Average No. Per Plant		Average No. Per Plant
Barley, Wild	2,420	Lettuce, Prickly	27,900
Beggar Ticks	7,000	Mallow	47,500
Buckwheat, Wild	11,900	Marsh Elder	82,150
Buffalo Bur	8,460	Mullein	223,200
Burdock	31,600	Mustard, Ball	490
Catchfly	1,800	Mustard, Common	2,700
Cinquefoil	42,600	Mustard, Dog	8,482
Cockle, Pink	4,300	Mustard, Hare's-ear	3,800
Coneflower, Long-headed	7,000	Mustard, Tumbling	80,409
Dandelion	15,000	Peppergrass	6,009
Dock, Curled	29,500	Pigweed, Creeping	14,609
Dodder, Field	16,000	Pigweed, Rough	117,400
Dodder, Hazel	7,000	Pigweed, Tumbling	129,000
Dragonhead	49,600	Plantain, Common	36,159
Evening Primrose	118,500	Purslane	52,309
False Flax (Large Seeded)	1,970	Ragweed	3,330
Flixweed	75,650	Shepherd's Purse	33,509
Frenchweed	7,040	Smartweed	19,309
Goldenrod, Stiff	3,290	Spurge, Leafy (1 Stem)	142
Grass, Barnyard	7,160	Spurge, Rhyme-leaved	2,670
Grass, Green Foxtail	34,000	Stickseed	2,129
Grass, Yellow Foxtail	6,420	Sunflower, Common	7,200
Grass, Stink	82,100	Sunflower, Narrow-leaved	2,609
Grass, Witch	11,400	Thistle, Canada (1 Stem)	630
Gumweed	29,700	Thistle, Russian	24,700
Hemlock, Water	5,500	Thistle, Perennial Sow (1 Stem)	9,750
Kinghead	1,650	Vetch, Narrow-leaved	150
Knotweed	6,380	Wild Oats	250/head
Lambsquarters	72,450	Wormwood	1,075,000

Blueweed (Echium vulgare)	Cariboo, Central Kootenay, Columbia-Shuswap, Okanagan-Similkameen, Thompson-Nicola
Burdock (Arctium spp.)	Bulkley-Nechako, Cariboo, Columbia-Shuswap, Fraser-Fort George, Kitimat-Stikine, North Okanagan, Okanagan-Similkameen, Peace River, Thompson-Nicola
Cleavers (Galium aparine)	Peace River
Common Tansy (Tanacetum vulgare)	Central Kootenay,  Columbia-Shuswap, North Okanagan
Green Foxtail  (Setaria viridis)	Peace River
Hoary Cress (Cardaria spp.)	North Okanagan, Thompson-Nicola
Kochia (Kochia scoparia)	Peace River
Night-flowering catchfly  (Silene noctiflora)	Peace River
Orange Hawkweed  (Hieracium aurantiacum)	Cariboo, Central Kootenay,  Columbia-Shuswap, East Kootenay
Oxeye Daisy  (Chrysanthemum Ieucanthemum)	Cariboo, North Okanagan, Peace River
Plumeless Thistle  (Carduus acanthoides)	Central Kootenay
Puncturevine  (Tribulus terrestris)	Okanagan-Similkameen
Quackgrass  (Agropyron repens)	Peace River
Russian Knapweed  (Acroptilon repens)	North Okanagan
Russian Thistle  (Salsola kali)	Peace River
Sulphur Cinquefoil  (Potentilla recta)	Columbia-Shuswap, Thompson-Nicola
Tartary Buckwheat (Fagopyrum tataricum)	Peace River
White Cockle  (Lychnis alba)	Peace River
Wild Mustard  (Sinapsis arvensis)	Peace River

Table 4

Weed Horses Cattle Swine Sheep Quackgrass + + - + Redroot pigweed + + + + Common Yarrow + + + + Cheatgrass - - - - Lamb's-quarters + + + + Ox-eye daisy + + + + Canada thistle - + + Wild carrot + - - - Cypress spurge - - - - Dog mustard + + + + Wild buckwheat - - Lady's-thumb + - Common purslane  - - + + Broadleaved plantain - + Curly dock + + + + Tall buttercup + + + + Wild radish - - Green foxtail + + + + Nightflowering catchfly + + + + Corn spurry - + + Chickweed + + + + Dandelion - - + + Common mullein - +

+ Germinated after passing through animal
- Did not germinate

PHYSICAL STRATEGIES

Tillage

Soil is the principle factor of the environment that the farmer manipulates in crop production. Soil tillage enables the farmer to attack many weed survival mechanisms. For annual weeds, the tillage objective is to prevent seed production and deplete current seed reserves in the soil. This is accomplished by encouraging weed seeds to germinate, then subsequently killing them. With perennials, destruction of the underground parts is sought, as well as the prevention of seed production and reduction of seed reserves. Tillage kills weeds by:

• Burial of the entire plant. If complete burial is not accomplished and a small portion of the plant is exposed, life may continue. Burial is usually only effective with seedlings.
  
