Preventive weed control involves all measures taken to forestall the introduction and spread of unwanted plants. Although preventive measures will reduce infestations, no program can eliminate the wide variety of weed species on a given piece of land. Success of a preventive program varies with the weed species, the amount and constancy of effort that you devote to prevention.

Key Principles of Weed Prevention

What weed you are dealing with? Correct identification of new weeds is essential to success in control measures. Local agricultural fieldmen know most weeds and if you have an unknown weed, they have resources (government specialists, agronomists, botanists and herbaria) to draw on for a positive identification.

  • Is it listed under federal/ provincial/municipal legislations? Local agricultural fieldmen are responsible for implementing weed legislation and are knowledgeable on regulated weed lists.
  • Alberta Weed Control Act
  • Canadian Food Inspection Agency Regulated Pests
  • How do I identify unknown species? Consult neighbours, plant identification books, agricultural fieldmen and search the internet.
  • How does the weed spread or reproduce? Method of reproduction will determine course of action needed for control.
  • Where and when to look for this weed? Often new weeds aren't noticed until flowers are present, always be diligent in scouting fields throughout growing season. Look in areas of high traffic i.e. roadways, new disturbance areas, field boundaries and near previous outbreaks.
  • Best methods to control outbreaks quickly? Research possible methods of control weighing cost and time limitations.

How do Weeds Spread

  1. Deliberate spread by humans - this includes the fodder trade, food plant trade, medicinal plant trade, ornamental plant trade, re-vegetation and forestry.
  2. Accidental spread by humans - this includes agricultural produce, construction and landscaping materials, human apparel and equipment, livestock movement, machinery and vehicles and waste disposal.
  3. Natural spread - this includes spread by birds, other animals, water and wind.

Start Right and Use Weed Free Seed and Hay

Any weedy plant in a seed or hay field, poses the risk that some weed seeds will find their way into the crop-seed or hay supply even with the best prevention techniques. Yet, obviously, the more thorough the prevention technique, the less the potential is for infestation.

Use certified seed that includes a weed seed analysis.

Feed hay in areas that can be regularly checked for weed growth in case the hay is contaminated with weed seeds.

Prevent Weed Seed Formation

A weed can produce a few hundred to several thousand seeds depending on the species and the growing conditions. These seeds add to the soil seed bank and will cause problems for producers and future users. The following table outlines the number of weed seeds that can be prevented from entering the soil through timely removal of weeds.

Seed production capacities of some common weeds found in Alberta:

Common name Approximate number of seeds per plant
Barnyard grass 7,200
Chickweed, common 12,000
Buckwheat, wild 1,200
Chamomile, scentless 200,000+
Foxtail, green 34,000
Kochia 14,600
Lamb's-quarters 72,000
Medic, black 2,300
Mustard, wild 2,000 -3,500
Oats, wild 250
Pigweed, broadleaf 117,000
Plantain, broadleaf 36,000
Purslane 52,000
Shepherd's-purse 38,500
Smartweeds 3,000
Sow thistle 10,000
Spurge, leafy 250
Thistle, Canada 700
Stevens, O. A. 1932. American J. Botany 19:784-794.

Control Strategies to Consider

  • Prevention. Prevention of seed production is an important control strategy.
  • Hand pulling. Small numbers of plants can be dug or pulled, but if they are setting seed then they should be bagged and burnt once plants are dry.
  • Mowing. Mowing some weeds is an effective control; however, other weeds with different growth habits are only slightly affected by mowing and simply continue to flower and produce seeds below the height of the mower.
  • Tillage. Tillage of fallow fields must be conducted before seed production.
  • Silage. Extremely weedy crops can be cut for forage before the weeds go to seed. If viable seeds exist, the feed should be put up for silage because seed viability is usually destroyed in the ensiling process.
  • Competition. Best practice is to promote the growth of desirable plants so they out compete the weeds for water nutrients and light.
  • Grazing. Grazing can be used for some weedy species but requires intense management, and is very time sensitive.
  • Mulches. Mulches can be used in certain situations to suppress weed growth through a physical barrier i.e. plastic, bark chips and straw.
  • Bio-control. Utilizing 'natural enemies' of a particular weed can help manage infestations. Bio-control agents can include insects or fungal pathogens. Not all weeds have an available bio-control agent available.
  • Chemicals. Herbicides can be very effective against weed infestations if applied according to label directions but are limited by their non use in sensitive areas.

Practice Fence Line and Headland Control

Fence lines and headlands serve as habitat for beneficial insects and wildlife. Disturbing these sites may not be necessary as long as they do not act as a refuge for weeds or insect pests. If fence lines, headlands and roadsides are sources of infestation, try planting them with native plants and grasses that are adapted to our climate and growing conditions, and thus are competitive with weeds. Mowing or grazing uncultivated wastelands helps to control weeds. If possible, delay mowing or intensive grazing until late July, to allow ground nesting birds to raise their broods.

Prevent Spread with Soil and Equipment

Weed seeds and vegetative parts of plants move with farm equipment and soil. Long distance transport is responsible for the introduction of new weeds to previously clean areas. Industrial equipment, seed and used farm machinery are the worst offenders. Equipment should be cleaned before moving from one area to another. Place a tarp over grain and soil when it is transported. In addition to preventing weed spread, tarps reduce unnecessary loss of a valuable product.

Maximum number of weed seeds for 500 grams of crop seed:

Total Weed Seeds Total Other Crops
Canada Registered #1 3 2
Canada Registered #2 6 4
Canada Certified #1 3 4
Canada Certified #2 6 10
Canada Common #1 10 25
Canada Common #2 20 50
These standards apply to barley, buckwheat, lentils, lupine, rye, sainfoin, etc. With minor variations, they also apply to wheat, canola, flax, and oats.

The next table shows the net loss of grain and associated weed seeds after a three-ton truck travelled 10 km at 80 km/hour.

Loss of grain while in transport

Method of box cover Crop Net loss lb Bushels
Full tarp Barley 0 0
Level box with baffle Barley 95 2.0
Level box only Barley 825 17.2
Level box with baffle Oats 95 3.0
Level box only Oats 745 23.2
Level box with baffle Canola 160 3.2
Level box only Canola 1,125 22.5

Prevent Spread Through Feed and Manure

Weed seeds may remain viable after passing through animals, resulting in contaminated manure. Screenings used for feed should be finely ground, cooked or pelleted to ensure destruction of all the weed seeds. Poultry are most effective in destroying weed seeds as their crops grind the seeds. In order of decreasing effectiveness are sheep, horses, swine, and cattle.

Once in manure, however, the rate of breakdown depends on the type of weed seed and the temperature of the manure. If the manure is frozen or cold, the seed will live longer.

Handling New Weeds in Your Fields

  1. Have the weed identified by a professional.
  2. Prevent seed production by removing the plant and seed head. Bag and bury viable seed or destroy the seed through incineration.
  3. If the plant is a perennial, destroy the root system by physical, chemical or mechanical means. Use cutting (discs) rather than tearing (cultivator) equipment on perennial weeds to prevent root spread to areas that are not infested.
  4. Check infestation sites before harvest to insure no more seed has set.
  5. Monitor sites for three to four years to avoid re-infestation from vegetative parts or seed that did not germinate in the first year.