By Chris Proctor, Roger Elmore, Brian Krienke, Dean Krull, Steve Melvin, Jenny Rees and Michael Sindelar

An increasing number of farmers are integrating cover crops into their cropping systems with goals of erosion control, increasing soil health properties, adding or capturing nitrogen, grazing for livestock, or aiding in weed suppression. Cover crop establishment after corn and soybean harvest can be challenging and is not always successful.

To adapt to this challenge, an increasing number of farmers are considering interseeding cover crops. To accomplish this, cover crops are seeded before the corn or soybean crop reaches canopy closure. Development of commercially available interseeding equipment (high-clearance drill, high-clearance broadcast air seeder) is one way earlier cover crop planting has been achieved.

In general, seed that is placed in-soil compared to surface broadcast results in more consistent establishment. The goal of interseeding is to produce more cover crop biomass in the fall and following spring than is possible when planting after cash crop harvest. The interseeded cover crop emerges and has minimal growth during the season because of limited light interception below the crop canopy. After the cash crop harvest, however, interseeded cover crops are able to produce more biomass than postharvest seeded cover crops.

This article highlights research available regarding interseeding and some considerations for farmers considering interseeding.

What the research shows

Contact your crop insurance agent with any questions.

Nebraska research

Research at the University of Nebraska-Lincoln tested broadcasting cover crops into corn —simulating aerial broadcast seeding. Several cover crop species were interseeded [60-pounds-per-acre rye; 20-pounds-per-acre hairy vetch; 40-pounds-per-acre soybean MG VIII; 10-pounds-per-acre radish; and a 38-pounds-per-acre mixture of rye, hairy vetch and radish (30, 5 and 3 pounds, respectively)] at corn planting and V8 development stage. The later seeding date had no effect on corn yields, but produced very little cover crop biomass. However, the interseeding at planting reduced yields by up to 160 bushels per acre.

In a 2018 on-farm research and Natural Resources Conservation Service demonstration study, farmers in Merrick County compared a fall dormant seeding vs. fall dormant seeding plus spring interseeding vs. check. In fall 2017, both the dormant-seeded treatment strips and the dormant-plus-interseeded treatment strips had a cover crop mix. The cover crop mix in both treatments was then terminated with glyphosate on May 10, 2018.

Corn was planted May 17 followed by a postemergence application of 32 ounces of Glyphosate and 5 ounces of Status per acre June 1. They used a Hiniker interseeder to plant the cover crop mix June 26, which included 6 pounds per acre of cowpea, 6 pounds per acre of soybean, 0.5 pounds per acre of crimson clover, 5 pounds per acre of sunn hemp, 2 pounds per acre of hairy vetch, 3 pounds per acre of buckwheat, 0.5 pounds per acre of chicory, 0.5 pounds per acre of flax, 0.5 pounds per acre of rapeseed-canola, 6 pounds per acre of Elbon cereal rye and 6 pounds per acre of spring oats. There were no differences in corn yield with any treatment; no biomass measurements were reported in the study.

In a 2017 on-farm research study, a Dawson County farmer compared planting various cover crops at the same time as planting corn in an organic system. Treatments included 12 pounds per acre of soybeans; 2 pounds per acre of clover; a 5-pound-per-acre mixture of phacelia, lentils, and turnips; twin-row corn; and check of monoculture corn. There were no yield differences among treatments. No cover crop biomass measurements were reported in the study.

On-farm research as part of Nebraska’s Central Platte Natural Resources District's Demonstration Education Project has been experimenting with interseeding cover crops into commercial corn since 2014. Different planting methods and cover crop species have been tested to find methods that work in this shaded environment.

Two primary conclusions have surfaced from these observation plots. First, seed-to-soil contact brings a higher consistency in emergence and establishment than other broadcasting methods, and second, timing of planting also is a very important factor. It has been found that V4 or slightly earlier renders the most success in emergence and early growth of the cover crop. 

These evaluations will continue in the future to help identify mixes that result in consistent success. The YouTube video, “Benefits of Cover Crops,” shows an overview of the 2019 demo plots.