• Depleting food reserves. This is accomplished by repeatedly removing top growth whenever it reaches sufficient size. Food manufacture can be halted by cutting off the plant tops or burning them. Tillage that breaks the underground parts into pieces is also very effective. This creates more growing points to use up food, and hastens food depletion.
  
• Exposure of underground parts to frost. The roots of most plants are killed when left on or near the soil surface during freezing temperatures. Fall cultivation followed by spring-tooth harrowing brings underground parts to the surface.
  
• Exposure of root systems to drying. If many roots remain under moist soil, growth will continue, therefore tillage must be thorough, with the plants fully exposed on the soil surface.
  
• Encouraging rotting of underground parts. Physical injury to underground parts due to tillage enables the entry of decay causing bacteria.

Spring Tillage: Don't till deeper than four inches. Deep tillage buries weed seeds and temporarily minimizes weed problems, but subsequent tillage will bring these seeds to the surface again. Seeds in the soil can germinate many years after burial - sometimes for up to 40 years or more.

Tillage should be designed to catch the first flush of weeds prior to sowing the crop. In moister areas tillage may be designed to control more than one flush of weeds prior to seeding.

Post-seeding tillage, if practical, should be done in dry, warm, sunny and perhaps windy conditions so that weed seedlings once disturbed are not able to transplant themselves.

Fall Tillage:Tillage after harvest can effectively kill biennials and suppress perennials. Under suitable conditions in the fall, weed germination may be stimulated making the new seedlings susceptible to frost kill.

Hand Weeding

Annual and biennial weeds and non-creeping perennials can be destroyed by simply pulling them out. This is best done when the soil is moist and before seed is produced. This is only practical of course for small patches or individual plants.

Mowing

When weeds are too numerous to hand pull, too large to effectively destroy by cultivation, or in an area where cultivation is impractical or impossible, they can be destroyed by mowing. This should be done before they produce seed and as close to the ground as possible.

Perennial weeds usually require several cuttings before the food reserves in the roots are exhausted. If only a single cutting can be made, the best time is just prior to blooming because (1) the reserve food supply in the roots is at its lowest level, and (2) viable seed is often produced just after blooming. Perennial sow-thistle begins producing viable seed only three days after blooming and after nine days, 73 percent of the seeds may be viable.

Grazing

The repeated removal of weed top growth by grazing animals, like close mowing, prevents seed formation and gradually weakens underground parts. Horses, sheep, goats, hogs and cattle are effective in destroying many weeds, if they are properly managed. Grazing animals should not be turned into pastures too early in the spring before the pasture grasses have made good growth. If grazing is permitted too early, the grasses do not develop a competitive stand and the weeds tend to take over. Ideally, pasture should be subdivided into two or more lots. This permits pasture areas to be grazed in rotation. If managed correctly, more weeds will be cleaned up than if livestock are allowed to roam over a large area and choose the more desirable forage, leaving the weeds to become rank, unpalatable, and over a period of time the dominant pasture plants. By rotating the pastures, desirable forage is encouraged during its rest period and results in healthier competitive pasture plants. Rotation also permits herbicide treatment with a safety margin, enabling the breakdown of the chemical before returning the pasture to grazing.

Burning

In situations when seed production has already occurred, some of the seeds can be destroyed by burning. The effectiveness of burning depends on the duration and intensity of heat produced, plus the maturity and location of the seeds. Mature, dry seeds are more heat resistant than green seeds, which have a high moisture content. Although intense heat will destroy most seeds remaining in plant heads, only a relatively small number of seeds on or below the soil surface can be destroyed by burning surface trash. Burning weeds over an extended area destroys valuable surface trash that would normally be returned to the soil through decay or cultivation.

In most farming operations, the most appropriate use of burning would be to selectively burn patches of weeds that have headed out by using a large propane torch. The flame can be directed at the mature heads of the weeds and the undesirable side effects of burning can be largely avoided. This is one of the few effective methods of preventing the dissemination of airborne seeds like Canada thistle, sow thistle, etc. Row crop producers sometimes use flaming machines which selectively burn weed top growth between the rows of specialty crops.

Mulching

The principle of mulching is to exclude light from the tops of the weeds until the reserve food supply in the roots is depleted and the weeds starve. Mulches include clean straw, hay or manure, tar paper, sawdust and black plastic. Black plastic is very effective. When the vegetation under the mulch has been destroyed, the resultant bare patch must be reseeded with competitive vegetation to prevent new weed introductions.

CULTURAL STRATEGIES

Cultural control uses plant competition or cropping practices to suppress weeds, either through use of smother or competitive crops and crop rotation.

