Summerfallow as a weed management strategy – pros and cons

B. Frick, E. Johnson - Scott Research Farm

Problem
Fallow has traditionally played a large part in weed control strategies. What are the advantages and disadvantages of this technique?

Background
Fallow can be used as a weed control method for both annual and perennial weeds. It can be used to reduce the weed seedbank by allowing weeds to germinate, then killing them before they set seed. This is especially effective on weeds with short dormancy periods, such as kochia, goat’s beard, hare’s ear mustard, Indian mustard, Russian thistle, cow cockle, green foxtail, downy brome, wild buckwheat or foxtail barley. Some reduction is possible for weeds with longer dormancy, but some seeds will survive.

Three to six tillage operations may be required for effective annual weed control during the fallow year. An early start is recommended for summerfallow tillage, perhaps by mid-May. Tillage operations should be as shallow as possible to avoid bringing new weed seeds to the soil surface. The initial operation should always be the deepest with subsequent ones progressively shallower. Tillage is most effective when the soil surface is dry and air temperature high. Tilling small seedlings when the soil surface is moist will usually produce poor results, as many of the seedlings are transplanted rather than being killed.

Fall tillage is an alternative to fallow tillage that can be used to destroy winter annual and biennial weeds. On weeds that over-winter, fall tillage is more effective than spring tillage. Fall tillage may encourage some summer annual weeds to germinate and most of these will be winter-killed. However, tillage also buries weed seeds that may become dormant, acting as a reserve for later years. Fall tillage should be shallow (less than four inches) to avoid deep burial of weed seeds. Alternating intensive tillage during fallow years with cropping and spring and fall tillage can be used to reduce severe perennial weed problems.

Fallow may also be used to control perennials such as Canada thistle and perennial sow-thistle. With these species, the first tillage should be done at the bud stage. Food reserves are at a low at this time, and the tillage is most effective. Once tillage begins, it should continue each time the plant reaches a height of about three inches, until freeze up. This approach will starve the root system and prevent it from forming any food reserves. The plants will enter winter in a very weakened state and many of them will not survive.

This late season tillage for perennial weed control may be used after an early maturing crop, or in a partial fallow situation. Appropriate crops include sweet clover, early maturing barley, fall rye, or oat cut for feed. Fall tillage after later maturing crops can also be effective against perennial weeds. Plants can be killed by exposing the roots, if freezing temperatures follow shortly after tillage. Fall tillage reduces stubble and trash cover -- reducing snow trapping. It also accelerates soil erosion. So fall tillage should be approached cautiously.

Quackgrass problems should be handled differently. Tillage to control this weed depends on physically damaging the root system. In dry years, a cultivator with narrow spikes will be effective, as this drags roots and rhizomes to the surface where they dry out and die. In wet years or areas, the first tillage operation should be with a disc implement that cuts the rhizomes into small pieces. Each smaller rhizone section will try to establish a new plant, which in turn should be destroyed by subsequent tillage. New plant growth should not be allowed to grow taller than three inches before being tilled. Tillage should be no deeper than required to do an effective job. Shallow tillage will concentrate rhizomes on or near the soil surface, resulting in a more uniform emergence and better control from future tillage.

Tilled fallow is a management tool to be used with caution. Extensive fallow for weed control has probably led to the evolution of greater dormancy in some weed species. Weed communities adapt to whatever practices are applied consistently. Repeated cultivation is detrimental to the soil, and practices that reduce the intensity or extent of tillage should be considered.

Conclusions
Summerfallow tillage can be used to reduce the seedbanks of annual weeds, and to attack persistent perennial weed problems. Tillage and fallow should both be used cautiously, since they can result in severe soil and environmental degradation.


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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