If you’ve just hopped on the “planting-green” bandwagon, or you are thinking of hopping on this season or next, experts will likely tell you to try soybeans since you’re likely to see better success — at least at first.

Corn, on the other hand, can be challenging because of poor seed-to-soil contact, allelopathy (the chemical inhibition of one plant by another) and lower soil temperatures associated with planting into crop residue.

Alison Robertson, a field crops pathologist at Iowa State University, says another culprit could be at work: corn seedling diseases.

At the recent Northeast Cover Crops Conference, held virtually earlier this month, Robertson says she got interested in corn yield drag in rye cover crop systems after a researcher at USDA’s Agricultural Research Service, Tom Kasper, asked her to help investigate the underlying causes of lower corn yields when seed is planted following rye, the most popular cover crop used in Iowa.

Seedling diseases are caused by soilborne organisms that affect corn seed and the root system, leading to plant death and reduced stands. But they can also lead to chronic infection of the plants once they have emerged from the soil.

“Seedling pathogens infect the radical and the seminal roots, and the plants struggle to emerge and then grow. Consequently, you get uneven emergence and reduced vigor, and then later on those plants then turn into weeds with smaller ears and even no ears at all,” Robertson said.

Rye able to host corn pathogens

One thing Robertson and her colleagues discovered was that rye could play a host to corn seedling pathogens. But could it provide a “green bridge” for these pathogens to then spread to the corn seed?

Robertson helped lead an experiment where she and other researchers looked at rye termination in relation to corn planting, and whether it led to more corn seedling disease. The group looked at plots where rye was terminated three, eight, 17 and 25 days before planting corn, and a plot where rye was terminated two days after corn planting. The trials were done in 2014-15.

“First of all, there was significantly more seedling disease in the corn that was planted after the rye, and then the most seedling disease was found in the corn that was planted within a week after terminating that rye,” she said.

It also affected yields, especially in 2015, when the average corn yield ranged from 209.7 bushels in fields where rye was terminated 21 to 25 days before planting, to 182.9 bushels in fields where spraying was done just after planting. She noted that rye biomass was also much higher in 2015 compared to 2014.

Her recommendation to growers was to wait at least 10 days after terminating winter rye to plant corn. But in Iowa — and for that matter, in many places across the Midwest, mid-Atlantic and Northeast regions — springs can be cold and wet, limiting growers’ access to fields.

This is where planting green becomes more of an option; but what is the potential yield penalty for waiting longer to terminate a rye cover crop after planting corn?

Trial to evaluate early corn growth

In a follow-up trial funded by USDA’s National Institute of Food and Agriculture (NIFA), Robertson’s plots evaluated early corn growth, delayed emergence, disease and yield in the following treatments:

The plots were in a corn-soybean rotation, no-tilled, and the rye was drilled the previous September. The plots had 50 pounds UAN applied at planting, and another 130 pounds sidedress nitrogen at V4.

The rye biomass was heaviest in the later-terminated plot and corn was taller in the planted green plots, which she said wasn’t surprising. But emergence was also uneven in the plots, with delayed emergence seen throughout. Disease was also higher in the planted green plots.

In terms of yield drag, the greatest yield drag was in the later-terminated, planting-green plot — 12 days after planting.

On-farm field trials

Robertson also worked with Practical Farmers of Iowa to establish on-farm field trials on four farms in the state in 2020 and 2021. The treatments differed, ranging from plots where rye was killed two days before planting corn to plots where rye was killed 25 days after planting corn.

While each farm reported a drag on yield in the later-killed plots, only one farm saw a significant reduction in yield in both years, Robertson said, and that farm seeded twice as much cereal rye as the others, resulting in more biomass.

More root rot in the planting-green plots was observed in two of the four farms in each year, one of these farms had the significant yield drag.

In 2021, yields were not significantly different for the three farms participating in the trial, whether planting green or not. But the biggest yield drag was the farm that had higher rye biomass. 

The experiments also shed light on some of the things farmers did to improve the planting-green conditions for the corn. One farm, she said, drilled rye between the corn rows, and left the corn row free of rye. She said prior research from her lab has shown that this method can significantly decrease the effects of root rot.

In a follow up trial funded by USDA NIFA, Robertson’s plots looked at the impacts of planting corn into a living cover crop — from suppressing weeds and reducing slug damage to favoring more seedling diseases, pests and possibly lowering yields. Preliminary results from 2021, the first year of field trials, suggest the amount of seedling disease, particular root rot, increased when planting corn into a living cover crop. But the results across the 16 locations weren’t uniform. Sites in Texas and Iowa also saw similarly high levels of root rot in the planting-green plots, but plots in Delaware, Maryland, New York, Pennsylvania and Vermont saw much lower impacts. Robertson said many other factors are likely at play and have not been evaluated.

“We plan to look at cover crop variety and biomass at termination, date of corn planting, precipitation and soil temperatures soon after planting, and if nitrogen was applied at planting and how much, corn hybrid, etc.,” she said.