Cover Crops Can Accomplish Many Important Tasks for Growers

This article by Lee Dean, Managing Editor, appeared in "The Vegetable Growers News," May 1999.

Michigan State University Extension specialists are working to 'cover' the state's vegetable growers with information about an important practice -- the use of cover crops.

Researchers say cover crops are planted to improve soil quality, enhance nitrogen management, provide erosion control, and suppress weeds, insects, and nematodes.

Cover crops can enhance nitrogen production and can also reduce nutrient losses. Grass can be used to take up excess nitrogen and help alleviate groundwater leaching. Cover crops can be used to reduce wind and water erosion. Maintaining ground covers through fall, winter, and early spring drastically reduces soil loss.

Soil texture affects both nutrient availability and water management. It also affects how much herbicide the soil will hold. All soils contain amounts of sand, silt, and clay particles. Soil texture is determined by the relative amounts of these three types of particles.

Cover crops provide organic matter to the soil system. They stimulate biosystems, microbial, and macro-organism activity.

Cover crops reduce soil compaction while increasing water percolation and retention. They help soils maintain a high organic-matter level, and help improve soil aggregation, porosity, infiltration, and bulk density.

MSU researchers are also looking at the use of cover crops for weed control without reducing crop yields. Cover crops can shade and interfere with weed germination and establishment, while cereal ryes actually produce chemicals that suppress weeds.

Researchers are trying to build on the small existing bank of knowledge about how cover crops fit into weed-control programs. For instance, some crops are good hosts for Trichograma wasps, which control European corn borer in seed corn. The researchers say that having cover crops in, especially in the early spring, helps provide habitat for beneficial insects.

A wide range of cover crops is available. Grasses include annuals, such as ryegrass, winter rye, oats, and sorghum sudangrass. Perennial legumes commonly used include alfalfa, while annual legume possibilities include hairy vetch, field peas, crimson clover, lupines, and clovers. Among the brassicas are rape, forage turnips, and oilseed radish. Other possibilities are buckwheat, medics, canola, and triticale.

When and how cover crops should be established depends on what a grower wishes to accomplish by their use. Four common methods are used for establishment. Overseeding is when cover-crop seed is placed between rows of a growing crop. In frost seeding, the cover crop is seeded into an established crop in late winter to early spring. Cover crops can be drilled or spread into crop residue following harvest. Aerial application is another option.

Cover crops should be managed in a way that prevents them from becoming weeds. They must be controlled in spring, to prevent them from competing with the crop.

A grower can choose cultural, mechanical, or chemical methods to manage a cover crop. Several cover crop species, such as oilseed radish, cannot survive Michigan winters, and can be allowed to die naturally. Mold-board plowing is a traditional mechanical control method. Legumes may be killed at flowering by flailing or mowing. The job is more difficult with reduced-tillage systems, where chemical solutions (2,4-D or Roundup) may be viable options. Researchers stress the importance of timing herbicide applications to the correct growth stage of the cover crop.