Weeds continue to be one of the most yield-limiting factors in no-till crop production, but thankfully, there are new ways of dealing with them when the right tools are sought out and used.
Research from Ohio State University shows when left unmanaged, weeds can cause up to 80% yield loss in the Midwest. And with growing concerns over government regulations for herbicides, as well as increased numbers of herbicide resistant weeds, cover crops become all the more important to utilize as a tool for weed suppression.
Alyssa Essman, assistant professor in the Department of Horticulture and Crop Science at Ohio State University, has been researching integrated weed management strategies and how cover crops can be used to supplement conventional herbicide programs.
The Big Five
Essman’s research has been focused on what her team is calling “the big five,” or the 5 most common weeds seen in Ohio and across a lot of the Midwest. Two of them are common ragweed and giant ragweed. Giant ragweed is a very tough weed that has a lot of biological advantages when it comes to seed size and plant size. It has a fast growth rate and is typically a very competitive weed.
Another of the “big five” is horseweed or marestail, which has a number of characteristics that make it hard to deal with. It can germinate throughout different points of the year, spread across long distances and grow in pretty much any season. Essman says she has seen this weed become an even bigger issue to deal with as the shift towards no-till production continues.
The two newer weed species that round out the big five are waterhemp and Palmer amaranth. Essman says she has seen a lot of issues with waterhemp in western Ohio, while Palmer tends to be more localized in certain regions. Regardless of location, these species are increasingly driving the weed management programs across Ohio and the Midwest.
Attacking the Problem
In recent years, farmers have dealt with weeds, particularly in soybean systems, through the use of herbicides along with herbicide-tolerant soybean varieties. Looking at the timeline of introduction, these systems allowed for excellent weed control and also simplified growers’ weed control strategies for a good period of time.
“So if we look at the introductions, of course, the first being the Roundup Ready crops in 1996, fast-forward to Liberty Link, Roundup Nine, and more recently, our Roundup Ready, Extend and Enlist systems, and when we compare this to some of the other crops, soybean adoption of these herbicide-tolerant varieties was really rapid,” Essman says. “That was due in part to the fact that we just had a lot less options for weed management and soybean compared to some of these other crops.”
TREND CONTINUES. This chart shows the number of herbicide resistant weeds by site of action, which is steadily increasing. Alyssa Essman says based on available research, there is nothing to suggest that this trend will not continue into the future. The Ohio state university college of food, agricultural and environmental Sciences
But as we look to the future of weed management and what's in the pipeline, Essman says the trend seems to be headed towards a scenario where there will be introduction of new varieties with different herbicide-tolerant packages, but a lot of these are stacked traits with resistance to the same herbicides that growers have used for a number of years. Which forces growers to ask themselves a tough question when thinking about the future of how they will manage weeds while knowing that they’ve relied on the same herbicides for a number of years.
The question being, “What is the result of the repeated use of the same herbicide sites of action, often applied multiple times a year and certainly over many years?
“And we know the answer at this point is certainly herbicide resistance,” Essman says. “This is the key issue that keeps weed scientists up at night and something we're certainly interested in investigating and figuring out how to manage.”
Understanding the Science
One thing that's inherent to weeds that differs from some of the other cash crops in our growing systems is that weeds are a lot more genetically diverse.
“What that means is there's a probability that one, or two, or a few individuals in a population have this naturally occurring gene for resistance to an herbicide,” Essman says.
“When we apply these herbicides in this intense selection pressure, we're essentially selecting for those few individuals until they become a larger and larger proportion of the population, and we can no longer control this population of weeds with a particular herbicide.”
Essman says the general lack of crop rotation on most farms plays a large role in compounding this issue, and narrows a grower’s herbicide options even in a corn and soybean rotation.
“Cover crop biomass is related to early season weed biomass, which means that the more biomass we can achieve, the greater consistency we will have in weed suppression…”
“It's not as diverse as we would like it to be, and it leads to a lot of overlap in product use,” Essman says. “Generally, there's a pretty limited selection of sites of action, there hasn't been a new introduction for many years, and when we get into situations where we have reduced rates, we know that contributes to the development of herbicide resistance as well.”
Essman says this effect is especially evident when looking at larger periods of time and when looking at the number of herbicide resistant weeds by herbicide site of action. This has steadily increased over time and researchers continue to see new species develop resistance to new sites of action.
What's potentially even more concerning, according to Essman, is the number of weeds that are developing resistance to more than one site of action.
“Within the same population, we have resistance to more than one product or site of action that was previously effective,” Essman says. “And so it's not all that uncommon anymore to have resistance to multiple sites of action — two, three or four sites of action within a single species. I know in many states, waterhemp has exhibited the ability to resist a number of sites of action, and we've seen that number just increase even more in recent years.”
