Organic Broadcaster

Interseeding cover crops in cash crops shows promise

By Erin Silva

Cover crops have long been recognized in organic agriculture for their many benefits, including reducing the risk of erosion, building soil organic matter, and fostering soil ecology and biology. However, in upper Midwestern organic grain rotations, finding windows to establish cover crops between cash crop phases can be difficult, limiting cover crop choices.

Increasingly, farmers are looking to interseed cover crops into cash crops. This promising management alternative lets farmers use a more diverse selection of cover crops in their cash crop rotations than would otherwise be possible in the short growing season remaining after the cash crop harvest. While more work needs to be done to finesse this practice for organic systems, some general guidelines can increase the success of cover crop establishment without interfering with cultivation or causing unintended competition with the cash crop.

Researchers at the University of Wisconsin-Arlington Research Station use a modified drill spanning four corn rows to plant cover crops between rows.

Cover Crops into Corn
Successful interseeding relies on understanding the relationship between cover crop and corn crop growth and management. The ideal management for interseeding cover crops into established organic corn likely varies a bit from region to region and farm to farm, depending on climate, soil type, and equipment resources. The competitiveness of the corn crop, heat units, and the tilth and water-holding capacity of the soil may each impact cover crop establishment and related fall (and potentially spring) biomass accumulation.

Interseeding cover crops in an organic system requires a balance between maintaining effective weed management and providing the environment that will allow the cover crop to germinate and establish. Unfortunately, the perfect conditions for cover crop seed germination—adequate soil moisture and sunlight reaching the soil surface—also promote weed seeds to germinate.

In conventional systems, interseeding cover crops into standing corn has shown to be most consistently successful when performed at approximately the V5-V7 stage. This is near same time as the last row cultivation of corn, or just prior. This past year (2017), the organic corn on our research plots in southern Wisconsin reached this stage approximately the first week of July. With good soil moisture and warm soil temperatures, the cover crop seed will be able to take advantage of the last few days that the sunlight can penetrate through the corn canopy and germinate. Interseeding at the V5-V7 stage of corn also aligns with the latter end of the critical weed-free period for corn, which lessens the risk of negative competition from the cover crop.

Different equipment can successfully be used to plant cover crops. Modified drills can be made or purchased; depending on row spacing of corn, drill units are removed to limit seeding to the width between corn rows, allowing the drill to pass over the 8” high corn without damage. Another way interseeding is often accomplished is by broadcasting using an air seeder or seed spinner mounted on the back of a cultivator. Higher cover crop seeding rates must be used when broadcasting versus using modified drills. Additionally, the risk of poor establishment is greater when broadcasting, as cover crop seed germination is dependent on adequate rainfall close to the time of seeding.

Cereal rye grows between the corn rows at the University of Wisconsin-Arlington Research Station. Researchers used a modified drill to interseed the three rows of rye between corn rows. Photo by Erin Silva.

Species Selection
When choosing a cover crop for interseeding into corn, look for types that are easy to establish, can tolerate (and germinate in) dry soil conditions and low light conditions. Several grass and legume cover crops fit this description, including annual ryegrass, cereal rye, red clover, crimson clover, and forage radish—which are all somewhat drought- and shade-tolerant, and relatively easy to establish. These can be planted either singly or in a mixture.

If you prefer a grass cover crop, cereal rye and annual ryegrasses are good choices, both addressing different management needs. Annual ryegrasses typically winterkill in the upper Midwest, facilitating spring management on soils that are colder and slower to dry out in the spring. Cereal rye, however, is a winter annual that will survive the winter and regrow in the spring. This crop can very effectively protect the soil from erosion and add additional spring biomass. Annual ryegrass can be seeded at 15 to 20 pounds per acre as a single species or 10-15 pounds per acre in a mix with clover. Cereal rye can be planted at approximately 2 – 2.5 bu/ac alone, and 1.5 bu/ac in a mixture.

Of the clovers, medium red clover has been most consistently successful, as it is fairly shade-tolerant, has good winter hardiness, puts on a lot of biomass (leading to some spring nitrogen credit), and costs a bit less than other clover seed. Plant medium red clover at 10-12 pounds per acre as a single species, and about half that as a component of a mix. Crimson clover is another option, potentially putting on more biomass in the fall; the trade-off is that it is a bit less wither-hardy. Crimson clover should be seeded a couple of lbs/ac greater than red clover.