Crop Rotations

Certain groups of weeds are almost always associated with specific crop rotations because:

• They are able to compete well with that crop.
• They are not destroyed by the herbicides and cultivations that normally accompany production of the crop.

Continuous cropping to small grains, for example, results in an increase in weed populations, chiefly annuals. Perennial forage crop plantings or permanent pastures favour development of perennial weeds like Canada thistle and quackgrass. Repeated plantings of the same crop favour the development of insects and diseases which result in weak or patchy stands which are easily invaded by weeds.

Summer annual weeds in grain plantings can be reduced by rotating to perennial forages or to row crop production where practical.

Once perennial forage crops are established and well managed, cutting for hay will suppress annual and perennial weeds, and eventually the competition will eliminate annual weeds.

Fall seeded crops such as fall rye or winter wheat provide increased early season competition to seedling weeds and the early harvest enables a partial summer fallow for the balance of the season.

Despite rotating crops, intensive summer fallow is often the only cultural alternative on land severely infested with perennial weeds such as Canada thistle and quackgrass. Summer fallow is designed to suppress growth, prevent seed production, deplete seed reserves in the soil and starve weed roots.

Plant Competition

The use of plant competition is one of the cheapest and most useful general weed control practices available to all farmers. Competition uses one of nature's oldest laws - survival of the fittest. Weeds are strong competitors by nature. If not, they would fail nature's survival tests. Certain weeds that can best compete under a certain set of circumstances always tend to dominate.

As mentioned earlier, weeds compete with crop plants for light, soil moisture, soil nutrients, carbon dioxide, and space. For example, one mustard plant from germination to maturity uses twice as much nitrogen and phosphorus, four times as much potassium, and four times as much water as one well developed oat plant.

As a general rule, for every pound of weed growth produced, about one less pound of crop growth is produced. Usually, early weed competition reduces crop yield far greater than late season weed growth. It naturally follows then, that early weed control is exceptionally important. Late season weed growth may not seriously reduce yields, but it can make harvesting difficult, lower crop quality, and add to the reservoir of weed seeds in the soil.

Crop Establishment

As a general rule, the first plants to germinate and emerge in an area tend to exclude all others. Therefore, it is critical when considering plant competition as a weed management tool to establish a vigorous dense crop. Important factors that affect germination and emergence include the viability of the crop seed (percent germination), soil temperature, availability of moisture, and physical resistance to seeding emergence by the soil. These factors are influenced by the soil type, physical condition of the soil, depth of planting, the firmness of soil around the seed, the degree of soil compaction above the seed, and the formation of surface crusts after planting. The final stand will also be influenced by post-emergent stress due to weeds, diseases, insects and adverse weather conditions. Crop germination, if planting has been correctly done, is generally rapid and predictable. If pre-seeding weed growth has been killed, at least a temporary advantage has been gained for the crop. This initial advantage can be lost however, if effective post-seeding herbicide application or tillage is not undertaken to control late emerging weeds.

FACTORS LEADING TO MAXIMUM CROP COMPETITION

Prepare Good Seedbed - Stale Seedbed Technique: A good seedbed is prepared, but no seeds are planted. After a good growth of weeds have emerged, they are killed using a non-selective herbicide with no residual effect in the soil. The crop is then planted with as little disturbance of the soil as possible to avoid bringing fresh weed seeds to the surface. This technique controls the all important first flush of weeds.

Rate of Seeding: Heavier seeding rates can be used to reduce weed competition in areas where sufficient moisture is available. This applies to seeding completed at the regular time, as well as in a delayed seeding program. The recommended heavy seeding rate varies from 25 to 100 percent more seed, depending on the crop and the location. However, it should be remembered that heavy seeding rates should be used together with other cultural and chemical control measures to be most effective.

Date of Seeding: Different weeds have different preferences for germination. Weeds such as many of the mustards and wild oats germinate best in cool conditions and are therefore more of a problem in early sown crops. Redroot pigweed and green foxtail prefer warmer soils and therefore germinate later. Late seeding allows for tillage or pre-seeding herbicide control of early germinating weeds. Early seeding results in crop competition with later germinating weed species. By varying crop seeding dates weeds have difficulty adapting.

Crop Variety: It is important to choose a variety of crop plant that is well adapted to local conditions of soil, water, climate and disease resistance.

Fertilization: The fertility of the soil affects both the vigour of crop plants and the vigour of weeds. Many weeds can utilize fertilizers as well as or better than crop plants. Nevertheless, if most of the weeds are suppressed or killed by tillage or herbicides, the extra vigour given to the crop by fertilizers will make them better competitors. Placement of the fertilizer in the crop rows has an advantage over broadcast fertilization because most of the fertilizer is directly available to the crop.

