Gordon Johnson, Extension Vegetable & Fruit Specialist; firstname.lastname@example.org
There is significant interest in cover crop mixtures, and in some cases up to 8 different species are being mixed together. As fall cover crop season is upon us, there are a number of considerations that growers interested in using cover crop mixtures should be aware of.
Cover crop species are commonly grouped into six major categories: 1) cool season grasses; 2) cool season legumes; 3) cool season broadleaves 4) warm season grasses; 4) warm season legumes; and 6) warm season broadleaves. In theory, a successful mixture will combine species from as many categories as practical based on the planting season. For late summer/fall planting we will be limited to 1, 2, and 3 above.
In addition, cover crop species can also be placed into groups based on the benefits they offer or functions they perform. This includes nitrogen fixation, nutrient (particularly nitrogen) uptake and recycling, compaction reduction, disease suppression, biofumigation, weed control, biomass accumulation, use as a mulch, winter killing to facilitate early spring plantings, and erosion control.
The first step in creating a mixture is to list the available species that can be used for the time of the year they will be planted in and the season(s) they will be growing in. For late summer and fall planting this would include small grains (wheat, barley, rye, winter oats, triticale), ryegrasses, rapeseed, other hardy Brassicas and winter annual legumes (crimson clover, hairy vetch, winter hardy field peas, subclover, many other clovers) for overwintering. If winter killed crops with extended fall growing seasons are desired then radishes, mustards, and spring oats would be examples of selections.
The second step would be to list what soil health attributes or other cropping system needs should be prioritized. For example, if a mulch for no-tilling vegetables into next spring is a priority then high biomass cover crops that decompose more slowly such as cereal rye or triticale should be in the mixture. Conversely, if early spring planting is the goal then winter killed cover crops should be in the mixture. If compaction needs to be addressed then radishes or other species in the Brassica family with strong tap roots should be in the mix. If nitrogen fixation is a priority then a high N fixing potential legume such as hairy vetch should be included.
The final step would be to develop seeding rates for each mixture component. This is critical because too much of one component can outcompete other components and limit their survival or limit their usefulness in the mixture. Unfortunately there is little actual science to guide seed rate determinations. Cover crop mixture research has been very limited. With that said, there are some guidelines to follow.
As a general rule, you should reduce the seeding rate from a stand-alone (one cover crop) rate by the percentage that you want to see in the final stand. For example if you want to have at least 50% small grain cover in a mixture with radishes once the radishes winter kill you would reduce you small grain seeding rate by 50% (from 120 lbs to 60 lbs per acre). Radish seeding rates would also be reduced by 25-50%. In a 6 species cover crop mix if you wanted to have equal amounts of all components in the final stand, then start with a seeding rate at 1/6 (17%) of a stand-alone rate.
While this is a good place to start, the rates may need to be modified additionally based on how competitive (or quick growing) each component is. Quick growing species such as ryegrasses or radishes may need to be reduced relative to slow growing species such as the winter annual legumes to allow the slower growing species to become established. The slow growing species are often are maintained 75%-100% of stand-alone rates in mixtures with quick growing species.
Overseeding rates will need to higher than drilled rates and adjusted also allow for the potential for establishment as a overseeding. Species with lower establishment rates as overseedings will need to have additional seed in the mixture to compensate for lower emergence.
Further modifications both in seeding rate or species selection may be necessary based on residual fertility or planned fall fertility programs. If there is significant residual nitrogen in the soil or if fertilization is planned (to give some components more growth potential) then non-legume species will be favored and rates should be adjusted or legumes should be left out of the mixture.
Another issue is timing for seeding. Some species should be planted 4-6 weeks before a killing frost in September. This would include rapeseed, radishes and other Brassica species as well as winter annual legumes. Small grains can be planted through October. The mixture needs to be planted in the time period that best meets the earliest needs of the species in the mix.
A practical consideration for cover crop mixtures is how they should be best seeded when seed sizes between species are considerably different. Drills with both large seed and small seed boxes are good tools to address this issue. Small seeded species would be mixed and placed in the small seed box and larger seeded species in the large seed box in the appropriate ratios. Alternating rows by blocking off some drill spouts is another option. Spinner spreaders may not work well where there are large differences in seed sizes. Do not forget to inoculate all legumes in the mixture. Seeding rates may need to be adjusted based on the planting equipment available.
Some seed companies offer mixes that have already been blended at certain proportions. While some of these may be suitable for Delaware, growers should look at the specific proportion of each component in the mix to see if it will meet the guidelines listed above. Consideration also should be given to whether or not the mixture can be effectively seeded without seed separation with available equipment
A successful mixture used in Delaware in the past has been to plant rye and crimson clover with hairy vetch (seeding rates of 30, 10, and 15 lb./A, respectively). An example of a potential September seeded cover crop mixture for Delaware with many winter hardy species is: rapeseed, ryegrass, cereal rye, crimson clover, and hairy vetch. A multi-species example with combinations of winter killed and winter hardy species is: radish, mustard, spring oats, triticale, crimson clover, and field peas.
Growers will need to do some experimentation on their own farms with different mixtures and seeding rates to determine what works best for their farm, growing conditions, planting windows and rotations.