Forage radish has also been successfully used as an interseeding cover crop. Rates of about 8 lbs/ac could be used, or a lighter rate (3 to 5 lbs/ac) in combination with a grass. Don’t expect the same degree of fall growth, however, as compared to an August-seeded radish cover crop. Even with lesser growth, however, the plants do produce some biomass and bio-drilling taproots. Radish can be a good option to further add diversity to your cover crop mix.

Impact of Harvest 
Typically, interseeding works best with corn grown for silage, since that is removed earlier in the fall, allowing a longer window for the cover crop to put on additional growth. While driving over the field may do some damage to the cover crop, it usually will quickly recover and, within two weeks with good growing conditions, will look none the worse for the wear. More risk of damage exists if wet soil conditions are present at harvest, leading to compaction by the equipment and inhibiting cover crop recovery.

Maintaining the cover crop while harvesting corn for grain can be a bit more challenging, not only due to the shorter window for enhanced cover crop growth, but due to corn stover covering the cover crop. To mitigate this risk, combines could be operated a bit higher to avoid shredding the stalks, while still harvesting the grain. Avoid mowing the corn stalks after harvest as this could smother the cover crop with corn stover. Leaving a high stubble in the field (versus mowing shorter) can also to reduce the amount of stover on top of the cover crop. The amount of cover crop biomass accumulation in the fall will ultimately depend on a number of factors, including cover crop species, success of cover crop establishment, harvest dates, soil fertility, soil moisture/rainfall, and heat units accumulated in the fall.

UW-Arlington Organic Research Plots 
This past season, we tried interseeding our organic silage corn after last cultivation for the first time. We selected three cover crops (cereal rye, forage radish, and red clover), each planted on its own. We used a modified drill that spanned four 30-inch corn rows to plant the three inside row widths between the corn with three rows of cover crop spaced 7.5 inches apart.

We managed the corn as we typically would: applying manure the previous fall, planting corn in late May at 32,000 plants per acre, tine weeding within 24 hours of planting, tine weeding or rotary hoeing 2-3 times depending on soil condition, and then row cultivating, first with narrow and then with wider sweeps.

At the V5-V6 stage, just after last cultivation on July 1, we drilled the cover crops. Cover crop seeding rates were 10 lbs/ac red clover, 8 lbs/ac radish, and 3 bu/ac cereal rye. (We chose to bump the seeding rate higher to assess the feasibility of integrating the strategy into no-till soybeans with rolled rye).

While we still haven’t harvested the silage, the cover crops established well under the corn canopy with no detrimental impacts on weed management. We will collect yield data at harvest, and measure cover crop growth the following spring. Stay tuned for more research updates as we continue with the study!

General Tips:

  1. Don’t set expectations too high—be conservative. As with many aspects of the organic system, cover crops are a biological management tool, and their performance will vary from year to year depending on soil and weather conditions.

  2. Choose cover crops that best align with your goals. Do you want to add carbon/organic matter back to the soil? Protect the soil from erosion during the winter? Have a late fall crop for grazing? Add fertility for the following cash crop rotation phase? Knowing what you want to accomplish will help you decide which cover crop or cover crop mixture best fits your desired cropping system needs.

  3. Have a plan for spring management. If you choose a cover crop that will overwinter, how will you manage it the following spring? Will it interfere with field activities and management of the cover crop? Remember, cover crops should be incorporated approximately 17 days prior to cash crop planting to avoid issues with seed corn maggot laying eggs on the decaying residue and impacting cash crop germination. (See Insect IPM in Organic Field Crops: Seedcorn Maggot, learningstore.uwex.edu/Assets/pdfs/A3972-01.pdf). Additionally, some cover crops, such as cereal rye, can begin growing and put on a lot of biomass quickly in the spring, making timely incorporation critical.

  4. If trying this (or any new technique) for the first time, limit the planting to an area of the field that is representative of the overall conditions. Replicate these blocks three times in the field, while keeping all other management the same. Then, monitor yield and quality to ensure there are not any unintended effects. 5. Don’t be afraid to ask questions and learn more from land-grant university organic research programs, organic education organizations, and other farmers

Erin Silva is an assistant professor at the University of Wisconsin-Madison. She leads the Organic and Sustainable Agriculture Research and Extension program.

From the September | October 2017 Issue

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