Smother Crops: A smother crop is defined as a thick stand of rapidly growing crop that competes with weeds to such an extent that their top growth is drastically suppressed and their roots are severely weakened. Alfalfa, when it becomes well established, is an excellent smother crop. Its extensive root system enables it to compete with most weeds for water, and its dense top growth smothers new weed growth.

The principle value of smother crops in weed control is that they severely weaken the underground parts so that weeds are readily killed by the cultivation that follows.

Soil - Water Relationships: Water relations, particularly the quantity of rainfall and its distribution, are critical to growth. Soil type, texture and the height of the water table are also important. Most land cropped with cereals and vegetables is sufficiently drained that any improvement of the overall drainage would have little effect on weeds.

Soil Reaction: Certain weeds tend to be associated with alkaline and acid soils. For example, wild barley and arrowgrass are more prevalent on alkaline soils. Wild oats do better on acid soils than most cereals. Amending soil reaction is suggested where indicator weed species prevail.

Limitations of Competition

The most that should be expected is suppression of weed growth and a measure of practical control. Competition is a good secondary tool and is most effective when used in combination with other control strategies.

BIOLOGICAL STRATEGIES

Biological control refers to the use of natural agents such as insects, nematodes, fungi, viruses or fish for the control of weeds. In some instances, grazing animals can be used to harvest and stress noxious weeds. Sheep have been used to successfully to control tansy ragwort and goats and sheep can be used to control leafy spurge. There have been a number of successes using biocontrol and this method is receiving increased attention in British Columbia.

The objective in biocontrol is never eradication; it is reduction of a weed's density to non-economic levels.

Biocontrol begins by first looking at the natural enemies of a particular weed in its native country. Most of our weeds are native to Europe and Asia. When a suitable control agent has been found, it is screened by the International Institute For Biocontrol in Switzerland in conjunction with Agriculture and Agri-food Canada, Biocontrol Section, Lethbridge, Alberta. Screening can take more than five years to complete as feeding trials must be performed to ensure the agent is specific only to the target weed and not to crops and other desirable plants as well. The usual procedure, where insects are being considered, is to starve the bug, then offer them a variety of host plants as potential food. This is done to ensure that the insects would rather die than eat a non-target plant. When screening is complete, the insects are increased and permission to release is received from North American Federal Governments. As a further precaution, new agents are not released to B.C. until a review and approval is obtained from the British Columbia Plant Protection Advisory Council.

Most biocontrol work to date has involved the use of insects. These act in many ways, including defoliation of the plant, boring into roots and eating seed, or forming galls in the seed head to prevent seed formation.

Biocontrol is a self-regulating type of weed control. That is, as the weed host increases so does the insect population. As the weed population decreases due to the insect, the insect population also decreases. A balance is hopefully attained where the weed and insect populations are held at a low level.

Biocontrol is designed for the most part for noncultivated lands where biennial or perennial weeds are troublesome. Cultivated lands are not usually suitable as the weed food source for the biotic agent is removed periodically. An exception is the recent discovery of a specific fungus disease which controls round-leaved mallow in wheat fields.

Biocontrol Status In B.C. - 1998

To date 57 bioagents have been released on 20 weed species on over 4000 sites in British Columbia.

Once approved for field release in B.C. the following procedure ensures provincial distribution:

1. first field release under controlled conditions. The majority of new agents are propagated at facilities maintained by the Ministry of Forests at Kamloops. Purpose - propagate agents.
2. caged release of offspring from propagated agents to protected areas. Purpose: to increase agents under diverse ecological conditions.
3. provincial distribution over the weeds range.

Weed Species Targeted For Biocontrol Research In B.C.

 

Biological weed control is the ultimate in controlling undesirable vegetation as it uses natural forces in a way which will least upset the environment. Much experimentation and screening has yet to be done, however, before this method can be used to the exclusion of alternate control measures for many of our noxious weedy plants.

CHEMICAL STRATEGIES

Man began to experiment with chemicals to control weeds in the 19th Century but it was the phenomenal success of 2,4-D, introduced commercially in 1946, that launched the present era of herbicides.

It is important to understand a few basics of herbicides and their use so that they might be used effectively as a component of an integrated weed management program.

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CLASSIFICATION OF HERBICIDES

Herbicides are classified according to their selectivity, mode of action and the timing of application.

SELECTIVITY

A non-selective herbicide kills or damages all plant life in a treated area (e.g., Roundup). A selective herbicide will kill weeds in a germinating or growing crop without harming the crop beyond the point of recovery (e.g., 2,4-D used to control broadleaved weeds in a grass pasture; Avadex to control wild oats in cereals.)

Factors Which Influence Selective Action of Herbicides

Nature of the Weed
Perhaps the most important consideration in selectivity is the nature of the weed itself. A listing of characteristics associated with 2,4-D susceptibility and resistance will serve as an example:

The first five characteristics in the previous list are associated with the amount of spray which will make initial contact with the plant and the plant's subsequent ability to retain toxic materials on leaf and stem surfaces. Studies with a large number of plant species have indicated that, in general, those having the characteristics on the left are injured more by a herbicidal application than are those having the characteristics listed on the right.

