Organic Broadcaster

Fall-planted cover crops build soil health, protect fields from erosion in winter

By Matt Leavitt, MOSES

As the growing season winds to close, our thoughts move towards harvest, fall field work, application of manure and other soil amendments, and cleaning up machinery for the winter. These shortening days also provide an excellent opportunity to think about planting a cover crop.

The multifaceted benefits of cover crops in a rotation are well understood. These crops contribute to the long-term health and maintenance of your most important farm resource: your soil. Cover crops can work in nearly every crop rotation and farming system in the Upper Midwest, but are especially well suited to organic systems where crop diversity and rotations are more complex and provide more windows of opportunity to successfully integrate cover crops. As organic operations are charged with maintaining or improving the natural resources on the farm, including soil and water quality, fall-planted cover crops can measurably achieve those goals on a year-by-year basis.

Unlike the short window in spring for planting, late summer and fall provides a wide opportunity to plant a diversity of cover crop species to suit a variety of production systems and rotations. Crops like small grains, silage corn, canning vegetables, dry edible beans and early soybeans are harvested early enough to accommodate a period of favorable weather for cover crop growth and development, keeping your soil biologically active until freeze-up.

Cover crop planting can also be a great compliment to fall field passes, giving producers a place to spread manure (and hold it in place), graze animals, harvest forage, help break down persistent crop residue, and can be timed with fall harvest or light tillage. However, as all Midwestern producers are well aware, fall weather can be erratic with early frosts and lack of reliable moisture.

While we strongly recommend cover cropping on a near-universal basis, the beneficial gains to your soil are typically related to good cover crop establishment, root growth, and biomass accumulation. Moreover, cash crops like corn and full season soybeans leave little time for adequate cover crop establishment and growth after harvest.

As with any planting window, there are critical questions to ask yourself when thinking about incorporating a fall cover crop into your rotation.
• What crop is coming off, and what crop will come next in rotation?
• What fall field passes need to be done on the field(s) in question (i.e., fall manure application, fall tillage due to drainage, amendments, tiling, etc.)?
• What are your primary goals with cover crops: protecting soil from erosion, fixing nitrogen, sequestering/scavenging nutrients, breaking up compaction, growing fall forage, etc.?
• How do you intend on seeding and terminating the cover crop?
• Do you desire a cover crop that will overwinter or winterkill?

Putting thought into the above questions can help guide which species or mixtures will be most successful on your operation as species, mixtures and end goals can be widely diverse.

Cover Crop Selection
There are a diversity of cover crops suitable for fall planting that generally fall into three major categories: cereal grains/grasses, cool-season legumes and brassicas.

Cereal grains are quick growing, cost-effective, easy to manage, accumulate ample biomass in a short time-frame, scavenge and hold nutrients, and have fibrous root systems. Winter cereal grains like winter rye and winter triticale are seeded in the fall of the year and reliably overwinter, providing the soil needed coverage during periods of high-erosion potential, continuing their growth right away in the spring. Spring grasses like oats, barley, and annual ryegrass are a good fit for producers in the Upper Midwest looking for cover crops that die out over the winter but still provide good biomass accumulation and rooting (though annual ryegrass can overwinter south of I-90 based on winter conditions). Cereal grains all provide excellent livestock forage, and should make up a portion of most fall cover crop blends.

Cool-season legumes like hairy vetch, common vetch, winter and spring peas, lentils, and faba beans form a symbiotic relationship with Rhizobium bacteria which fix atmospheric nitrogen and covert it to a plant available form. Given adequate establishment, legumes can provide a critical nitrogen source in an organic rotation.

However, legumes are often more expensive than grasses or brassicas and must grow to or near reproductive maturity (flowering) to reach peak nitrogen-fixation potential. Most fall-planted legumes will winterkill in the Upper Midwest with the exception of hairy vetch, which can reliably survive given an early planting date and adequate establishment before the snow flies. Hairy vetch has a penchant for producing hard seed, so ensure complete kill the following season, especially if small grains are a routine part of your rotation. As with any legume, make sure they’re inoculated with the proper strain of Rhizobium at planting.

Brassicas like radishes, turnips, forage rape, and kale capture and hold nutrients (especially nitrogen), compete well with weeds, break down rapidly and will winterkill given normal conditions in the Upper Midwest. Deep-rooted brassicas like daikon radish can mitigate compaction, and the residue breaks down completely over the winter, leaving a very mellow soil in the spring. It is important to note that one season of cover crops will not solve persistent compaction issues, but they can be part of a long-term solution if utilized year-after-year. Turnips, forage rape and other leafy brassicas also are excellent for livestock grazing and are a cost-effective component to cover crop mixes.

Late summer and fall are ideal for planting brassicas as the cool weather prevents the plant from going to seed, allowing it to devote all its energy to biomass accumulation and rooting. A little goes a long way with brassicas in a mix; keep seeding rates reasonable to prevent brassicas from overtaking a mix. Avoid brassicas altogether if you grow a lot of production brassicas for harvest (broccoli, kale, cabbage, etc.).

Cover crop mixes tend to give you the best return on investment and greatest chance of success given variable soil, moisture, environmental and weather conditions. Adequate species diversity in a cover crop blend can maximize resource use, rooting depths and synergistically enhance growth of the entire mix.

