Although some weed scientists doubt the beneficial allelopathic effects of cereal rye in no-till soybeans, independent research agronomist Jim Stute says data from his latest trial makes the case for the rye’s ability to hold back weeds, even with minimal biomass.
Allelopathy is the beneficial or harmful effects of one plant on another plant from the release of biochemicals by leaching, root, exudation, volatilization, residue decomposition and other processes, according to cover crop expert Steve Groff. Many weed scientists believe cereal rye biomass works to suppress weeds because it acts as a physical barrier to prevent germinated weed seedlings from emerging, reduces light interception to prevent weed seed germination and buffers soil temperature fluctuations that trigger weed seed germination.
Stute, who no-tills in East Troy, Wis., has been working with local farmers Tom Novak, Nick Kau, Tyler Troiola and Tom Birmingham over the past 4 years on research trials assessing if cereal rye could be used to suppress herbicide-resistant waterhemp, marestail and giant ragweed and the best time to terminate the cereal rye cover to maximize weed suppression while also minimizing yield loss. The second generation of the study, funded by North Central SARE, took place during the 2023 and 2024 growing seasons, a dry year and a wet year that ended dry. They evaluated rye termination before planting and weekly through anthesis beginning at soybean emergence.
In the dry year of 2023, weed suppression increased by nearly 2% for every 100-pound increase in cereal rye biomass, starting from 41% relative suppression, according to Stute’s data. For this work, suppression was calculated as the percent reduction in weed population (density measured at time of postemergence herbicide application) compared to a no rye control. In the wet year, suppression started at 82%, but each 100-pound gain in biomass only resulted in a 0.9% gain in weed suppression.
Stute says this helps make the case for the allelopathic effect of cereal rye. He says the difference was in the initial amount of biomass at the preplant termination timing. Doubling the biomass more than doubled the suppression, and the residue decomposed rapidly after planting, disappearing completely by the time of postemergence application.
“As recently as 2 years ago when I delved into the scientific literature, allelopathy had not been demonstrated at the field scale,” Stute says. “We see it in nature and in greenhouse studies, but we can't document or demonstrate it. As a result, weed scientists tend to discount allelopathy as contributing to overall weed suppression. But we're seeing pretty good indications that something's going on other than the physical barrier, light interception and temperature fluctuations that cereal rye biomass causes.”
The indications that Stute and his research cooperators are seeing are illustrated in the photos below.
Figure 1. Residue from cereal rye terminated before planting at soybean emergence and the previous year's no-till corn residue in Jim Stute's East Troy, Wis., research plots
Figure 1 shows cereal rye biomass after termination at the time of soybean emergence in 2023. Stute says the small amount of cereal rye biomass isn’t acting as a physical barrier, intercepting light to prevent the germination of weed seeds or buffering temperature fluctuations.
“In fact, the corn residue was doing more to buffer the fluctuations, and yet we still saw a greater suppression of our target weeds in this treatment with cereal rye terminated before planting, compared to the control of no-till without a cover crop,” Stute says.
Figure 2. No-till soybeans with a cereal rye cover crop terminated before soybean planting (left) and no-till soybeans without a cereal rye cover crop (right).
Figure 2 shows the obvious difference between the rows that had cereal rye terminated before soybean planting (left) and the control rows that had no rye but the same residual herbicide treatments (right). The rows without rye have late-season weed pressure, while those that had the small amount of rye biomass have virtually none. Stute says before planting a cereal rye cover crop, his no-till farmer cooperators would apply a residual grass product with their post-emergence weed control for late-season grass suppression. But after they added cereal rye as a cover crop, they haven’t made that application.
“So what's going on there? Is it the allelopathy?” Stute says. “We tend to think that it is.”
Stute says that evidence raised the question of whether no-tillers should manage rye for maximum early season biomass and always terminate pre-plant to take the guesswork out of the termination timing decision to combine termination and residual application into one trip and take the yield risk out of the equation. Over the 4 years of Stute’s trial, the cereal rye produced an average of 170 pounds of biomass per acre and an average 48.5% weed suppression compared to the control. In the wet year of 2024, the biomass production increased to 433 pounds per acre with 82.3% weed suppression.
“This represents a nearly 70% gain in suppression with the gain in biomass production,” Stute says, “but when we break it out by site, some really interesting interactions separate out.”
Figure 3. Aboveground cereal rye biomass as by location in 2023.
At the pre-plant termination, the cereal rye at the two Palmyra, Wis., sites (sites 1,2 in Figure 3) achieved roughly double the growth as Stute’s sites in East Troy (site 3 in Figure 3). Stute says the difference is due to planting timing for the rye after fall harvest. Reliant on custom corn harvest, Stute plants rye 3-6 weeks after the other cooperators.
“That's reflected in not only the initial amount of biomass yield, but also how my rye behaves,” Stute says. “My rye tends to take off in the spring, but it's later in the season, so I can achieve the biomass yields that they do at anthesis termination. I'm slower to get there, but I have compensatory growth.”
Another observation from the site-specific data shows the impact of the dry year in 2023. The cereal rye used up all the moisture in the soil profile of the sandiest site (site 1 in Figure 3) and limited its own biomass production.
Figure 4. Aboveground cereal rye biomass as by location in 2024.
The cereal rye had unlimited moisture in 2024, and the rye planted earlier (sites 5 and 6) started the season with higher biomass production and weed suppression. Sites 4 and 7 with rye planted later started with lower biomass production and weed suppression, but Stute’s site caught up with compensatory growth later in the spring.
In terms of weed suppression, the relative gains beyond those achieved with preplant termination are slow to develop and are not maximized until rye nears anthesis, according to Stute.
“It’s really the rye biomass and growth later in the season that accounts for the additional suppression,” he says, “but that’s also when competition with growing rye can also cause the yield reduction.”
Stute says all of this data supports the group’s recommendation to use the amount of cereal rye biomass as an indicator of the baseline for initial suppression in a wet year.
“If you've got a lot of biomass there, our recommendation is to terminate, looking at the relationship between suppression and yield loss,” Stute says.
In a dry spring, the group recommends terminating early, which will protect yield and still capitalize on the rye’s allelopathic benefits.
“Based on what we’ve seen and our historical experience with this system, if it looks like it’s going to turn dry, practice adaptive management and terminate right away,” Stute says.
If you'd like to see Stute’s 2025 National Cover Crop Summit presentation with the complete results of the 2-year trial and specific data about the “sweet spot” for cereal rye termination and more, register for the 2026 National Cover Crop Summit All-Access to receive full access tot eh video archives!