Biochemical Selectivity Some plants can tolerate a given amount of herbicide within their tissues, whereas others will succumb due to the differing chemical makeup of the plants concerned. It is thought that the resistant plant is able to break down the herbicide to nontoxic components, whereas the susceptible plant cannot.

Comparing corn, which is resistant to atrazine, with oats, which is susceptible, the greater amounts of atrazine which accumulate in the oat leaves and eventually kills the plant are due to three factors: oats absorb more atrazine, translocate it more effectively, and cannot break it down as readily.

It is important to note that biochemical selectivity can be lost if excessive herbicide rates are applied.

Depth of Rooting: Most germinating weeds arise from seeds in the top one-quarter to one-half inch of soil. A soil applied herbicide will be taken up by weed roots in this zone, whereas crop seeds planted deeper than this will germinate in a herbicide free zone.

Influence of Herbicide on Selectivity: The herbicide itself can influence selective action due to:

a. Nature of the chemical molecule;
b. the concentration applied;
c. the formulation used;
d. surface tension and spreading qualities.

Selective action is observed for example where MCPB (a 4 carbon chemical) is used for weed control in clover or peas. The weeds are able to change the MCPB to MCPA (a 2 carbon chemical) which is much more toxic than the original herbicide. The crop cannot do this, so in essence the weeds commit suicide.

Exceeding recommended use rates can kill or injure normally tolerant non-target plants. Use of oil soluble ester formulations of 2,4-D are often more effective on many weeds than water soluble amine formulations as they can more easily penetrate the waxy coating on plant leaves.

MODE OF ACTION(or how the herbicide works to kill a weed)

Contact herbicides - kills plant parts covered by the herbicide and are directly toxic to living cells. There is little or no translocation or movement of the material through the plant. Contact herbicides are effective against annual weeds but they only "burn off" the tops of perennial weeds - chemically mowing them. Contact herbicides may be selective, such as Torch (bromoxynil) which kills broadleaved weeds in cereals without damaging the crop, or nonselective, such as Gramoxone (paraquat) which kills any green plant material.

Systemic herbicides - absorbed by either the roots or above ground parts of plants, these herbicides move or are translocated in the plant. They exhibit a chronic effect; that is, the full effects may not show for a week or more after treatment. An overdose on the leaves may kill the leaf cells more quickly, thus preventing translocation to the site of action in a plant. The old axiom "If a little is good, more will be better" does not apply to herbicide use rates. Systemic herbicides can be selective, as in the case of 2,4-D, MCPA, Banvel and Tordon or non-selective, as with Roundup.

Soil Sterilants - chemicals which prevent growth of plant life when present in the soil. These products will prevent plant growth for periods of a few months to a number of years. Examples include bromacil, tebuthiuron and atrazine at high rates.

TIMING OF APPLICATION

Pre-Plant Soil Incorporated - these herbicides are applied to the soil before the crop is sown. They are incorporated in the soil to prevent loss due to vapourization and breakdown by sunlight. Trifluralin and EPTC are examples.

Pre-Emergence - applied to soil prior to seeding or after the crop is sown but before crop and weeds emerge. In most cases, the weeds germinate in treated soil while the crop germinates below the herbicide zone.

Post-Emergence - sprayed directly on the weeds after they are up and growing.

PERSISTENCE OF HERBICIDES IN THE SOIL

The length of time that a herbicide remains active or persists in the soil is extremely important as it relates to the length of time that weed control can be expected. Also, residual activity is important as it relates to phytotoxic after-effects that may prove injurious to succeeding crops or plantings.

Seven factors affect the persistence of a herbicide in the soil;

1. micro-organism decomposition,
2. chemical decomposition,
3. adsorption on the soil colloids,
4. leaching,
5. volatility,
6. photodecomposition, and
7. plant uptake.

Micro-Organism Decomposition

The principal micro-organisms in the soil are algae, fungi, actinomyces, and bacteria. They must have food for energy and growth. Organic compounds of the soil provide this food supply, except for a very small group of organisms that feed on inorganic sources. Micro-organisms use all types of organic matter, including organic herbicides. Some chemicals are easily decomposed (easily utilized by the microorganisms), whereas others resist decomposition.

Chemical Decomposition

Chemical decomposition destroys herbicides through interaction with the soil constituents of oxygen, hydrogen or water. Hydrolysis (interaction with water), for example, is responsible for inactivating atrazine in the soil.

Adsorption of Herbicide by the Soil

Herbicides tend to leave the soil solution and are adsorbed by clay and organic matter particles making the herbicide unavailable for uptake by the weed.

