Organic Broadcaster

Spring offers tight window for planting cover crops—follow these tips

By Matt Leavitt, Albert Lea Seed

Cover crops are an integral part of any organic farming system. They protect a farm’s valuable soil resource from wind and water erosion, capture and hold nutrients, compete with weeds, break cycles of pests and disease, manage excess moisture, and contribute to organic matter formation. They provide living roots in the soil that feed complex soil microorganisms and contribute to a resilient farming system.

Most cash crop rotations keep the soil covered only three to four months and rarely during times of peak soil erosion—the spring and fall of the year. Cover crops can plug the gaps in these rotations.

Cover crops are a required part of a certified organic operation. Ideally, all organic farms should be working towards having as tight a rotation as possible with limited periods of bare soil. In practice, though, this can be difficult for a variety of reasons.

Farmers in the Upper Midwest (growing zones 2-4) face unique challenges incorporating cover crops into their farming operations that other, more temperate areas of the country don’t face. Wet and cool spring weather conditions can hamper planting times and make early season tillage, field prep and planting very difficult. Winters are severe and prevent all but a select few species from overwintering into the following season.

Windows to incorporate cover crops into a rotation can be very tight and dependent on favorable weather conditions and maximizing a short growing seasons for marketable crops. Farmers understandably favor ideal planting conditions for their cash crops versus trying to sneak in a cover crop with uncertain returns.

On the whole, the benefits of cover crops are typically related to how long they are allowed to grow. There are additional expenses to every field operation, and cover crops are no exception. Once seed, fuel, labor, and machinery (for planting and termination) costs are all calculated, there should be a feasible benefit to your farm to justify the expense; though on balance, cover crops are always a good investment.

Growers who have experimented with planting cover crops right away in the spring prior to cash crops have had inconsistent success and sometimes have to take an additional step to control the cover crop prior to planting. However, there are others that make spring-seeded cover crops part of their farming rotation when the weather allows.

Bob Yanda, from Midwest Bio Ag, recommends an early season planting of oats and peas as soon as the ground is fit to plant in the spring. This can activate the soil with living roots right away, feed soil bacteria and fungi, and protect it from excessive rainfall. He tells his growers to work this green cover crop down with shallow tillage (<2’’ disturbance) before a corn, soybean
or other row crop.

Cover Crop Selection
When adding cover crops to your rotation, it is important to think about what species will best meet your end goals for the farm and your soil. Cover crops consist of many different species and plant types, but can roughly be grouped into cool- and warm-season grasses, legumes, brassicas, and broadleaves.

When selecting the best-adapted cover crop(s) for your farm, it is important to establish what your overall goals are first. Are you looking to cover your soil? Build up organic matter? Fix nitrogen for the following crop? Provide early season forage for livestock? Provide a place to spread manure? All of these goals can be met by utilizing various cover crops seeded alone or in multi-species blends.

Grasses such as oats, spring wheat, spring barley, spring triticale, and annual ryegrass provide quick growth, thorough fibrous rooting, nutrient scavenging/sequestering, and make excellent livestock forage. Grasses have very economical seed costs and are easily planted by many different methods, including broadcasting. As they mature they have a higher carbon/nitrogen ratio which can help stabilize leachable nutrients in the soil. Grasses should typically be a part of any cover crop blend.

Spring annual legumes like forage peas, lentils, fava beans, chickling vetch, and lupins allow for early season nitrogen-fixation for following cash crops and provide deeper rooting and quicker breakdown after incorporation. Perennial legumes like red clover, yellow blossom sweet clover, alsike clover, and alfalfa provide more complete soil coverage after establishment and excellent nitrogen-fixation potential with adequate growth time. Legumes have a lower carbon/nitrogen ratio and are a good component in a mix, but seed costs are typically higher than grasses and brassicas. Always inoculate your legumes with the proper strain of rhizobium for maximum nitrogen-fixation potential. A word of caution: it is unlikely that any spring-planted legume would fix enough nitrogen to satisfy fertility demands for the following crop in a month or two—keep your normal fertility plan in place.

Most brassicas are not a great fit for spring planting as early season growing conditions encourage the plants to bolt and set seed as opposed to producing root and leaf biomass. The exception to this is spring-seeded mustard. Mustards establish very quickly, cover the soil, and are excellent competitors with weeds. They can even be frost-seeded, avoiding potentially costly and time-consuming early planting passes. The biomass decomposes rapidly and can suppress pathogenic soil fungi and nematodes. Mustards are not a great constituent in a mix; they are better suited for planting alone. Klaas Martens, of Lakeview Organic Grain in upstate New York, will frost-seed mustard before soybeans when appropriate. Ensure you mow your mustard at the first sign of flowering to prevent volunteer seed set.

