same area
There's a population of redroot pigweed, but a community of weeds (pigweed, nightshade, lambsquarters, etc.)
The long-term study I'm showing data from had a number of treatments, but we're going to focus in on just three of them:
A) glyphosate (Roundup) every year, and nothing else.
B) isoxaflutole (Balance) every year, and nothing else.
C) annual rotation of those 2 herbicides; Roundup one year, Balance the next, and so on.
Each applied continuously (8 years in Wyoming, 12 years in Nebraska).
B) isoxaflutole (Balance) every year, and nothing else.
C) annual rotation of those 2 herbicides; Roundup one year, Balance the next, and so on.
Each applied continuously (8 years in Wyoming, 12 years in Nebraska).
After 8 years applying those treatments in Wyoming, here's the effect we saw on pigweed emergence in year 9 (no herbicide was applied in year 9). Pigweed density was significantly higher where glyphosate was used continuously.
Which means we're getting glyphosate-resistant pigweed, right?
Nope. Those pigweed plants were every bit as susceptible to glyphosate as they'd ever been.
Continuous glyphosate use increased pigweed, but not because of resistance. Continuous glyphosate use was selecting for something different.
It relates to the weed community, and the requirements for growth of the different species within that community. Specifically, the seed germination requirements.
Different weed species evolved under different climates and environments, and have therefore evolved different germination requirements. So different crop management practices (like herbicide use) can select for (or against) different species based on those germination reqs.
Redroot pigweed tends to emerge much later than most other weed species in our study. And this is what we selected for in this particular study - ALL of the weeds were susceptible to glyphosate. Glyphosate (applied in May/June) killed ALL the emerged weeds.
Redroot pigweed had greater capacity to emerge AFTER the herbicides were applied compared to any of the other species at this site. Repeated use of an effective herbicide selected for the latest emerging weed species - which was redroot pigweed.
We observed this with glyphosate, but we would have seen a similar shift in the weed community with any other weed control practice, as long as it was similarly effective and applied at around the same time of year.
Now - to the Nebraska location. Same study, same 3 herbicide treatments. Here we're going to look at kochia emergence in year 13 after 12 years of continuous herbicide treatments (no treatments applied in yr 13).
Kochia density was MUCH greater where we used isoxaflutole every year. Which means we must have isoxaflutole-resistant kochia, right?
Things you need to know:
* kochia tends to emerge EARLY. Like it will emerge underneath snow. It has a very low temperature requirement for seed germination.
* kochia tends to emerge EARLY. Like it will emerge underneath snow. It has a very low temperature requirement for seed germination.
* isoxaflutole is applied to the soil before any weeds have emerged, around the time of crop planting. It remains in the soil and kills germinating weeds for a few weeks.
* glyphosate is kind of the opposite, it has no effective soil activity, only kills emerged weeds.
* glyphosate is kind of the opposite, it has no effective soil activity, only kills emerged weeds.
And based on all that, we'd expect that herbicides applied early would be pretty effective at killing kochia. And we'd also expect herbicides applied late would be effective at killing kochia. Because kochia emerges early! So what's going on?
My friend Gustavo spent like 2 years trying to figure this out. The kochia was very susceptible to isoxaflutole. It was also very susceptible to glyphosate. We couldn't blame herbicide resistance for this dramatic increase.
A community shift to kochia (as we saw with the pigweed earlier) didn't make any sense, because kochia is an early emerging weed species.
But what if kochia *wasn't* an early emerging species?
Gustavo started doing germination studies on the kochia at different temperatures, comparing the kochia plants from the isoxaflutole treatment to populations from neighboring fields.
And what he found, in my opinion, is evolution in real-time. The blue line is a 'typical' kochia germination pattern; rapid even at low soil temperatures. The red line shows germination patter from kochia seed collected in the isoxaflutole plots.
In just 12 years, we had selected for a kochia pattern with a completely different germination pattern than we would typically see in this species. At temperatures that would typically cause >70% germination, this population hadn't even BEGUN to germinate.
The optimal germination temperature of this population had increased by probably >20 degrees C. All because we continuously used a herbicide that killed off the early emerging individuals every year for 12 years.
And that's all I got for #WeedScienceWednesday. If you want to read a story about my friend Gustavo, who discovered the cool germination patterns, you can start here. It involves him getting sprayed head to toe with shit. https://twitter.com/WyoWeeds/status/961096466348625920
