As late summer rolls around, many growers will start planning ways to seed cover crops after corn or soybeans. Meanwhile, some will already be interseeding — whether aerially or with a high-clearance machine. When it comes to establishing cover crops early by interseeding, Keith Glewen, Nebraska Extension educator notes, "I've seen successes, and I've seen failures."

"The factors involved are moisture after application, timing, and getting covers seeded during the right window," Glewen says. "If you get it on too soon and don't have enough sunlight, cover crops will germinate and die. You can get it on too late, and then you don't have enough growing season left to benefit from the growth of the cover crop."

The advantage, of course, is overcoming the limited amount of biological time in a corn-soybean rotation, and being able to seed a bigger variety of cover crops, including brassicas and legumes that normally wouldn't overwinter in Nebraska.

"The biggest benefit is cover crops are established earlier than you can with a drill. That's where aerial applications shine," says Ryan Krenk, agronomist at Roth Aerial in Milford. "If we can get out there earlier, maybe we can catch a few more rains, and maybe we can get the cover crop established sooner. If you want to run cattle on the stalks right after it's harvested, you already have a cover crop established and growing. From a soil loss standpoint, the longer you have something growing on that soil the better off you'll be."

Germination is key
It may not be worth seeding early if the crops aren't able to germinate, or if they simply germinate and stop growing, notes Dan Gillespie, a Natural Resources Conservation Service no-till specialist, who farms near Battle Creek. Aerial application can be subject to inconsistency depending on timing and weather. Moisture — from rainfall or irrigation — is needed for cover crops to germinate, and more moisture is needed for seeds on the soil surface. When surface-seeding, seeds can get caught in the canopy and never make it to the ground. If cover crops are seeded too early, especially in soybeans, the canopy may be too thick to give the seeds the sunlight they need to germinate and grow.

"Unless you're in an irrigated system, you are largely depending on Mother Nature. The optimum aerial seeding window sometimes occurs at a time when aerial applicators are still applying insecticides. You can be subject to a wait," says Gillespie. "One of the reasons I went to drilling was I found with the variability that comes with surface-seeding in stand establishment, at the end of the next spring's growing season, early establishment doesn't guarantee you'll have more biomass than you would if you would have just drilled it and gotten good seed-to-soil contact and a uniform stand."

Improve success rate
There are, however, a few avenues to help increase the chances for success when surface-seeding.

Aerial seeding after silage or seed corn harvest provides a wider window of establishment, and a number of growers have opted to use high-clearance applicators fitted with air seeders and drop tubes, like those available from Hagie, to place the seed below the canopy. However, unless using a method to incorporate the seed, there's still a chance the seed will germinate and die. Some operators have used Lilliston spider gangs with high-clearance machines.

The height of the applicator also makes a difference. Jody Saathoff, farm representative for CHS in Minden, first tried aerial-seeding over five years ago, but has since moved to seeding with a no-till drill or a high-clearance applicator with about 5 feet of clearance — compared to ultra-high clearance applicators that can clear 10-to-12-foot corn.

"It works well if I'm interseeding cover crops on seed corn. I can clear the canopy with 5 feet of height, and blow the seed straight down," he says. "But if I were seeding with an airplane, it doesn't matter if the plant is 4 feet tall or 12 feet tall, the seed will trickle down through the canopy. I can't do that. Since most of my cover crops are seeded with an air-seeder, they will have to go on de-tasseled seed corn."

Pick the right cover
The cover crop species also makes a difference. A small percentage of cover crops that are not as light-sensitive, like crimson clover, will still germinate beneath a thick canopy. Small-seeded cover crops tend to work better for surface-seeding, since they can fall through the canopy more easily.

"Cereal rye can grow in about any condition as long as it has enough moisture. Turnips and radishes are small-seeded, and they also work well," Krenk says. "The small seeds bounce off the leaves a little easier too, and it's easier to get down into the soil."

It's best to surface-seed on corn or soybeans once the canopy starts to lose color and leaves start dropping. This way, leaves don't catch as many seeds and more sunlight can penetrate the canopy.

It's also important to consider moisture and good seed-to-soil contact. The seed needs a moisture buffer to germinate, and more moisture is needed when surface-seeding compared to planting one-half inch in the ground. Larger-seeded crops also need more moisture to germinate; another reason smaller-seeded cover crops increase the chance for success.

"Seed-to-soil contact has more to do with moisture buffering than it does with getting seed in a position where it's in the soil so it's buffered from cold temperatures," adds Saathoff. "The more moisture we can get around that seed, the higher its chances for survival are."