April 14, 2022
Your cereal rye should be greening up by now, and if not, it will be soon.
If you like to plant corn green — planting into a growing cover crop — you’ll likely be on the sidelines for a while until things start warming up and your cover crop continues growing. But that’s in a “normal” year, and this year has been anything but normal, especially with high nitrogen prices.
Heidi Reed, agronomy educator with Penn State Cooperative Extension, says you may want to think twice about allowing your cover crop to get too big and then planting corn into it, especially if you’re worried about tying up nitrogen in the soil when your corn could benefit from it. Bottom line, waiting too long might not pay this year, so keep early cover crop termination as an option.
All about C:N ratio
Growers who plant green laud the system’s ability to hold back weeds, and to provide a nice, healthy soil for the cash crop. But with non-legumes, and especially if you’re planting corn into it, the potential downside is that if you let the cover crop grow to boot stage or anthesis, it will inhibit the amount of N that’s potentially available to the cash crop.
“The carbon-to-nitrogen ratio [C:N] is the amount of carbon relative to the amount of N in plant tissues. This ratio varies depending on cover crop species and maturity at termination. Grass and other non-legume cover crops, including cereal rye, have a lot of C relative to the amount of N, or a high C:N,” Reed co-wrote in a recent Penn State Field Crop News alert.
In contrast, legumes have comparatively less carbon relative to nitrogen in their tissues. They have a lower carbon-to-nitrogen ratio. Immature, lush, vegetative tissues also have less carbon and a lower carbon-to-nitrogen ratio compared to mature, tough, reproductive tissues.
Simply put, the higher the carbon-to-nitrogen ratio is, the longer the residue will last, and the chances that nitrogen immobilization will become an issue can grow — forcing more upfront nitrogen application.
Check out this graphic provided by Penn State Extension that shows carbon-to-nitrogen ratios of various cover crops and how that ratio increases as they are left to grow taller.
As the newsletter explains, when a cover crop is terminated, the residue is broken down by microbes in the soil. These microbes use nitrogen and other nutrients in the cover crop residue as fuel for the breakdown process. But when there isn’t enough nitrogen in the residue to complete the process, the microbes will use nitrogen from the soil instead — N immobilization. This tie-up reduces the amount of nitrogen available to the next crop.
The carbon-to-nitrogen threshold for nitrogen immobilization is about 20-to-1, and by boot stage it can reach 30-to-1, and 40-to-1 by anthesis or greater, depending on the cover crop.
Reed explains that recent research at Penn State on this topic showed that letting a rye cover crop mature from a carbon-to-nitrogen ratio of 15-to-1 to 40-to-1 can increase the nitrogen requirement of corn by 20 to 50 pounds per acre.
Waiting until flowering is beneficial for growers who roller-crimp the cover crop, and you can still roller-crimp when the cover crop is smaller, but all you’re doing is knocking the cover crop over, not killing it, Reed says. And with glyphosate at a premium, you may have to look at alternatives.
“Basically, gramoxone, plus 2,4-D, plus metribuzin/atrazine is the best alternative to glyphosate for corn burndown,” she says. “In this situation, make sure to maximize atrazine rate [2 pounds if possible] and full rate of Gramoxone to help with rye kill. Spray ASAP after planting.
“But if using 2,4-D in the mix, it is best to wait a few days [three to five] after planting to make application in order for the seed to get a head start at forming a root to help reduce injury. Rainfall soon after can cause problems, so make sure the planter slits are closed and well covered. If possible, plant as deep as possible, greater than 1.5 inches or so.”
Staying the course?
William Thiele, who farms 300 acres and raises 85 cows in Cabot, Pa., is planting 90 acres of corn green this year. He’s planning on staying the course, but he’s making changes to deal with the high price of nitrogen.
“So for this year, with N prices being so high, we will still probably put down N at planting whenever we plant green,” he says. “We usually would topdress the corn when the time comes, but since N prices are what they are, we may not topdress corn at all. We may also consider putting down less N at planting to save a little also. We will see how it goes for this year.
“The only time we wouldn't plant green would be if we are planting into old sod,” he adds.
Growers who have been no-tilling with cover crops, and are experienced with planting green, should stay the course with what they are doing, says Steve Groff of Holtwood, Pa. But if you are new to no-till and cover crops, this might not be the best year to try planting corn green, he says, especially with the prospect of having to spend more on nitrogen.
“If you’re new to no-till or cover crops, and your soils haven’t achieved a good sense of soil health, do it early, terminate early,” Groff says. “The biology in the soil is not functioning correctly as of yet.”
But there are caveats. If this spring ends up drier than normal, you may want to think about terminating early anyway because a living cover crop will take up water from the soil.
On the flip side, if it’s a wet spring and you terminate early, and it stays wet, you may not be able to get back into the field for a few weeks because of the moisture in the field, he says.
Whatever Mother Nature decides, growers soon will have a decision to make.
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