Southeast Pennsylvania growers are blessed with the right conditions to get cover crops planted after corn in fall.

But just a few hours north, the growing season can shorten drastically, making it much more challenging for growers to plant cover crops and get the benefits they can provide.

Still, if you have the right equipment, can plant cover crops early and have the right weed residual program, interseeding cover crops into corn can be a viable option to get a cover crop established. That’s according to a recently published paper by the Weed Science Society of America.

“Our results show that interseeding cover crops early, at the V3 corn-growth stage and in 30-inch row spacing, can balance cover crop and corn production management goals, while placing cover crops at a relative fitness advantage over weeds,” says John Wallace, assistant professor of weed science at Penn State, who led the project.

“Compared to interseeding at the V6 corn-growth stage, interseeding at the V3 corn-growth stage lengthens the cover crop establishment period before rapid corn canopy closure, and thus contributes to a relative fitness advantage of cover crops over weeds.”

In the project, funded by the Sustainable Agriculture Research and Education program, Wallace and others evaluated cover crop interseeding programs at three locations: two Penn State research farms in Landisville and Rock Springs, Pa., just outside State College, and at a Cornell research farm in Aurora, N.Y.

Cover crops evaluated were cereal rye, annual ryegrass and red clover.

The interseeder used was developed by Interseeder Technologies, a company started after the development of the high-clearance grain drill at Penn State. The machine is designed to interseed three cover crop rows on 7.5-inch spacing between 30-inch corn rows. It can clear 36 inches.

At each location, cover crops established best in the V3, 30-inch systems.

"We were kind of going into it thinking that for production regions, or locations where you have a more competitive corn crop, maybe a higher yield potential, that those extra two weeks or three weeks that you might get by interseeding around the V3 stage might be important," Wallace says. "It's really a race to canopy closure.”

EMERGING COVER CROPS: In the study, which compared interseeding cover crops at either V3 or V6 corn, interseeding at the V3 corn-growth stage lengthened the cover crop establishment period before rapid corn canopy closure.

He notes the project involved measuring the amount of photosynthetic radiation getting through the corn canopy to the surface.

Wallace says the project showed that cereal rye was easier to manage the following spring for burndown or roll-down for planting green. But it was much less consistent than annual ryegrass when interseeded in corn. Annual ryegrass is more challenging to manage the following spring.

"In all our studies, we had cereal rye emerge in the first couple of weeks,” he says. “It would take off prior to canopy closure. It would be well ahead of annual ryegrass. But in almost all the cases, we saw a really significant decline in the stand through the hotter part of the year. It just doesn't persist across a range of conditions under the corn canopy like annual ryegrass does."

Red clover in a 30-inch system and interseeded early also did well.

"It's a nice complement to annual ryegrass if you want to get a legume established," Wallace says. "I would recommend trying to use it in a mixture though. I think that typically probably helps with establishment. It's less vigorous in the fall, so as far as weed suppression and nutrient scavenging, having a grass in the mixture would be important."

Challenges persist

Still, many factors should be considered before diving in and interseeding.

You must have the right equipment and be patient to see consistent results, Wallace says. More importantly, it will likely require a change in residual herbicide programs.

Wallace says that many corn premix residuals have long enough residual activity that can potentially lead to injury of small-seeded cover crops. One option is to take the residual out and go with a post application. But weeds can persist and compete too much with the establishing cover crop, he says.

Early research focused on residuals with a single active ingredient, but recent work has focused on more broad-spectrum weed control programs. If annual grasses such as foxtail or crabgrass, or small-seeded broadleaf weeds like pigweed, are a problem, Wallace suggests using a Group 15 herbicide — such as Outlook — in the mix.

“These are the first ones to dissipate and are the safest to interseed,” he says. “Verdict as a premix with Sharpen and Outlook in it is also good.”

If larger-seeded weeds such as burcucumber or common cocklebur are problems, using atrazine in the premix is good, but watch the amount being applied. Wallace suggests between a half-pound and a pound.

While the study also looked at flex ear and determinant corn hybrids, the type of corn hybrid used wasn’t a big determinant of cover crop establishment.

"So that's not to say that hybrid selection is not important. It's just to say that based on kind of how we did the studies, we didn't find a dramatic effect from the hybrid," Wallace says, adding that the 60-inch rows did show a 22% corn yield drag.

The big question is, when does doing this make sense? Wallace says that depends on what the end goal is.

"If the management goal is really driven by nutrient management concerns, I think that postharvest drill seeding a cover crop after corn silage, or even after corn grain harvest, in a region where it is possible will maybe be more consistent over the long term," he says. "So, it depends on what the management objective is, and that's going to affect how you're thinking about short- and long-term return on investment.

“It's also the case that we're likely to see cover crops continuing to be incentivized, and those are based on outcome-based models in those regions where we're just not consistently using cover crops because of that growing season window. Those are really the areas that are ripe for adoption or continuing to investigate this practice."

The full article, “Light partitioning strategies impact relative fitness of weeds and cover crops when drill-interseeding in corn,” can be found on the Weed Science Society of America website.