Using allelopathic and cover crops to suppress weeds

B. Frick, E. Johnson - Scott Research Farm

Problem
How can allelopathy and cover crops be used to suppress weeds?

Background
Allelopathy in plants is the production of compounds that inhibit the growth of other plants. It may be direct, by living plants, or indirect through the products of plant decomposition. Allelopathy may be mediated by micro-organisms. It is a challenge to separate the effects of resource competition and alleopathy. Resource competition occurs when one plant utilizes a necessary resource from a habitat, and excludes access from neighboring plants. In plant-plant interactions, allelopathy is generally used to denote the process by which plants release phtyotoxic compounds (allelochemicals) in the soil environment, resulting in a harmful effect on neighboring plants.

Both crops and weeds have been found to contain compounds that can be considered allelopathic. These include crops such as barley, oat, wheat, rye, canola, mustard species, buckwheat, red clover, white clover, sweetclover, hairy vetch, creeping red fescue, tall fescue, and perennial ryegrass. These crops, in rotation, may suppress weeds in subsequent crops; however, these crops’ weed suppression effects cannot be solely attributed to allelopathy. As with all other techniques, caution must be employed. Crops with allelopathic properties may suppress subsequent crop growth.

Cover crops may be sown to protect soil from erosion, for snow trapping or to increase soil organic matter. When the cover crop fixes nitrogen or otherwise improves soil properties, it is often referred to as a green manure. Both cover crops and green manures can have weed suppressing qualities. They may shade the ground, thus reducing temperature fluctuation and the weed seed germination that depends on it. They may compete with weeds, and thus reduce their vigour, or they may have allelopathic properties. Cereal cover crops with a high C:N ratio may immobilize soil nitrogen allowing nitrogen-fixing crops to be more competitive. Tillage to kill the cover crop will also suppress weeds.

Cover crops can be sown into existing crops. If so, the timing should correspond to the time when weeds no longer cause yield losses (the end of the critical period). Cover crops may also be sown after harvest, or in place of a fallow. Successfully established cover crops can develop sufficiently dense canopies in the fall to interfere with growth of perennial and winter annual weeds. Most tests of cover crops involve fall or winter cereals sown in the late summer and killed by herbicides the following spring. Research at Lethbridge found that a well established, vigorous fall rye cover crop that was killed by herbicides or tillage in the spring suppressed weeds for the remainder of the fallow season. The cover crop protected the soil from erosion and provided about a 50% reduction in weed biomass in the fall compared to bare fallow.

The fall rye cover crop was particularly effective in reducing populations of dandelion and Canada thistle. Wheat yields following the cover crop were equal to yields obtained on bare fallow. Fall seeded and spring tilled winter hardy rape substantially reduced lambs quarters and pigweed growth in a subsequent potato crop, and may have suppressed nematodes and diseases as well. Another alternative is to use species that are not winter hardy. Tests with sorghum and oats showed weed suppression. These tests showed less effect than those where a winter hardy crop was killed chemically in the spring. Winter-killed cover crops form a mulch in the spring that further suppresses weed establishment and growth. Mulches may suppress weeds, but they are generally inadequate to control perennial weeds. Crop suppression is generally less than that of weeds, in part because crops generally have larger seeds. Water use by the mulch crop is partially offset by greater snow-trapping.

Allelopathic mulches have potential problems as well as advantages. They may deplete moisture and immobilize nutrients, especially nitrogen. Including rapidly decomposing legume crops in the mix helps alleviate the latter problem. The allelopathic effect may inhibit germination of smallseeded crops. Cover crops may be more effective when you eliminate tillage, to concentrate residues more at the soil surface.

Weed suppressing mulches need not be crop residues. Small areas of perennial weeds can be mulched with substances like manure. You need a substantial amount of manure for effective control - three feet or more deep, at least four feet beyond the patch. Other mulches: tar paper or black polyethylene, and mulched wood. Mulches must be maintained for at least one year for good weed suppression. Check with a certifying agent which nonplant mulches are allowed.

Conclusions
Allelopathy is one plant’s production of chemicals that suppress another. Cover crops are sown for the soil, and may have potential as a fallow substitute.


Funding
Provided by the Canada-Saskatchewan Agri-Food Innovation Fund


Contact information
Brenda Frick, Ph.D., P.Ag.
Prairie Coordinator
Organic Agriculture Centre of Canada
c/o Department of Plant Sciences
University of Saskatchewan
51 Campus Drive, Saskatoon
Saskatchewan, Canada S7N 5A8
Tel: (306) 966-4975
Fax: (306) 966-5015
Email: brenda.frick@usask.ca


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