With the challenge of growing cover crops, particularly after corn harvest, interest in interseeding cover crops into living corn and soybean has increased in recent years. Goals for doing so include using the cover to: grow nitrogen for the crop, remove excess nitrogen (in the case of seed corn), aid in weed and erosion control, increase biodiversity, determine any soil health benefits, and desire for fall biomass for grazing. Some are also concerned about increasing regulation and wanting to figure things out ahead of the curve. Planning is key when it comes to interseeding cover crop into corn or soybean. Planning needs to include the goal of why interseed, the cover crop species interseeded, how the cover will be interseeded, the corn/soybean crop development stage for interseeding, and the herbicide program used.

A few years ago, we wrote an article sharing what was known about interseeding cover crops. The following is information we’ve learned as an interseeding project between The Nature Conservancy, Upper Big Blue NRD, Nebraska Extension, 11 farmers, NRCS, and Kellogg’s.

Timing: In corn, we’d recommend aiming for V4 (four leaf collars). V5-V6 is almost too late in years where canopy closure occurs quickly. The literature says there’s no yield loss after V2. For soybean, aerial interseeding around senescence (leaves turning yellow) is one option. From plots interseeded at V4 in 2020, we felt that was almost too late for aiding establishment. This year we will be trying at planting through V2.

Species: Penn State has a mix that’s considered the interseeding standard; it includes annual ryegrass, red clover, and hairy vetch. From the 12 species mixes we tried, the annual ryegrass, vetch, red clover all survived and were growing this spring. Thus, most likely why it’s considered the standard.

In corn, we’ve tried multispecies mixes because of the growers’ and partners’ goals and testing what came through different herbicide programs. We found the first species to emerge were the buckwheat and cowpeas. The farmers liked the species that provided more of an understory like the annual and Italian ryegrass, collards and other brassicas, and buckwheat. Cowpeas grew up to the corn tassels and provided the greatest biomass. Most of the species went to seed. Cowpeas, forage soybeans, and sweet clover were fixing nitrogen in season.

For diversity, the flax and buckwheat upon flowering drew many beneficial insects to the field. Pests like grasshoppers ate the covers in the interseeded strips and left the corn alone from what I observed.

In soybean, wheat was planted in the soybean management field day trials last year with some success. This year we’re considering wheat + red clover for the fields that will be interseeded from planting through V2.

Herbicide Programs: This is the difficult part. I think ideally (and I’m unsure if this is even realistic yet), a cover between rows aiding in weed control, adding nitrogen, providing fall biomass, and regrowing the following spring to aid in weed control again with only needing to add herbicide in a band, would be pretty cool.

In the wet year of 2019, Callisto-type (Group 27) products did their job and kept re-activating. This led to covers dying in one field. So in 2020, I suggested no residuals in post- apps. The guys went with me on this with most doing a pre- with residual followed by a post- of only glyphosate or Liberty prior to interseeding. The July 9^th, 2020 windstorm causing plants to greensnap and/or bend caused problems with the canopy opening up and weed control in addition to biomass growth became a problem in these fields in competing with the corn crop. I was just sick about this and the guys extended much grace to me.

This year, for corn, some guys are sticking with last year’s program because it worked well for them, particularly in no-till with heavy residue. Another thing some may try is to apply a pre- with residual, interseed at V4 and then upon 1-2” growth of the covers, apply Dual II Magnum or Outlook (no grazing restriction with Outlook) to provide residual to aid in weed suppression. One farmer who applied generic Lexar pre-plant in some fields and did split app in others in 2020, still saw cover crop growth and emergence from the split applied. The cover crop growth in the split-applied was just stunted and thin compared to the fields where he applied the full rate pre-. He’s testing herbicide programs this year.

For soybean, there’s even more risk. For those who wish to plant and interseed at the same time, we’re trying a burndown immediately prior to interseeding (if they hadn’t applied an early pre- already), allowing the cover to get 1-2” tall and then go with a Group 15 chemistry. The other option we’re trying is going with their pre- with residual followed by interseeding at V2 and application of Group 15 herbicide after cover reaches 1-2” of growth. We also have guys who are planting soybean green into rye and will try interseeding after rye termination. We have no idea how all this will work and if others have ideas, please feel free to share! Next week I’ll share the yield and biomass results from the past few years.

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The above pics were post-harvest. The covers showed good at that time still but they seemed to disappear pretty fast upon more hard frosts.

The above pictures were taken in March 2021 of spring growth. Annual ryegrass, hairy vetch, red clover, and collards survived the winter. As time went on, one could easily ‘row’ where the covers were interseeded in June 2020. The taproots on the red clover were extra impressive to me! Also, pretty much always saw earthworms when I dug up one of these plants.