A cover crop mix can be as diverse and variable as you desire, but 3-5 species typically minimizes seeding issues and keeps costs reasonable. A grass/brassica mixture (e.g., oats/radish, oats/turnips) is a low-cost way to achieve diversity that can fit in multiple rotations. A 3-way grass, brassica, legume (e.g., oats/peas/radish, winter rye/hairy vetch/radish) mixture will provide benefits of all three species given a well-formulated seeding rate.

Managing Winter Rye
Winter rye is one of the most useful and ubiquitous cover crops for producers in the Upper Midwest due to its winter hardiness, adaptability to a variety of soil types and growing conditions, competitive ability with weeds, and ability to produce significant biomass in the spring of the year. In fact, winter rye can put on so much growth in so little time that terminating and incorporating it can be a challenge, depending on your machinery and timing.

Winter rye competes very well with weeds; its root system and young tissue exude water-soluble allelopathic compounds into the soil that can suppress the germination and growth of small-seeded weeds. It provides excellent forage quality for livestock and can be seeded basically until the ground freezes. (The seed can germinate at 35F!) However, the later you seed into the fall, the slower the stand is to establish and grow the following spring.

It is important to fit winter rye into the right rotation. If planted after full-season corn (before soybeans), winter rye has very little time to establish before winter. Growing winter rye before next season’s corn can also be problematic. Recent research from USDA and Iowa State has shown that winter rye roots can harbor Fusarium, Pythium, and other pathogenic fungi that can infect young corn seedlings. Moreover, winter rye can overutilize soil moisture and nitrogen in nutrient- or moisture-deficient soils, especially as it reaches the reproductive stage.

While it can be difficult to ascertain what characteristic of rye is actually stunting corn growth in the field, experts recommended waiting at least two weeks after incorporation to plant corn after winter rye. You can definitely get a poor stand of corn following rye without the right rotation management. Soybeans after winter rye, however, are much more forgiving and are a nice rotational fit.

Winter rye is the most popular choice for the organic no-till soybean system due to its vigor, tall growth, winter survival, competitive ability, allelopathic qualities, relatively early maturity, and resilient residue after termination. Research from University of Wisconsin and other institutions have helped growers be more successful with this system as it can combine cover crop benefits with an active row crop rotation with limited to no yield drag in optimum years.

However, there are some general guidelines to follow. Ensure you plant your winter rye early enough in the fall so it gets well established (typically before Sept. 30 in the Upper Midwest, but it can vary by latitude). Also seed enough so you have a complete stand (at least 3 bushels/acre or ~175 lbs/acre).

Seeding Recommendations:
After Corn (grain), Soybeans
As described above, there is little time after a full-season corn or soybean grain crop to get reliable establishment of most cover corps with the exception of winter rye.

Winter rye can be successfully drilled after harvest or flown into standing corn when the crop reaches physiological maturity (black layer for corn, leaf yellowing for soybeans). Time these seedings with rain to increase your chances of success. Also, seed prior to Nov. 1 (or even better, before Oct. 15).

Winter rye going into soybeans (produced under conventional tilled or no-tilled management) is a great rotation as soybeans are planted later than corn, which gives the winter rye time to establish. There is also a wide window to terminate the winter rye prior to soybeans, and soybeans don’t seem to mind being a little short on nitrogen going into the season.

It is not advisable to plant winter rye after corn going into small grains as it will be challenging to kill the rye before the optimal time to plant spring small grains. Having rye in your oats, wheat, or barley will reduce the desirability of your crop to marketers.

After Corn (silage), Soybeans, Dry Beans, Vegetables
When corn silage, early soybeans, sweet corn, vegetables or dry edible beans come off before the middle of September, there is an excellent window to seed a variety of cover crops beyond winter rye like brassicas and legumes. Brassicas and fall legumes should be seeded before Sept. 15 (can vary with latitude) in the Upper Midwest to provide the best chance for optimum growth and return on your investment; seeding the first week in September would be more ideal.

Oats or winter rye with radish, turnips or forage rape provide excellent late-fall forage and can hold onto spread manure. If you’re rotating into a higher-need crop like wheat or vegetables the next season, seeding peas with oats can help fix some nitrogen. Hairy vetch with oats or winter rye also work well in rotation if you have a cash crop seeding later (June).

Drilling the cover crops in after the cash crop is harvested will always give you the best establishment vs. broadcasting or other seeding methods. Always pay close attention to seeding depth when seeding multi-species mixes.

After Small Grains, Vegetables
If you’re harvesting a cash crop in early to Mid-August, you have ample time to seed a wide variety of cover crops. Annual legumes in the Upper Midwest like crimson clover, berseem clover, faba beans, sunn hemp, cowpeas, and common vetch could be seeded with success in a mixture and put on reasonable growth before the weather turns cool. Diverse multi-species mixes of cool-season cover crops (like oats, winter rye, peas, radish, etc.) and warm-season cover crops (like buckwheat, sorghum/sudan, cowpeas, etc.) could be explored if the mixture were seeded around Aug. 15.

Depending on the investment you choose to make in cover cropping, you could even plant a series at this stage. For example, you could plant buckwheat directly after cash crop harvest then follow that with a cool-season blend of oats/peas/radish or oats/radish. You could also save money by allowing a small grain crop to volunteer seed after harvest and drill in some higher-value cover crop species like clovers, legumes, or brassicas with it.

Matt Leavitt is an organic specialist with MOSES.


From the September October 2018 Issue

Comments are closed.