Research from other states

Researchers at Penn State University have conducted numerous studies regarding interseeding cover crops and have even commercialized an interseeder. A fact sheet available from Penn State provides a summary of their research and their recommendations.

Researchers at the University of Wisconsin seeded radish, red clover and winter rye into V5 corn. The cover crops achieved good growth up to grain harvest. Biomass of the cover crop species varied after harvest due to being covered by the corn residue.

Rye was the only crop that needed to be terminated the following spring. There was no corn yield reduction and no visible stress to the corn in the two years of their study. A fact sheet summarizes findings from the University of Wisconsin trials.

Researchers at Michigan State compared interseeding annual ryegrass, crimson clover and tillage radish into corn from the V1 to V6 development stages. Weed pressure was greatest at V1 interseeding and decreased with each successive interseeding stage. Corn yield was negatively affected at the V1 interseeding timing but other interseeding timings had no effect on corn yield. Interseeding at V4 to V5 corn development stages resulted in the greatest cover crop biomass.

An Ohio State University researcher compared a monoculture corn crop to corn interseeded with individual species treatments of radish, annual ryegrass and red clover. The cover crop species were interseeded using a high-clearance drill and via broadcast seeding at V6. Cover crop establishment was considered evenly distributed with both methods. There were no yield differences among the treatments.

Biomass of red clover and radish after grain harvest was significantly higher than that of annual ryegrass. The biomass of the red clover the subsequent spring was significantly higher than that of the annual ryegrass.

In a 2017 on-farm research study in Ohio, 13 pounds per acre of annual ryegrass vs. 30 pounds per acre of cereal rye were interseeded into V6 corn and compared to a check treatment. Both grass cover crops had 4 to 6 inches of growth at the time of corn harvest. There were no yield differences among treatments.

In a similar 2017 on-farm study in Ohio, 13 pounds per acre of annual ryegrass, 30 pounds per acre of cereal rye, and 27 pounds per acre of oats were interseeded at soybean development stages of R3, R5 and R7 in twin-row soybean. The soybeans interseeded with oats R5, and soybeans interseeded with cereal rye and annual ryegrass at R7 yielded significantly less than the check (no cover crop).

All other cover crops and seeding dates did not result in soybean yield loss compared with the no-cover crop control.

Interseeding for nitrate management

Nitrate contamination of ground and surface water is a critical issue in Nebraska. In agriculture, this often results from fertilizer-N applied to corn in simplified cropping rotations (i.e. continuous corn or corn-soybeans). To alleviate N loss and contamination, cover crops have been integrated with some success into these cropping systems.

Cereal rye is commonly used for its associated benefits (low cost, cold tolerance, rapid biomass accumulation, N capture, carbon cycling and weed suppression). However, several detrimental effects (N immobilization, N leaching and yield penalty on the cash crop) are associated with mono-crop cereal rye.

To alleviate some of the detrimental effects from cereal rye related to N immobilization and compensatory fertilizer-N applications, researchers suggests targeting a 1-to-1 biomass ratio of a grass-legume at the time of termination. To achieve a 1-to-1 biomass ratio, current seeding recommendations suggest a 20-to-80 grass-legume seeding ratio when planted after corn harvest.

However, the high cost of legume seed makes the relatively high recommended seeding rate often not worth the investment for farmers. If legumes are established earlier in the season, they are more competitive, which requires a lower proportion of seeding ratio to achieve desired benefits.

Interseeding provides an opportunity to obtain the ideal 1-to-1 biomass ratio with a lower proportion of legumes and a reduced seeding rate, which would reduce costs to the farmer.

Chris Proctor is a weed management Extension educator; Roger Elmore is a professor in the Department of Agronomy and Horticulture; Brian Kreinke is a soils Extension educator; Dean Krull is a research technologist; Steve Melvin is an irrigated cropping systems Extension educator; Jenny Rees is an Extension educator; and Michael Sindelar is a cropping and water systems Extension educator at the University of Nebraska-Lincoln.