An increasing number of growers are integrating cover crops into cropping systems to reduce erosion, improve soil health, fix or sequester nitrogen, graze livestock or suppress weeds.
However, cover crop establishment after corn and soybean harvest can be challenging with mixed results. So, an increasing number of growers have asked about the potential for interseeding cover crops into an established crop.
Interseeding is primarily done before crop canopy closure or before harvest when the crop canopy begins to reopen. After harvest, the hope is the cover crop produces greater biomass, since it's already established, compared with seeding postharvest.
Here's a look at takeaways from studies in Nebraska and the Midwest:
Yield impact. Most studies show minimal corn yield impact by interseeding cover crops during the V5-V7 growth stages. Few studies exist on interseeding into soybeans.
Herbicide injury. Research consistently shows that rimsulfuron (Resolve, Basis) and safluenacil (Sharpen) tend to result in little to no herbicide injury on interseeded grasses, legumes, brassicas or mixes. Penn State University lists additional herbicide injury ratings.
Common options. Most successful legume options include clovers such as red, crimson, berseem and balansa. Cereal rye and annual ryegrass are the most common grasses interseeded. Buckwheat and phacelia were used in organic studies to attract beneficial insects.
Equipment. Many types of equipment have been retrofitted or bought to interseed cover crops.
Be prepared. Have your plans together a few growth stages before you interseed. From grower conversations, once the crop takes off, it is hard to catch up if interseeding plans aren't already in place. If you hope to interseed by V5-V8, have your seed and interseeder ready by V3, so you can get the cover crop interseeded at V5-V8 when the weather allows. The window is narrow and closes quickly.
Crop insurance. The USDA Risk Management Agency addresses interseeding in its cover crop guidelines. In a Q&A, RMA notes that coverage of an insured grain crop would not be affected by "seeding a cover crop into an existing crop at a time that will not impact yield or harvest of the insured crop. These examples do not affect the insurability of the insured crop, but if not done properly, may impact the indemnity received in the event of a loss on the insured crop." Contact your crop insurance agent with any questions.
As part of her Ph.D. research at the University of Nebraska-Lincoln, Angela Bastidas investigated interseeding by broadcasting cover crops into corn. She interseeded several cover crop species at different seeding rates, including rye, hairy vetch, soybean and radish at the time of corn planting and at the V8 stage.
The later seeding date had no effect on corn yields but produced little cover crop biomass. However, interseeding at planting reduced yields by up to 160 bushels per acre. As summer cover crop and weed biomass increased with the interseeding at corn planting, corn yields decreased.
In a 2018 on-farm research and Natural Resources Conservation Service demonstration study, farmers in Merrick County compared a fall dormant seeding versus a fall dormant seeding plus spring interseeding versus a 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 terminated with glyphosate on May 10, 2018. Corn was planted May 17 followed by a postemergence application of glyphosate and Status on June 1. They then used a Hiniker interseeder to plant the cover crop mix June 26, which included cowpea, soybean, crimson clover, sunhemp, hairy vetch, buckwheat, chicory, flax, canola, Elbon cereal rye and spring oats. There were no differences in corn yield with any treatments.
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. This included soybeans, clover, a mix of phacelia, lentils, turnips, twin-row corn and a check of monoculture corn. There were no yield differences among treatments.
Testing via on-farm research
If you are interested in testing interseeding via on-farm research, the following protocols can be found at go.unl.edu/kvjf. The two-treatment protocol compares interseeding versus a check.
The three-treatment protocol can be altered a number of ways, including:
- Interseeded species/mix 1, versus interseeded species/mix 2, versus check
- Broadcast interseeded, versus drilled interseeded, versus check
- Timing 1, versus timing 2, versus check
Nebraska Extension encourages growers to contact a local on-farm research Extension educator if interested in trying to interseed cover crops.