Published Apr 19, 2019
If you’ve considered using cereal rye as a cover crop in front of corn, you’ve probably been warned that your yield would suffer.
And the truth is it might. Research has shown that corn yields can take a hit after cereal rye, but it doesn’t happen every time.
Unfortunately, it’s hard to pinpoint what exactly is causing these problems, and predict when they may occur. Alison Robertson, a plant pathologist at Iowa State University, believes it’s a combination of ‘green bridge,’ allelopathy, nitrogen and the environment that are involved.
Pathogens at Play
One of the reasons corn may suffer after cereal rye is if the rye is serving as a “green bridge,” where pathogens that were infecting the rye move onto the growing corn as the rye dies.
Research conducted by Robertson and colleagues confirmed that this phenomenon can occur. In a 2018 Iowa Learning Farms (ILF) webinar, she explained that researchers first confirmed that rye can host several pathogens, including:
- Fusarium graminearum — which causes several diseases, including stalk rot and ear rot of corn, head scab of wheat, etc.
- Pythium sylvaticum — which causes corn and soybean seedling disease
A field experiment funded by the Iowa Nutrient Research Center was conducted in 2014 and 2015 to determine whether the rye could serve as a green bridge in transferring those pathogens to corn.
Immediately after soybean harvest in late September, researchers drilled 60 pounds of rye. The following spring, the rye was terminated with glyphosate either at 25, 14–17, 8–10 and 3 days before planting (DBP), as well as 2 days after planting (DAP). A plot without rye served as the check plot.
They found the less time there was between rye termination and corn planting, the more likely it the rye served as a green bridge. The percent of corn seedlings with root rot planted into the check plot and where rye was terminated at least 17 DBP was between 8% and 25%. But where rye was terminated between 8 DBP and 2 DAP, the number of seedlings with root rot increased to above 80%.
In her research, Robertson found that Fusarium was always associated with corn seedlings, regardless of whether rye was previously there. But there was consistently more Pythium infecting corn seedlings when rye was in the system, compared to when no rye was present.
The shorter the time between when rye was terminated and corn planting, the more incidence of radicle root rot. Photo from Alison Robertson’s ILF webinar.
Lower corn yields were also measured in the experiment. According to Robertson, the check plot yielded the highest at 221.4 and 224.5 bushels in 2014 and 2015, respectively. In the terminated rye treatments, yields decreased as the time between rye termination and corn planting got shorter with the lowest yield being where corn was planted into rye terminated 1 to 2 DAP, which yielded 185.9 and 182.9 bushels, respectively.
The roots of corn following no cover crop (left) were much cleaner than the roots where cereal rye was terminated three days before planting (right). Photo from Alison Robertson’s ILF webinar.
It’s important to note that in the 2015 experiment there was a high amount of rye biomass and the researchers did not apply nitrogen at planting, which Robertson acknowledges is not a practice farmers would typically do. Nitrogen was not applied because researchers were concerned it could affect seedling disease.
“We have a lot of questions about nitrogen,” she says. “We know that, especially with older rye that has a higher carbon-to-nitrogen ratio, as it’s decomposing, there’s probably nitrogen being immobilized in the soil. How is that impacting the germinating seedling? If that germinating seedling is not getting enough nitrogen, is that predisposing it to Pythium infection?”
What About Other Small Grains?
Given the negative effects seen with cereal rye, you may be wondering if similar results would occur with other small grains.
Most of Robertson’s research has been specific to cereal rye, which she says is because it overwinters so well in Iowa and is grown by 90% of farmers who use cover crops, so she can only speculate on what would happen if a species like oats or winter wheat was used instead.
She says it’s likely there could still be a green bridge effect because they’re also hosts of the Pythium pathogen. However, she doesn’t believe all the blame should be placed on the green bridge.
Allelopathy — when a plant releases chemicals to inhibit the growth or germination of other plants — may also be playing a role, Robertson says.
“Rye releases these chemicals to inhibit the growth of the corn, but these chemicals break down very quickly and get absorbed to the soil particles,” she explains. “I think that may have something to do with the variability in the response you see in corn planted after a rye cover crop.”
Robertson has very preliminary data, she says, to suggest that allelochemicals from rye are interacting with some Pythium species, and this may result in corn disease.
“So when we terminate the rye too close to the corn, depending on the soil type, soil temperature and moisture, and how much soil organic matter and rye biomass there is, that affects how much of the chemicals are released, how quickly they degrade and how much effect they have on the corn and the pathogen,” Robertson explains.
Given that other small grains aren’t allelopathic to corn like rye, it’s possible growers may have better results with those grain species in front of corn.
