Author: Nick Andrews, OSU Extension and Nathan Harkleroad, Agriculture and Land-Based Training Association (Salinas, CA)
Publish Date: Summer 2016
The benefits of cover crops are widely touted, but in the real world of farming it is often challenging to work cover crops into your rotation. Crops like late sweet corn, pumpkins and winter squash, tomatoes and peppers, fall brassicas, etc. are usually in the ground until October or November. By then fields are often too wet to work, and even if you can prepare a seed bed (i.e. on a coarse textured soil), cover crops planted in late October or November don’t normally establish well or protect soil from erosion over the winter.
Short season crops are usually planted in succession with small blocks going across a field. Often the same piece of ground will be double or triple cropped. In this situation it is possible to squeeze in small blocks of cover crops, but they can be awkward to manage, and delay your ability to get back into that part of the field.
Successful cover cropping requires stepping back and taking a slightly longer term perspective on your crop rotation. The National Organic Standards emphasize cover cropping, and certified organic vegetable growers are generally required to include cover crops in their rotations, but cover crops usually have to pencil out for a non-organic farmer to incorporate them in rotations. Some benefits of cover crops are relatively easy to quantify. Legume covers can provide plant-available nitrogen (PAN) to subsequent crops. Work in Salinas, CA also indicates that high seeding rates can sometimes pay off in terms of improved weed control. Other cover crop benefits are less easily quantified. Cover crop leaves intercept raindrops during wet winter months and reduce their impact on the soil surface. Fibrous cover crop roots hold soil aggregates together and reduce the risk of erosion, while deep tap roots can improve water infiltration and potentially loosen hard pans. Roots and their exudates also support the growth of microorganisms when soil would otherwise be bare. For example, cover crops have been shown to significantly increase winter survival of mycorrhizal fungi and other beneficial organisms. Many of these organisms release glomalin and other compounds that hold sand, silt and clay particles together in strong soil aggregates. Increased aggregate stability and the channels created by plant roots help to increase water infiltration rates, which in turn can reduce runoff and erosion during storms. High biomass cover crops can also maintain or increase soil organic matter, which increases soil water holding capacity and nutrient mineralization, and can help reduce the risk of some soil-borne diseases.
One of the biggest trade offs in traditional cover cropping is that ground is not in a cash crop during the period of time that the cover crop is growing and for up to four weeks after the cover crop is incorporated while the residue is decomposing. There are also the direct costs of seed, ground preparation, potentially irrigation, and incorporation. Depending on your crop rotation, some cover crops can increase the risk of pest damage, for example, brassica cover crops could increase the risk of clubroot in a field. Pest relationships are complex and very site specific. The benefits of cover cropping have to be balanced against potential drawbacks related to your crop rotation.
Some farmers assume the benefits of cover crops can easily be replaced by purchasing and applying compost and organic fertilizers. This may be true with regard to crop nutrient demand, but cover crops are less expensive than most other purchased soil amendments, and can provide superior soil protection during heavy rain.
Cover crops can help with infiltration of rain water and slow or prevent surface runoff. In areas dependent on groundwater, this can be especially helpful for recharge of aquifers. However, a maturing cover crop can also begin to transpire groundwater resources, resulting in a net groundwater loss in arid regions. The best management technique in such a situation may be to terminate the cover crop early.
The best way to plant most cover crops is with the use of a grain drill. Ideally, this results in cover crop seeds being planted to a uniform depth and at the same rate across a field. Under dry conditions seed can be placed deeper into moisture. Other manual and tractor-assisted options include broadcasting the seed with a tractor mounted PTO-driven spin spreader, or a crank-style manual belly broadcaster for smaller areas. Broadcast seed should then be lightly buried with a disk, roller or harrow to prevent desiccation and depredation by birds and other animals. Research in Salinas indicates that burying with an offset disk results in a more uniform planting than a tandem disk. On finer textured soils in western Oregon rollers have been successfully used to incorporate seed. For a manual option in small areas, seed can be buried with a rake. Most farmers use what they have available and choose to purchase a better option when consistently cover cropping larger areas and finances allow.
Inter-seeding cover crops
Researchers at Pennsylvania State University developed a seed drill that applies fertilizer and herbicide, and inter-seeds cover crops in one pass. Interseeder Technologies in Lancaster Co., PA is now commercially producing this equipment. Ed Peachey (OSU Extension) started working with this drill in 2015. About eight farmer collaborators are working with Ed. They are interested in establishing cover crops in late harvested sweet corn, silage corn and winter squash. Some are on highly erodible soils that flood each winter near the Willamette Valley, and they are mainly trying to establish cover crops that will form a root ball that help keep soil in place even if the cover crop gets flooded out. Other farmers are interested in improving soil organic matter, competing with weeds, and the nitrogen contributions of legume covers.
With this drill, farmers can seed into soil moisture. This avoids the need for extra irrigation for inter-seeded cover crops broadcast onto the soil surface, and reduces seed predation. Ed has been working with several cover crops to investigate a wide variety of objectives with cooperating farmers. Covers include Cayuse oats, black oats, barley, triticale, cereal rye, winter wheat, Sudan grass, orchard grass, tall fescue crimson clover, red clover, hairy vetch and common vetch. He has been evaluating the best time to interseed cover crops (crop stage and time of year), cover crop species, seeding depth, and herbicide compatibility. In sweet corn the cover crop is typically seeded at V6 growth stages (6 visible collars). Establishment is less complicated in organic fields where herbicide compatibility isn’t an issue. All the cover crops did well there. Trials are expanding in 2016 from Junction City to Brooks. Subscribe to OSU Veg e-Notesto learn about summer field days and to read more about project results.
