A weed is any plant growing somewhere that it is not wanted, or it is unwanted plant because of some undesirable qualities. Weeds cause economic loss to farmers by:

- decreasing crop yield and crop quality

- harvesting difficulties

- aesthetic issues

- future weed management difficulties due to weed seed production

Weeds vary greatly in their biological characteristics, in order to control them you need to understand their biology and take advantage of a weakness in a particular plant. It is for this reason that weed identification is the prerequisite in any weed control strategies.

There are many different types of weeds (e.g. grasses, broadleaf), they can be classified according to their life cycle

Table 1. Weed classification of commonly occurring weeds in Alberta 

Weed Populations in Different Tillage Systems Different tillage systems have an effect on the number and the type of weeds present. Weed surveys conducted in western Canada have shown that weed species and numbers were affected by Environment, Crop Rotation, and Tillage Intensity (Table 2) in descending order of importance. 

Table 2. Species response-increase in infestation-- to tillage intensity

After Blackshaw 2001. 

Weed Control Strategies in Minimum or Zero Tillage
Farmers in western Canada rely heavily on herbicides for weed control, however, in recent years, some of the most serious weeds (wild oats, green foxtail, kochia, cleavers and chickweed) have become resistant to commonly used herbicides (Fact sheet: FS640-11). This has prompted the move towards the use of an integrated weed management approach in weed control strategies.

What is an Integrated Weed Management (IWM) Strategy? 

1. Prevention and Sanitation – prevent the movement of weed seed into fields. This can be best achieved by:

- planting certified, vigorous, high quality seed

- cleaning farm machinery and harvest equipment before moving from one field to other

- tarping grain truck

- spreading manure that is well composted

- controlling weed infestations along road sides, fence rows, grain bins and other non crop. Areas such as shelter-belts, hay and pasture areas adjacent to cultivated fields.

2. Cultural Control - measures involve practices that increase the competitive advantage of the crop. This can be achieved by:

- planting competitive crop and cultivars

- using optimum seeding rate and row spacing

- seeding early – early sown crops result in competitive stand establishment and a jump start on the weeds

- seeding shallow – generally, shallow seeding results in uniform seedling emergence that quickly covers the ground

- using diversified crop rotations that include perennial forages ,and winter and summer annuals that include oil-seed and pulse crops in rotation with cereals.

- band nitrogen near the seed to give your crop an advantage over weeds

3. Biological Control - involves the use of natural enemies such as crop specific insects and other pathogens, however, at present, there is nothing available for intensive cropping situations. 

4. Physical Control - ranges from summer-fallow and tillage to hand pulling, mowing, grazing, and chaff management. In minimum or zero tillage systems, tillage for weed control can not be used, as it leads to soil erosion. Mowing can be used successfully in non-crop areas such as hay or pastureland, fencerows and shelterbelts. Mow the weed patches before they seed.

5. Chemical Control - because of the lack of spring tillage in direct seeding systems, chemical weed control is an important part of an IWM strategy, however, herbicides should only be used when necessary. Before using chemical weed control, determine the economic threshold level for chemical control. (Link to Integrated Pest Management –Knowledge tid bits).

6. Fall Weed Control – in a minimum tillage or direct seeding system, certain winter annuals such as stinkweed, shepherd’s purse, narrow leaved- hawk’s beard and downy brome will increase. These weeds can be best controlled with 2,4-D or MCPA (Link). If perennial weeds such as Canada thistle, quackgrass, dandelions, and toadflax are present, they should be controlled in the fall, provided there is sufficient weed regrowth at the time of herbicide application.

7. Pre-seeding (Spring Burn-off) Weed Control – because of lack of tillage, a preseeding burn-off treatment with glyphosate (Roundup Original, Roundup Transorb, Vantage, or Vantage Plus) may be required. This will control most of the annual weeds present at the application time. This treatment will also provide suppression of most of winter annuals. Depending on the glyphosate rate, effective quackgrass control can also be achieved at this time.

8. In-Crop Weed Control – After crop emergence, grasses (wild oats and/or green foxtail) and broad-leaved weeds should be controlled with selective in-crop herbicides. As a part of a resistance management strategy, avoid repeated use of chemicals from herbicide groups with the same mode of action.

9. Preharvest Control - Preharvest applications of glyphosate provide excellent control of perennial weeds (quackgrass, Canada thistle, toadflax, dandelions, milkweed and perennial sow thistle in wheat, barley, canola, pea, dry beans lentils and flax. The best time to control these weeds is when they are actively growing and have lots of green leaves. The grain should contain 30% moisture or less (7 to 14 days before harvest). Pre-harvest applications are more effective and more consistent as compared to post-harvest applications. An example using a Roundup application for quack grass control is given in Table 3. A pre-harvest application was generally more effective and more consistent.

Table 3. Quack grass control (%) with pre-harvest compared to post-harvest 

Source: M. N. Baig, Monsanto Canada Inc.