Cover Crops to the Rescue
The question then becomes how do growers combat this current situation of resistance and also attempt to prevent future instances and development of resistance?
“When we look at the data and the literature over time, the consensus among weed scientists is that it's really going to take an integrated approach to weed management,” Essman says. “This means utilizing what we call lots of ‘small hammers’ to tackle this issue. So, combining the factors of biological, cultural, mechanical and chemical control to have a number of tools to attack this issue. And research both at universities and on farms by experienced cover croppers suggests that cover crops can be one of these many small hammers needed to fight herbicide resistant weeds.”
While cover crops are often discussed under the lens of soil health and regenerative agriculture, their weed suppression properties are just another benefit on a long list of reasons why they can be so useful in cropping systems. But there are several different factors to consider when selecting a cover crop species that will be good for weed suppression; the first of which is physical.
BURNDOWN. The graph shows ground cover from the cover crop 10 weeks after planting as it relates to the population decrease or percent decrease of horseweed at the time of the burndown. As more ground cover from cover crops is achieved, there is a greater reduction in the percent of horseweed population at the time of burndown. The Ohio state university college of food, agricultural and environmental Sciences
“Cover crops generate a lot of biomass. This biomass then becomes this mulch-like layer of residue on the soil surface, and that really hinders the germination and growth of our weeds in the cropping system,” Essman says. “When they're actively growing, they compete with our weeds for vital resources like light, nutrients and water.”
When growers terminate their cover crops, that can also be a direct way to fight weeds, depending on the method of termination. Oftentimes growers will spray, mow, till or use a roller crimper to terminate their covers. All of these methods can have an effect on helping keep any weeds present at the time of termination at bay.
Cover crops can also alter the environment of the seed in the soil or the area right around where the weed seed exists. Covers influence soil moisture, soil temperature, light, and even some of the predators within that system. All of this can help change the factors that contribute to how well weeds can germinate and grow.
Biomass & Ground Cover
One of the two main drivers of weed suppression when it comes to cover crops is biomass production, or in other words, how much green vegetative matter is being produced above the soil surface.
“Cover crop biomass is related to early season weed biomass, which means that the more biomass we can achieve, the greater consistency we will have in weed suppression,” Essman says. “We have lower weed biomass levels as we get higher cover crop biomass levels. At those low cover crop biomass levels, we have a lot more variability and the potential to have a lot greater weed biomass than we do when we can achieve higher biomass levels.”
The second major factor for weed suppression when using cover crops is overall ground cover. One example that Essman has seen in her research is ground cover from a cover crop 10 weeks after planting as it relates to the population decrease or percent decrease of horseweed at the time of the burndown.
“We have to combine the factors of biological, cultural, mechanical and chemical control to have a number of tools to attack weeds…”
The research shows that as more ground cover from the cover crops is achieved, there is a greater reduction in the percent of horseweed population at the time of burndown. But how do these factors of biomass and ground cover interplay and what do they actually do for development of herbicide resistance?
“We talked about trying to be proactive and using these tools to not only manage our herbicide-resistant weed populations, but also potentially delay the development of resistance,” Essman says.
“What this means for our weed populations is that when we have these high-biomass situations where we're seeing a lot of weed suppression from the biomass and ground cover, we have a reduction in the number of weeds present at the time of our herbicide applications and also in the size of the weeds present at the time of our herbicide applications.”
Essman notes that when there is a lower weed density, those are less weeds exposed to herbicides. We know that herbicide resistance development happens often as a game of chance related to genetic diversity and the small probability that some of those individuals within a population have this naturally occurring ability to resist the herbicides being applied.
“If we're applying these herbicides to a lower number of weeds, that's a greater chance that we're not going to select for those individuals,” Essman says.
Systems Approach
What Essman’s cover crop research supports, along with a lot of other studies that have looked at cover crops and different management factors and how they influence weed management, is that cover crops can be one part of a comprehensive weed management program.
Using integrated strategies, along with chemical control strategies, supplements the suppression and allows growers to have potentially more effective herbicide programs that might be effective longer if resistance development is being delayed in some way.
“It's part of the systems approach that we need to manage herbicide-resistant weeds and potentially the development of herbicide resistance,” Essman says. “The effectiveness and the success of our cover crop is really going to be dependent on the different management factors, as well as some environmental factors and things like weather that are outside of our control.”
For the management of cover crops for weed suppression, just like the management of weeds, Essman says they're going to be most successful when implemented in a very tailored plan. This might look different year by year or even field by field based on the characteristics of those fields and potentially what weed species might be the biggest threat in each field.