Broadleaves such as flax and phacelia are shallow-rooted species that can add diversity to a cover crop mix, provide forage for pollinators, and terminate easily. They are useful as an additional species in a mixture but are rarely planted alone strictly for cover.

Cover Crop Mixes
Generally speaking, a mixture of cover crop species will best adapt to varying soil, landscape, and moisture conditions and maximize your investment. Species diversity will almost always result in a more resilient system and provide synergistic effects between the cover crops and the soil environment.

There is functionally no limit to how diverse and complex one can make a cover crop blend. The question becomes, how do you seed it? A critical consideration when formulating a mix is ensuring proper seeding depth and maximum seed-to-soil contact. Cover crop seed mixes can consist of a multitude of seed sizes and seeds per pound which all require differing depths and rates.

A drill or a brillion seeder is the preferred method, but they must be in good condition and properly calibrated. Broadcast seeding can also work and cover a lot of ground quickly, but plan on dragging, harrowing or lightly disking to cover the seed for the best chance of success. Many times you’ll need to do a couple of trial passes to try out your settings for a complex mixture and ensure you’re not getting too much or too little seed planted.

Broadcast seeders with an agitator can be advantageous for mixes with a range of seed sizes to prevent mixes from separating in seeding passes across a field.

Try to get your spring mix seeded as early as weather and soil conditions allow. Choose cover crop species that have some degree of tolerance for cold or frost since early spring weather conditions can be unpredictable.

While there is no limit to how many species are viable in a mix, mixing 3-5 species tends to work well and is manageable from a cost and seeding perspective. When appropriate, try to incorporate species from each of the cover crop groups in your mix. For example, if you’re looking for a cover crop to plant in the spring prior to full-season tomatoes or corn, consider a mixture of oats, peas, barley, and lentils.

Optimum Seeding Practices
Spring-planted small grains, peas, lentils, and chickling vetch are all a feasible fit before row crops planted in May/June. Oats alone or oats and peas are probably the most typically planted in the Upper Midwest due to their cost-effectiveness, forgiving growth, and frost tolerance. This green material has a relatively low carbon-to-nitrogen ratio than more mature residue, and activates soil bacteria when tilled in and rapidly decomposes. Incorporate the cover crops 10 days to 2 weeks prior to planting cash crops to allow for adequate soil digestion of the biomass, limit nutrient tie-up, and prevent any carryover pest issues. Shallow tillage or disturbance is preferred as opposed to any deep tillage passes.

Avoid rotating a legume cash crop in after a legume cover crop to minimize disease transfer.

A very common and successful way to incorporate cover crops into a rotation is to underseed spring grains (oats, wheat, barley, triticale) with clovers or alfalfa for plowdown. Small-seeded legumes like medium red clover, alsike clover, yellow blossom sweet clover, or alfalfa can be seeded at the same time as spring grains, don’t compete with the crop, and will be well established to continue growing after the grains are harvested.

Since rains after small grain harvest can be inconsistent, spring-seeded legumes are an easy way to ensure soil remains covered all season long. The legumes provide good growth and nitrogen-fixation potential for the next crop in rotation. You can allow the legume cover crop to grow until well into the fall or even into the following spring before termination, depending on your soil type and rotational needs.

Swathing small grains and drying in a windrow will dry down any taller clover/alfalfa biomass that has snuck through. Avoid cutting the clover off for forage and if you do mow it, leave the residue in the field.

Small-seeded legumes like medium red clover, yellow blossom sweet clover, and alsike clover can be frost seeded into standing winter small grains like winter wheat, winter triticale, or winter barley early in the spring when the ground is still frozen but free of snow (typically late February-early March). This system works in much the same way as underseeding spring grains with plowdown legumes except that the winter grains are established the previous fall. If you have winter grains in your rotation, this is an excellent way to maintain soil coverage nearly the entire season.

Since clovers are small, they can be broadcasted with ease even using small equipment like an ATV with a spreader.

Grazing Cover Crops
Incorporating animals into your farming operation is one of the easiest ways to make cover crops pay. With very few exceptions, nearly all cover crops can be grazed or harvested for forage. Spring grains will provide good tonnage for grazing or chopping and have good feed quality if harvested before heading. The optimal time to harvest to maximize quality and quantity would be boot stage.

Legumes like peas and lentils add marginal tonnage but have excellent forage quality and are a great addition to a forage mix. Again, spring grains with peas are an excellent early season forage option. About the only cover crop not recommended for spring forage is mustard.

Spring cover crops for forage provide a place to hold your animals, spread manure, and stretch your pastures.

Cover crops play a vital role in an organic crop rotation and there are ways to make them work for you. So if weather allows this spring, consider putting in a spring cover crop.

 

Matt Leavitt is an agronomist with Albert Lea Seed.

 

From the March | April  2018 Issue

 

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