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Tillage for weed control -- an introduction
B. Frick, E. Johnson - Scott Research Farm
Tillage is often seen as the organic alternative to chemical weed control.
Tillage can be very effective at reducing weed populations, but can it
be embraced as the solution to the weed problem? What cautions are necessary?
Tillage should be considered an external input – it requires the expenditure
of fossil fuels. Reducing tillage reduces dependence on limited non-renewable
resources. Spring tillage loosens and dries soil, thus warming it. Tillage
can pulverize soil, making it more susceptible to erosion. Tillage reduces
stubble and trash cover, thus reduces snow trapping, and also accelerates
soil erosion. Tillage also speeds decomposition and loss of organic matter,
and increases salinization. It increases nitrogen volatilization to the
atmosphere and potential for nitrate leaching.
Tillage also affects life within the soil. It can reduce the survival
of beneficial invertebrates, soil microfauna and microflora. Tillage reduces
the populations of weed-seed eating carabid beetles and field crickets.
Tillage exposes soil and weed seeds to the light. For some species, this
triggers germination. When cultivation was performed at night or if the
implement was covered, weed populations were reduced by up to 50%. Tillage
places weed seeds in better contact with the soil, also facilitating germination.
In terms of weed control, deep tillage is a mixed blessing. It may bring
up dormant seeds buried in the soil, and bury other seeds for later retrieval.
Tillage equipment may spread perennial plant parts throughout the field.
In this way, a small patch can become a general problem. Tillage favors
some species over others, and thus is one of the management tools that
can be used to alter weed communities.
Tillage is not appropriate in some situations. Where risks of erosion
or salinization are high, avoiding tillage is important. In other circumstances,
tillage’s negative effects can be limited by careful management, or seeking
alternatives that reduce the need for tillage.
The type of tillage itself makes a difference. Some implements, such as
the Noble or Victory blades, that leave stubble standing, will alleviate
some fall tillage risk. These two equipment types are less effective in
cool wet conditions. Tilling at slower speeds can reduce the erosion potential.
The amount of tillage can be lessened in various ways. You can leave uncultivated
strips between cultivated areas. Leaving strips may also help trap snow.
Tilling weedy patches rather than entire fields may also reduce risk.
If erodible knolls have few weeds, leaving the knolls untilled will conserve
residues and limit erosion. Where fallow tillage is planned, black summerfallow
can be replaced with partial fallow, for instance, after plowdown of an
underseeded crop, or mowing green feed.
Mowing, in place of tillage, can be an effective part of weed control
where tillage is undesirable. Done early enough, mowing may prevent weed
seed set. To be effective, mowing should be done before flowering, as
many weeds can set seed very quickly after flowering, using the reserves
left in the portion of cut stem that remains attached to the flowers.
Many weeds, such as wild oats or Russian thistle, can be used in green
feed if cut before seed set. To control perennial weeds, mowing might
be delayed until the onset of weed flowering when food reserves are at
a low point. The weed will respond by sending up new stems, further depleting
its reserves. Mowing at about three week intervals can severely weaken
or even kill the weeds.
Mowing is also important in conjunction with perennial forages. Mowing
can be useful in giving an advantage to perennial forages over weeds.
By preventing annual weeds’ seed set, and depleting reserves of perennial
weeds, mowing may be the most important component of weed control in perennial
Mowing, like tilling, can interfere with beneficial creatures. Delaying
mowing until mid to late July can reduce nesting birds' mortality. This
fits well with perennial weed control, but may compromise annual weed
seed set prevention.
Every weed control technique has benefits and detriments. Non-chemical
methods are not automat-ically environmentally friendly. Techniques can
sometimes be altered to reduce detriments without greatly reducing benefits.
In developing effective and efficient weed management strategies, growers
must consider advantages, disadvantages and limitations of all the tools
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