Using allelopathic and cover crops to suppress weeds
B. Frick, E. Johnson - Scott Research Farm
How can allelopathy and cover crops be used to suppress weeds?
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 cropsí 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
Allelopathy is one plantís production of chemicals that suppress another.
Cover crops are sown for the soil, and may have potential as a fallow
Provided by the Canada-Saskatchewan Agri-Food Innovation
Brenda Frick, Ph.D., P.Ag.
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