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Impact of Tillage Intensity on Weed Populations and Associated Weed Management Strategies on the Canadian Prairies
R. E. Blackshaw, Weed Scientist, Agriculture and Agri-Food Canada Research Center, Lethbridge, AB Email: blackshaw@em.agr.ca

Conservation tillage has revolutionized cropping in western Canada, resulting in reduced soil erosion, greater soil water conservation, improved soil quality, and higher crop yields. Fallow has decreased by 40% over the last decade and oilseed and pulse crops are now grown in areas that traditionally only produced cereals. On the Canadian prairies, direct seeding accounts for 50% of all crop production, split evenly between low- and high-disturbance systems. Low-disturbance (zero-till) systems leave more than 80% crop residue on the soil surface while high-disturbance systems often leave very little surface crop residues.

Farmers adopting no-tillage often cite adequate weed control as their biggest concern. Concerns include an overall increase in the number of weeds, an increase in perennial or difficult-to-kill weeds, and reduced herbicide efficacy due to increased surface stubble.

Weed species shifts

Research studies conducted across western Canada have allowed comparison of 77 site-years of data to examine the impact of various cropping practices on weed populations. The most noteworthy finding was that weed species and numbers were affected by Environment > Crop Rotation > Tillage Intensity.

In years of low rainfall and high temperatures, species such as kochia and Russian thistle increased regardless of crop rotation or tillage. In situations where high rainfall was received in fall or early spring, winter annuals such as downy brome and flixweed and perennials such as dandelion and quackgrass increased in all cropping systems.

Crop rotations with a high frequency of winter wheat had high populations of the winter annual weed downy brome while systems dominated by spring wheat had high numbers of summer annual weeds such as wild oat and green foxtail.

Nevertheless, changing tillage practices can result in marked shifts in weed species over time that may pose problems for producers. Table 1 depicts species associations with tillage intensity from these long-term cropping studies. Minimum- or zero-till often resulted in an increase in many perennial, biennial, and winter annual weeds. Notable perennials would be foxtail barley and dandelion that are generally well controlled by tillage but increased in reduced tillage systems. Examples of biennial weeds that have increased in reduced tillage systems include goat's-beard and biennial wormwood. Due to the snow trapping effect of standing stubble, species such as redstem filaree and cleavers now survive as winter annuals where previously they would have died with the cold temperatures of winter. Weeds that overwinter are often difficult to control with herbicides or tillage and they tend to become greater problems with time. Species that have seed that are wind disseminated and that can easily germinate at or near the soil surface tend to proliferate in reduced tillage systems.

Several annual weeds and large-seeded volunteer crops were strongly associated with conventional tillage (2 to 3 tillage operations) and become less problematic as growers move to conservation tillage (Table 1). Volunteer peas, lentils, and sunflower posed less of a problem in zero than conventional tillage. Green foxtail would be the best example of a weed that has decreased dramatically with direct seeding on the Canadian prairies.

A number of species, including the ubiquitous wild oat, were relatively unaffected by tillage intensity (Table 1) and can be more affected by climate or choice of crop grown. Volunteer canola, flax, and wheat were equally prevalent across tillage systems.

Table 1. Species response to tillage intensity determined from 77 site-years of data in western Canada.


 
Minimum-Zero Tillage
 
       
Perennials Biennials Winter annuals Annuals
       
Foxtail barley Biennial wormwood Downy Brome Kochia
Dandelion Goat's-beard Narrow-leaved hark's-beard Prickly lettuce
Quackgrass Sweetclover Common peppergrass Hemp nettle
Perennial sowthistle   Redstem filaree Common groundsel
Field horsetail   Wood whitlowgrass Canada fleabane
Smooth brome     Annual sowthistle
Wild rose      
Canada thistle      
Conventional Tillage
   
Annuals Volunteer crops
   
Green foxtail Pea
Common lambsquarters Lentil
Oak-leaved goosefoot Barley
Wild mustard Rye
Wild buckwheat Sunflower
Smartweed  
Cleavers  
Wild tomato  
Thyme-leaved spurge  
 
Unaffected by Tillage
 
     
Perennials Volunteer crops Annuals
     
Common milkweed Flax Wild oats
  Wheat Flixweed
  Tame mustard Shepherd's-purse
  Canola Redroot pigweed
    Barnyardgrass
    Chickweed
    Round-leaved mallow
    Prostate knotweed
    Cow cockle
    Bluebur

Implications for weed management

Farmer experience indicates that herbicide use may increase during a 3- to 5-year transition period from conventional to conservation tillage. Growers then report that their herbicide use often declines in zero tillage systems. This may be due to greater weed seed mortality when seeds remain on the soil surface due to predation by rodents, birds and insects, attack by fungi and bacteria, and dessication over the winter period. Timing of herbicide use may change with conservation tillage with reduced emphasis on in-crop herbicides and increased use of herbicides in fall or before crop seeding to control winter annuals and early germinating summer annuals. Preharvest glyphosate is an effective treatment for many perennial weeds.

Crop selection and crop rotation sequence have been shown to have a greater impact on weed community composition than tillage system. Diversified rotations should include perennial, winter annual, and summer annual crops. Annual crops can be further diversified by growing oilseeds and pulses in rotation with cereals.

Another key factor to effectively manage weeds is improving the competitiveness of crops with weeds. This can be accomplished by using competitive cultivars, higher crop seed rates, altered crop seeding dates, and selective fertiliser placement. These practices can be supplemented with the use of cover or green manure crops that inhibit weed growth through physical and allelopathic interactions. Sweetclover (Melilotus officinalis) has shown good potential for this purpose on the Canadian prairies.

Experience has indicated that adoption of zero tillage cropping systems need not be limited by concerns over effective control of weeds. Certain weeds will proliferate with reduced tillage but they can be managed with timely use of herbicides and adoption of integrated weed management practices. Conservation tillage is a key component of a more sustainable cropping system.

References

Blackshaw, R. E., F. J. Larney, C. W. Lindwall, and G. C. Kozub. 1994. Crop rotation and tillage effects on weed populations on the semi-arid Canadian prairies. Weed Technol. 8:231-237.

Blackshaw, R. E., G. Semach, X. Li, J. T. O'Donovan, and K. N. Harker. 1999. An integrated weed management approach to managing foxtail barley (Hordeum jubatum) in conservation tillage systems. Weed Technol. 13:347-353.

Derksen, D. A., R. E. Blackshaw, and S. M. Boyetchko. 1996. Sustainability, conservation tillage and weeds in Canada. Can. J. Plant Sci. 76:651-659.

Derksen, D. A., G. P. Lafond, A. G. Thomas, H. A. Loeppky, and C. J. Swanton. 1993. The impact of agronomic practices on weed communities: tillage systems. Weed Sci. 41:409-417.

Moyer, J. R., E. S. Roman, C. W. Lindwall, and R. E. Blackshaw. 1994. Weed management in conservation tillage systems for wheat production in North and South America. Crop Protect. 13:243-259.