Ed’s work is very relevant to smaller farms who might not be able to justify the expense of a specialized seed drill, but could broadcast covers into late summer crops and rake them in with cultivation equipment. Nick and others have had some success broadcasting covers into sweet pepper, eggplant, tomato, winter squash and sweet corn in small demonstration plots. Important factors seem to be seeding covers early enough so they could establish well before the vegetable canopy closes, and more frequent irrigation for germinating cover crop seedlings left near the surface during hot, dry weather.
High-density cereal-legume mixes for weed suppression
Surveys of small-scale growers frequently rate weed management as one of the top production challenges, and poorly managed cover crops can increase weed problems. Many winter weeds (e.g., annual nettle and shepherd’s purse) can set seed in less than two months, whereas a full-term cover crop is typically growing for a period of three to seven months. Increased weeds can have a negative impact on an organic farm’s profitability through reductions in yield and/or high weeding costs. Fast-growing cover crops can effectively smother weed competition and reduce weed populations in subsequent cash crops. If weeds are a concern, it is important to make cover crop decisions that can help reduce weed pressure over time.
Cereal-legume mixes have been very popular as cover crops among organic and small-scale farmers, partly due to the addition of nitrogen through nitrogen-fixing legumes. However, the use of legumes may not always be the best for winter weed control due to low plant densities at typical seeding rates and slow emergence in cool temperatures. According to USDA-ARS research in Salinas led by Eric Brennan (Research Horticulturalist focused on organic vegetable production), when cereal-legume cover crops are planted at high-densities (2-3x typical commercial seeding rates) they can reduce weed populations compared to standard seeding rates. Extra seed equals extra seed costs; however, this may pay off in reduced subsequent weeding costs in a cash crop. Good weed suppression can be achieved with cereal-only cover crops, as well, without needing to increase the seeding rate. See Eric’s YouTube Are legume-cereal cover crop mixtures a good fit for organic vegetable production? for a discussion on cereal vs. cereal-legume cover crop mixtures in California.
Furrow cover cropping in organic strawberries
A furrow cover cropping system has been developed by Brennan and Richard Smith (Farm Advisor with the University of California Cooperative Extension) for organic strawberries grown with plastic mulch on the Central Coast of CA. The system involves planting ‘Ida Gold’ mustard in the strawberry furrows before the winter rains come. This mustard is allowed to grow through the winter and is then easily mow-killed with a weed-whacker when the cover crop reaches the height of the beds. While this system doesn’t offer all of the benefits of growing a cover crop on an entire field, it does minimize erosion and surface run-off, and promote rainwater infiltration. These are serious resource concerns, especially in drought affected regions like Monterey County, CA where strawberries are planted on over 10,000 acres of land each year.
Growers in the Willamette Valley who are interested in adapting this system should be careful not to allow mustards to flower especially if they are near fields used for specialty brassica seed production. Pollen transfer could destroy your neighbor’s seed crop. Also during the current blackleg epidemic in Oregon, ODA Rules require that all brassica seed planted in Oregon is tested and found free of blackleg.
Nitrogen release from legume cover crops
Dan Sullivan (OSU Extension) and Nick studied nitrogen (N) release from cover crops in the lab and the field from 2006-2011. They found that a N mineralization model published in 1998 (J.T. Gilmour) accurately predicted N release after ten weeks from a wide variety of cover crops in Western Oregon. They incorporated this model into the OSU Organic Fertilizer and Cover Crop Calculator. The calculator uses cover crop biomass, percent dry matter and total percent nitrogen to predict plant-available nitrogen (PAN) release after about ten weeks. To use the calculator, sample your cover crop just before incorporation, and send a sample to a lab for percent dry matter and total nitrogen analysis. You can also use the calculator to estimate PAN release from organic fertilizers and the cost of your cover cropping and fertilizer program. Estimating plant-available nitrogen release from cover crops is a PNW Extension publication that explains the research used to verify this model, and how to sample cover crops. The publication also describes a shortcut method for generating rough estimates of PAN release if you prefer not to send samples to a lab. Figure 1 shows that as the nitrogen content of a cover crop increases (i.e. from increasing percent legume in the stand), the proportion of total nitrogen released as PAN during decomposition also increases.
Finding a cover crop niche can be difficult for small-scale vegetable growers, but it definitely is possible. There will always the tempting reasoning that cover cropping just doesn’t pencil out for the farm. However, many of the benefits of cover cropping are hard to quantify and/or require a longer term approach. With the right management, cover cropping can even save a grower money on fertilizer and weeding costs. Innovative techniques like planting high-density cereal-legume mixtures, furrow and relay cover cropping may make cover cropping more feasible and increase the benefits to a farm operation. Perhaps try out some of these techniques over a small area this season to gauge their suitability to your farm. There are financial and technical resources to help a grower integrate cover crops into rotations, including the USDA Natural Resource Conservation Service’s Environmental Quality Incentives Program (or ‘EQIP’)and Oregon State University Extension. Don’t be afraid to ask for assistance to get the most out of your cover crops!
This article is based upon work supported by the Natural Resources Conservation Service, U.S. Department of Agriculture, under NRCS Conservation Innovation Grant 69-3A75-16-003, USDA’s Western Sustainable Agriculture Research & Education program, the Oregon Processed Vegetable Commission, the OSU Agricultural Research Foundation and Meyer Memorial Trust.