Editor's Note: Learn more from Joe Ailts during his upcoming session at the 2026 National Strip-Tillage Conference, Aug. 6 in Springfield, Ill. 

Looking back on the 2025 growing season in northwest Wisconsin, the corn crop in late July was pretty much where it has been all season. In this our area of northwest Wisconsin, it was hovering right on top of the 30-year average, with 1184 heat units (GDU) accumulated. That put 2400 GDU 98-day hybrids exactly at the halfway point to maturity.

No-till soybeans planted into heavy corn residue looked really tough during the spring and summer. While they do every year, 2025 seemed especially pronounced. Why? And is there anything we can do about it?

I Have a Theory

Bear with me as I lay out this argument and then decide if there’s any merit to it. If we turn back the clock to the previous September and October, many locations in northwest Wisconsin had no meaningful moisture even with an above average corn crop. This translates to a lot of corn residue biomass.

Without water, very little corn residue began the decomposition process in September and October. Once temperatures drop below 50 degrees, the residue breakdown “pause button” gets pushed until favorable temperatures resume the following spring. 

For argument’s sake, let’s say the normal decomposition process resumes in mid-to-late April. Under these circumstances, an above average corn residue biomass has very little time under optimal moisture and temperature conditions to break down. Contrast that with a more normal harvest season, where there would be ample moisture and days above 50 degrees to jump start the decomposition process in early fall.

There is a general consensus that heavy surface residue slows soil warming and can contribute to excessive soil moisture. This is due to the absence of evaporation, which can impact soybean emergence. I believe these factors contributed to the less-than-appealing soybean stands that were observed the following spring. This happens every year, but I think there may be more to the story.

Corn Residue Allelopathy?

My theory is purely speculative, and I would be happy to be proven wrong. I don’t have a way to confirm this idea with laboratory analysis, but only with a few in-field observations and Internet searching. So take my theory with a grain of salt.

Corn residue has a scientifically documented and published allelopathic effect in which chemicals produced in the corn plant have a negative effect on the germination of various weed species. This allelopathy concept is most commonly associated with seeding cereal rye, but there are many other plant species that produce these compounds, including corn.

Here’s one example: Allelopathic Effects of Corn Straw and Its Water Extracts on Four Weed Species and Foxtail Millet, Dong, et al, 2024.

Unfortunately, no research studies have explored the effect of corn allelopathy on soybeans, but I believe it is quite likely that any potential allelopathic effects of corn on soybeans beans simply do not routinely manifest because:

1  Corn doesn’t produce high enough quantities of these compounds to be toxic year over year,.

2  Soybean seeds are large and fairly impervious to allelopathy.

3  Adequate moisture and temperatures allow for the leaching and breakdown of most allelopathic chemicals.

Despite these points, I wonder if something is going on in concentrated residue that “might” extend beyond temperature and shading effects.

Think about how hard it is to kill or suppress ditch weeds. County crews spraying herbicides only seem to slow the growth of these weeds.

Yet a modest layer of corn residue has provided nearly 100% weed control. Is this due purely to a temperature, moisture and light effect? Or is there potentially an allelopathic contribution from all that stover that didn’t release any allelopathic compounds until early May when rain and rising temperatures permitted the system to do so? 

Don’t Have the Answers

Those who have followed my ramblings on allelopathy in rye may notice that I’ve consistently taken the position that rye allelopathy is not a major contributor to corn and soybean emergence and vigor. I remain firm in that belief.

So how does one reconcile the absence of an allelopathic effect in the most allelopathic species (rye) and maybe the presence of allelopathy in a low allelopathic species such as corn?

Context and numbers, of course. Corn biomass at full maturity greatly outweighs rye biomass at an 8-10-inch termination. More biomass equals a greater concentration of potentially allelopathic compounds. This is the numbers component of my thought process: Think of how many tons per acre of corn biomass remains in a state of suspended animation until early May from a 180-200 bushel per acre crop. 

Many growers in my area have noticed an extra burden of residue at planting time. With no meaningful rain and zero stover degradation until late spring, any allelopathic compounds in the residue weren’t released until just before or shortly after the soybeans were planted.

This is a significant departure from normal when fall rain and moisture kick-start the degradation and soil flushing process. This likely leads to a meaningful contribution of allelopathic compounds the following spring.

So what can be done to improve soybean stands and plant vigor in heavy residue no-till systems?

The most straightforward answer is good no-till row cleaning. While this is easier said than done, any effort to remove residue from the row is likely to improve no-till soybean stands. 

My advice is to run your row cleaners aggressively if emergence and vigor are a historical issue. This is especially true for growers who no-till beans early ahead of corn. 

Some growers have experimented with strip-till rigs or strip fresheners. Others have sprayed residue-managing biologicals. I like the idea, but in  northwest Wisconsin we don’t typically have a fall or spring window of optimal conditions for them to work successfully. These products also require 50-degree  temperatures and adequate moisture to work effectively. 

Lastly, I think in-furrow additives and seed treatments warrant discussion. It is possible that various goodies placed on or around the seed may help jump start germination and vigor. These could include sugars, plant growth regulators, (PGR)  starter fertilizer, bio stimulants, plant growth-promoting microbes and other products. All are available, but few, have a long-standing track record of success in helping overcome the circumstances discussed here. On-farm, split field trials are the way to find out. 

Among these options, I’m excited by combinations of PGRs and sugars that can stimulate the seed to wake up faster and feed soil microbes some sugar to jump start the nutrient cycling process.

To summarize, soybean emergence and vigor is influenced by corn residue. We have the ability to use physical, mechanical and biochemical tools to help get no-tilled soybeans off to a stronger start.