Tillage for weed control -- an introduction

B. Frick, E. Johnson - Scott Research Farm

Problem
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?


Background
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 crops.

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.


Conclusions
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 available.


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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