Chapter 6

Cover Crops for Organic Farms

Cover Crop Strategies with Crop Rotations

Cover crop strategies refer to how the cover crop fits into a crop rotation plan. Cover crops offer many benefits for agriculture that include erosion control; reduced compaction and nutrient leaching; increased water infiltration; improved soil biodiversity; pest suppression; increased carbon sequestration and maximum nutrient recycling; and improved air, soil, and water quality. One of the biggest challenges of cover cropping is to fit cover crops into your current rotations, or to develop new rotations that take full advantage of their benefits. A wrong combination of cover crops may exert negative attributes, so a thorough understanding of cover crops selection and management is needed to minimize negative outcomes. Cover crops can be used in a variety of ways with field crop and vegetable rotations as discussed in the following sections.

Replacing Fallow with Cover Crops

Replacement of fallow with cover crops may break the disease cycle, sequester soil organic carbon, conserve soil, and improve soil properties. Fallow is the stage of crop rotation in which the land is deliberately not used to raise a crop. Fallow can be accomplished with a year-long, cycle of cover crop planting and incorporation or by a summer fallow, a practice of allowing land to lie idle during the growing season.

Year-long Fallow with Cover Crops

Year-long cover crops are an excellent way to rebuild fertility and accumulate plant biomass over a longer period. This approach will especially benefit fields with low fertility and farms with limited access to manures and other sources of organic amendments. Planting perennials with a deep tap root can have tremendous soil improving benefits when allowed to grow for several years.

Summer Fallow with Cover Crops

Summer fallow cover crops are more common in vegetable than field crop rotations. Typically, a fast growing cover crop such as legumes, cowpeas, or soybeans are planted during this period. Other plants, such as sorghum-sudangrass, can break up a compacted soil through its extensive root system.

Winter Cover Crops

Winter cover crops are planted after harvest of a cash grain, oilseed, or vegetable crop before the next crop is planted the following spring. Winter cover crops are planted at least six weeks before a hard frost. In colder climates, the winter cover crops selected need to possess enough cold tolerance to survive hard winters. Rye, wheat, triticale, barley, and spelt are all winter cereal grains that offer similar functionality in a crop rotation but have some important differences for management. There are differences in winter hardiness and planting dates. Winter hardiness and ability to establish the cover crop late in the fall goes in the order rye > spelt > triticale > wheat > barley.

Annual Ryegrass

Annual ryegrass is a unique grass species in its characteristics and management. It requires an earlier planting date than the cereal grasses and reaches maturity more slowly in the spring. Annual ryegrass has a deep, fibrous root system that is excellent for improving soil structure, increasing soil organic matter, and capturing nitrate. Annual ryegrass has a particularly high forage quality and can produce more tonnage than the cereal grasses when multiple cuttings are made.

Interseeding

Interseeding, also known as relay seeding, is the planting of a cover crop into a growing cash crop. This strategy promises to help annual cash crop growers incorporate cover crops into their crop rotations with late harvested cash crops. By not waiting for the cash crop to be removed, earlier seeding provides quicker canopy cover, reducing weed pressure and providing over-winter erosion control. The cover crop will grow rapidly once the cash crop is removed or dies back and allows more sunlight to reach the cover crop. This can lead to an increase in cover crop biomass production, and presumably, better erosion control and soil organic matter enhancement because the cover crop is in place longer. Seeding cover crops during the growth of cash crops is especially helpful for the establishment of cover crops in areas with a short growing season.

Cover Crops Interseeded into Vegetables

Some vegetable growers, especially those living in colder climates with short growing seasons, broadcast cover crops into established vegetables just before a final shallow cultivation to remove existing weeds and incorporate the cover crop seed. Sowing should be delayed enough to minimize competition with the vegetable crop, but early enough so the cover crop can establish well and then withstand the harvest traffic. Typically, a good time to sow is at last cultivation, before the crop canopy closes. Essentially, this strategy utilizes the time after the vegetable crop’s minimum weed-free period to begin growing a cover crop in lieu of late-emerging weeds.

Cover Crops Interseeded into Corn and Soybeans

Growers can interseed cover crops into standing corn and soybeans before harvest (See Figure 6.7). Interseeding into soybeans or corn should be completed before leaves drop, when leaves yellow and begin to droop down. Grass cover crops such as rye, wheat, triticale, spelt, barley, and oats are typically planted in a crop rotation between corn and a summer annual legume, such as soybeans.

Smother Crops

Smother crops are effective weed suppressors because they produce large amounts of biomass in a short time, and it’s easy to reap these weed-suppressing benefits while extending the crop’s use to forage. For a smother crop to be effective it must establish fast and completely. It must be inexpensive to use, and should be managed to minimize volunteers. Fast growing buckwheat and sorghum-sudan, hairy vetch, and sweetclover help control weeds by growing a thick canopy that reduces the amount of sunlight available to help weed seeds germinate and grow.

Strip Cropping

Strip cropping involves growing cover crops and cash crops simultaneously in alternating rows. Strip cropping provides a simple method for rotating a cover crop with a vegetable planting. Alternate field strips or beds of a fall- or spring-planted cover crop with strips of early-planted vegetables like potato, onion, cabbage, lettuce or peas. Adjust the width of your fields to accommodate easier cover crop seeding (using your seeding equipment as the standard of measure). Strip cropping is a low-cost, low-input way of getting the benefits of a cover crop. For agronomic crops planted on a larger acreage, the strip-cropping concept can still apply, but the strips will be large, field-sized strips with the width adjusted for your particular farm planting and harvesting equipment.

Click on the following topics for more information on cover crops for organic farms.