Weed Prevention

Preventive control involves all measures taken to forestall the introduction and spread of weeds. 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 and the amount and constancy of effort that you devote to prevention.

Use weed free seed
Any weedy plant in a seed field poses the risk that some weed seeds will find their way into the crop-seed supply even with the best cleaning techniques. However, the more effective the cleaning technique, the less the potential for infestation. A 1986 - 1989 weed survey in Alberta identified the sources of seed used by the average producer.

The greatest potential source of weed seeds comes from seed that is not inspected. The sources of these seeds are "home grown" and "other farmers". Alberta has a good record of sanitation because 98 per cent of this seed was subsequently cleaned at a co-op seed cleaning plant (64%) a private commercial plant (15%), on farm (6%), or in the local elevator (6%). With the exception of the "on-farm" and the elevator system, which are primarily designed for dockage removal, the available cleaning systems remove the majority of weed seeds. Pedigreed seed, while not weed free, comes with a certificate of analysis that identifies the species and number of weed seeds found in a seed sample. Seed dealers must supply a certificate to buyers upon request. An example of the seed standards is shown in the next table. The higher the grade, the fewer weed seeds allowed.

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.

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. 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 selected weeds:

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.

Prevention of seed production is an important control strategy. 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 of fallow fields must be conducted before seed production. 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.

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.

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

• Have the weed identified by a professional.
• Prevent seed production by removing the plant and seed head. Bag and bury viable seed or destroy the seed through incineration.
• 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.
• Check infestation sites before harvest to insure no more seed has set.
• Monitor sites for three to four years to avoid re-infestation from vegetative parts or seed that did not germinate in the first year.