Research has shown that:

• Soils high in organic matter require relatively large amounts of pre-emergence and soil sterilant herbicides for weed control.
• Soils high in clay content require more herbicide than sandy soils for pre-emergence or soil sterilant weed control.
• Soils high in organic matter and clay content tend to hold the herbicides for a longer time than sands. The adsorbed herbicide may be released so slowly that the chemical is not effective as a herbicide.

Leaching

Leaching is the movement of a herbicide in solution through the soil. Leaching occurs in any direction (downward, upward, sideways).

The extent to which a herbicide is leached is determined principally by:

• Solubility of the herbicide in water.
• Amount of water passing through the soil.
• Adsorptive relationships between the herbicide and the soil.

In general, those herbicides which are completely water-soluble are most easily leached. Salts of 2,4D are water-soluble and leach readily through porous, sandy soils whereas esters of 2,4-D are low in solubility and do not leach easily.

Herbicides have been known to move upward in the soil. If water evaporates from the soil surface, water may move slowly upward. The water may carry with it soluble herbicides. As the water evaporates, the herbicide is deposited on the soil surface.

Volatility

Herbicides may evaporate and be lost to the atmosphere as volatile gases. The volatile gases may or may not be toxic to plants. The volatile gases may drift to susceptible plants such as those from the ester forms of 2,4-D causing injury to susceptible crops such as tomatoes or grapes. Eptam, Eradicane and Treflan are examples of volatile herbicides which must be incorporated into the soil immediately following application to prevent loss of herbicide to the atmosphere.

Rain or irrigation water applied to a dry soil will usually leach the herbicide into the soil, or aid in its adsorption by the soil particles. Once adsorbed by the soil, the loss by volatility is usually reduced.

Photodecomposition

Ultraviolet light from the sun decomposes many herbicides applied to the soil surface. Some herbicides such as Treflan are recommended for soil incorporation as they break down readily when exposed to sunlight.

Plant Uptake

The uptake of herbicides by plant roots results in their removal from the environment; hence, reduced concentrations in the soil.

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FACTORS AFFECTING HERBICIDE EFFECTIVENESS

The Weed Leaf

For foliar active herbicides, the chemical must be retained on and penetrate through the leaf surface. Wax cuticle, hairs on the leaf and the angle of the leaf all determine retention and adsorption. Surfactants or surface active ingredients can be added to herbicide formulations to increase their wetting ability and penetration into the leaf.

Climate

Under dry or cool conditions, plants grow slowly and translocation of systemic herbicides is slow. Cloudy conditions interfere with food production in the plant thereby slowing translocation and reducing effect of systemic herbicides. High temperatures and low humidity increase evaporation of the herbicide from the weed leaf. The weed leaf surface is also difficult to penetrate under these conditions as the plants tend to develop a heavy wax cuticle. High humidity often damages the leaf cuticle allowing for a high absorption of herbicide into the weed. Warm temperatures and high humidity are generally favoured for best control with systemic herbicides. Avoid spraying in wind as herbicide will be lost to drift. Rainfall during or after application can wash the herbicide off the leaf, hence, reducing effectiveness; but rain or irrigation is required for many soil applied herbicides such as Dual and Primextra. Always read the herbicide label to determine under which environmental conditions the product will work best.

Time of Day

This factor relates both to temperature and light. Generally carbohydrate levels in the leaves are lowest in the morning and highest in the late afternoon. Spraying early in the morning is more effective.

Age of Weed

As a rule, weeds in the seedling stage are usually most susceptible to herbicides. Young weeds are growing actively and there is a great deal of movement of food and water throughout the plant. Systemic herbicides, once inside the plant, will be moving with this material to the growing points.

With established perennial weeds, such as Canada thistle, treatment at the bud or early flower stage is usually more effective. This is because the root food reserves are expended and newly produced food along with herbicide is being moved back down to the root system to again build up reserves. With Canada thistle and other perennials, it is the root we must attack for control.

It is critical to follow herbicide label directions regarding stage of weed growth to obtain optimal control.

Nutrition

Herbicide injury to weeds is greatest when nutrient levels are high and the weeds are growing vigourously. Adequate fertility also improves weed control by increasing competition by the crop.

Cultivation

Chemical weed control is more effective in fields that are cultivated because more weed seeds are brought to the surface to germinate. With perennial weeds, cultivation lowers the root reserves making them more susceptible to herbicides.

Soil Organic Matter

Soils with high O.M. such as peats or mucks require higher rates of chemical than do sandy soils. This is because more of the chemical is bound to the soil colloids in high O.M. soils.

Soil Texture

Higher herbicide rates are needed on silts and clays because they provide more surface area for chemical binding. Sandy soils require the lowest rates.

Soil Moisture

For soil applied herbicides, even mechanically incorporated products, some moisture is required to place them in the soil solution for weed uptake. Soil herbicides generally work best in a warm, moist soil.