Environment Likely a Key Factor
Even though Robertson’s research has shown corn yield reductions can occur, especially when following cereal rye, she knows that’s not always the result.
Last year she worked with Practical Farmers of Iowa (PFI) to sample some farmers’ fields. Termination dates varied from two weeks before planting to right before planting and so did the results. One farm had similar results to Robertson’s field experiments, but there were also farms where she didn’t see an increase in seedling disease, even though the farmers terminated close to corn planting, and there was no difference in yield.
The key factor involved, she believes, is the environment.
The environment is one side of the “disease triangle” — the other two sides include the host, which would either be the corn or the rye, and the pathogen itself, which exists in the soil. The right conditions is the last thing needed for infection to take place, Robertson explains.
“If you don’t have all three sides of that triangle, you’re not going to get seedling disease, even though we know the pathogen is there and we know the rye and corn are there,” she says. “If we don’t have the right conditions, we won’t get the disease.”
Plant in Ideal Conditions, Time Your Termination
Because the environment plays such a big role in whether disease occurs following cereal rye and other cover crops, one of the best steps growers can take in preventing a yield drag is to plant in warm, dry conditions.
“If we have a warm, dry spring, it is unlikely seedling disease will occur, even if you terminate the rye one day before planting the corn,” Robertson says.
Even if conditions aren’t perfect during planting, if a grower can wait to plant until there’s a warming trend, that gives the corn a chance to emerge before seedling disease can take place.
“If conditions are such that the soils are warming up quickly, if we’re going to accumulate those growing degree days very quickly and that corn is going to pop out of the ground, it’s going to escape infection by the pathogen,” she says. “I think the longer the corn sits in cool, wet soils, the more at risk it is for seedling disease.”
Of course, sometimes it’s not possible to plant in ideal conditions. Which is why Robertson also advises terminating the cover, especially cereal rye, at least 10 days before corn planting, as that will help reduce the risk of seedling disease. If there’s no host when the rye dies and releases the pathogen, then that population will decrease over a period of a few days to a couple weeks, she explains.
Consider Seed Treatments, Reducing Rye Biomass
Growers can also try seed treatments to help protect the corn a little more. Robertson says that currently, on every single corn seed that is planted, there will be either metalaxyl or mefenoxam fungicides, which are both active against most Pythium species. Ethaboxam is another fungicide that is proven to provide protection against Pythium species. Syngenta will also be releasing another seed treatment with the active ingredient picarbutrazox, which is expected to be available in 2020.
“The seed treatments are definitely going to help, but they are not a silver bullet,” Robertson says.
Since the amount of biomass may play a role in the likelihood of seedling disease, Robertson adds that growers may consider reducing the biomass before termination, whether it’s through grazing the cover crop or harvesting some of it.
“If that rye is hip high when you terminate it and then you plant your corn into it, you may have more of a problem than if that rye is only ankle high,” she explains, noting that again, if the conditions are ideal for corn to emerge rapidly, growers can probably get away with more biomass and terminating closer to planting.
Worth the Risk
Despite the risk of reduced yields following a rye cover crop due to seedling disease, allelopathy or another factor, Robertson doesn’t want farmers to use that as an excuse to avoid cover cropping, because it doesn’t occur consistently and there’s a good chance a grower won’t experience these problems.
Research from ILF and PFI confirm that not only do yield drags not always occur, but sometimes yields are increased.
The two organizations have partnered with farmers since 2008 to determine cereal rye’s effect on corn and soybean yields, and according to their Year 10 report, in 61 of 68 site-years, farmers have reported that “properly managed cover crops had little to no negative effect on corn and soybean yield.” In fact, they note that corn and soybean yields increased in 3 and 8 site-years, respectively.
The report also says that after the farmers first introduced cereal rye to their operations, they made adjustments to their planter settings to handle more residue and also planned to terminate cover crops 10–14 days before planting to minimize negative impacts on yield.
Reducing rye biomass through grazing or harvesting may help reduce the risk of seedling diseases and lower yields in the following corn crop. Photo by Stephen Ausmus, USDA Agricultural Research Service.
“If farmers are able to terminate 10–14 days before planting their corn, that would be good,” Robertson says. “It would really reduce their risk. But if they’re not able to do that because of cruddy conditions in the spring, they should just go ahead and do their best, especially if soils are warming.”
She adds that if farmers are interested in using cover crops but really concerned about the potential yield drag in corn, they could try planting soybeans instead.
“Soybeans are far more resilient than corn and it is extremely rare to see reduced yields after a cover crop,” Robertson says.