Chemical Compatibility

Some chemicals, when present in combination with other chemicals, are more toxic to plants and animals than when present alone. If the field has been sprayed with a herbicide previously, spraying with another chemical may cause great crop damage. Read the label to determine compatibility with other pesticide products.

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SOME PROBLEMS ASSOCIATED WITH HERBICIDE USE

Injury to the crop.

Faulty applications and environmental factors can lead to crop damage from herbicides.

Residues in the crop.

Herbicide residues in crops cannot exceed the safe and acceptable levels set by the Federal government. In many cases you will see recommendations telling you not to apply chemicals within a certain number of days of harvest or excessive chemical levels in the food will result.

Residues in soil.

Some chemicals persist in the soil in phytotoxic concentrations. The land manager must be aware of this if susceptible crops are to be planted subsequently.

Compatibility with other pesticides.

Two chemicals mixed together might cause crop injury where either one would not when applied separately.

Occupational hazard.

Herbicides as a rule are quite safe to use, having a low toxicity to man and animals. All precautions must be taken, however, to protect the applicator and persons who may come in contact with the chemical.

Air pollution.

a) Spray droplet drift.
b) Vapour drift (volatile gases)

Water Pollution.

Herbicides may enter water courses and groundwater either by direct application through accident or indirectly via leaching and runoff from sprayed areas.

Disposal of used containers.

Irresponsibility in disposal of used pesticide containers sometimes leads to poisonings of animals and man.

Resistant weeds

Weed populations contain a very small number of individual plants that are naturally able to withstand a particular herbicide (or herbicide group) which is registered to control the weed. This naturally occurring resistance is not due to weather conditions or application technique but is an inherent characteristic of the genetic makeup of the resistant plants.

Because of their low frequency of occurrence, these resistant plants go undetected. The continual use of the same herbicide (or herbicide group) year after year allows them to set seed and multiply. Futhermore, by removing the susceptible weeds, there is more opportunity for the resistant strain to flourish.

It is important therefore to know the herbicide groups so that herbicides with different mechanisms of action can be selected as part of the strategy in preventing a build up of resistant weeds. For information on herbicide groups refer to the Field Crop Guide For Commercial Growers.

Managing Resistance

• rotate crops. This usually results in using a diversity of herbicides.
• rotate herbicides with different modes of action.
• use tank mixes that control the target weeds by different modes of action.
• use short residual herbicides whenever possible. Use long term residual herbicides wisely and not continuously on the same field.
• integrate herbicide use with other weed control strategies.
• Practice good sanitation practices to prevent movement of weed seed with soil, machinery, crop residue, etc.

Effects on wildlife

Some animals depend on weeds as a food source. Altering the botanical flora could have detrimental effects on some animals but in turn could lead to a better environment for others.

Great care must be exercised when using herbicides in or adjacent to riparian areas. Read labels or contact habitat management personnel for information on setbacks from watercourses and riparian zones.

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WEED MANAGEMENT PLANNING

In any weed management program it is often the little things that can spell success or failure and it is often these same items that effect the cost of weed control.

As with other aspects of successful farm management, weed control requires an equally high level of management to obtain optimum control while minimizing costs.

A successful weed management program requires the producer to undertake three basic steps:

Step 1. Diagnosis of the problem

Identify the weed

Know your enemy! Just as a doctor cannot prescribe a cure for disease until he has identified the causative agent, so can you not choose a cure for your weed problem without first knowing the plant you are dealing with. Failure to properly identify the problem leads to dollars unwisely spent on control options. For example, Basagran is registered on peas and beans for control of hairy nightshade. It will not, however, control its relative, black nightshade. As another example, Treflan provides generally good control of many annual broad-leaved and grassy weeds in peas. It does not however, control any mustard family weeds. If you fail to identify shepherd's purse (a mustard) you won't get control.

It is also important to know something about the biology of the weed. Is it a perennial or an annual? If perennial, is it taprooted or creeping-rooted? If annual, is it a winter annual or a summer annual? This knowledge is critical to choosing control strategies and timing.

Identify the problem

Listen to your weeds. Weeds in crops are more often the result of a problem and not the cause. They often simply fill in environmental voids caused by such factors as:

• poor seedbed preparation results in uneven crop germination and therefore less competition for weeds.
  
• poor land preparation. Don't seed into a weed problem. If land to be used for crop production is infested with tough perennials like quack grass, failure to control prior to seeding invites disaster. You must control it (chemically or mechanically) prior to seeding or rotate to a crop where selective control is available. Black nightshade for example is not easily controlled in peas and beans but is effectively controlled by registered herbicide options in corn. Use of perennial forages for a number of years will also reduce a black nightshade problem.
  
• poor drainage. Buttercups and bog rush (slough grass) have a preference for wet soils so are indicative of poor drainage.
  
• poor fertility.
  
• soil pH. Weeds such as bracken fern grow best in acid conditions while others such as foxtail barley show a preference for higher pH soils. Amending the soil reaction can reduce the competitive advantage enjoyed by such indicators.
  
• diseases/insects. Pest organisms attacking beneficial plants weaken their competitive ability and predispose an area to weed invasion. In past years dandelions were thought to be the cause of failure of new seedings of alfalfa. Investigation showed that the disease organism, Verticillium wilt, was attacking the crop and dandelions were just filling in the environmental void left by a non-competitive crop.
  
• resistant weeds. Routine cultural practices and repeated use of the same herbicide hastens build-up of weeds resistant to those practices. Over reliance on atrazine for weed control in corn has resulted in a build-up of resistant weed populations (particularly groundsel, redroot pigweed and lamb's-quarters). Some of these weeds are now resistant to 20 times the rates that once controlled them. This creates another problem in that these resistant weeds can limit control options in rotational crops. Metribuzin, used in processing pea production, is in the same chemical family as atrazine. It could not be considered as an effective option when rotating from corn to peas if resistant weeds were present. (See section on Managing Herbicide Resistance, page 28).
  
• weed population shifts. Continued use of one management strategy can result in changes in the weed population. Atrazine herbicide, for example, has controlled most broadleaved weeds in corn but by so doing has created more space for grasses such as green foxtail and barnyardgrass which are only partially controlled.
  
• microbial degradation. Annual use of the same herbicide can result in huge increases of soil microorganisms that use the product as an energy source. Eradicane, continually used in corn production, has in some areas been quickly decomposed by soil microorganisms, resulting in poor weed control.
  
• weather effects. Failure to be aware of weather effects on herbicide activity often results in poor weed control and crop damage. Herbicides are less effective in cool, wet conditions. Heavy rains soon after a foliar application wash the herbicide from the leaf before they can enter the plant. Excessive rains can also leach the herbicide into the soil and result in crop damage and poor weed control. Wet conditions make soil preparation difficult and result in poor herbicide distribution over the soil and again poor weed control.
  
• lack of Total Farm Weed Control. Weeds growing on fence lines, farm roads, irrigation ditches, manure piles, in your stock yards and around farm buildings provide an annual nursery of weed populations. (See Table 2) Carried by wind, water, vehicular traffic and livestock, they travel with ease to infest "clean" land.

These "non-crop" weeds are also important habitat for insects and diseases which attack your adjacent crop. As an example, huge populations of the green peach aphid build up on common broad-leaved weeds such as lambsquarters, pigweed and mallow. This aphid can transmit over 100 plant viruses including potato leaf roll virus. Tomato Spotted Wilt Virus, a new and serious disease of greenhouse crops in south coastal BC, can be spread to field grown vegetables by thrips. Creeping yellow-cress, an increasing weed problem in the Fraser Valley, is an important source of disease carrying thrips.

Remember

1. Learn your weeds to properly select the right control strategies. Your local Ministry of Agriculture and Food office can assist you in identifying weeds.
   
2. Rotate herbicides and cropping practices to break resistance cycles and shifts in weed types, to prevent soil microorganism decomposition of herbicides and to prevent soil residue build-ups.
   
3. Don't use herbicides until weather and soil conditions are suitable as indicated on the label.
   
4. Solve your tough weed problems before seeding.
   
5. Be aware of and control weeds in areas adjacent to your cropland.
   

Step 2. Planning the program

Once you have properly identified the weed and determined the problem, the next step is planning.

Draw a map of your operation, including non-cropped areas. Mark on the map crops that you are growing, cropping sequence and weed that are present. Determine if the weeds are caused by underlying factors such as poor drainage, poor fertility, pH, etc., and plan to correct these first.

Obtain information on weed control strategies and write them on the map along with notes on timing of control operations. If chemical control is chosen you must ensure that the herbicide is registered for use on your crop and that the weeds you have identified are listed as controlled on the label. List all registered herbicides on your map, along with their cost per hectare (acre).

Obtain information on potential environmental impacts. Information on chemical toxicity to fish and wildlife, herbicide use in or near riparian areas and recropping restrictions that might impact rotational crops are critical to deciding management options.

Choose an appropriate management strategy based on effectiveness, cost and environmental considerations.

Step 3. Implementing the program

Once you have diagnosed the problem and planned your attack, you must now put the plan into operation. This is where the majority of mistakes are made! Accurate herbicide application saves money. Recent surveys in the U.S. have shown that 63 percent of growers had a 10 percent or greater application error when applying herbicides. Over-applying results in crop damage and a waste of money. Under-applying results in lower crop yields caused by poor weed control, and again therefore, a waste of money. Sprayer maintenance, particularly of the delivery systems, is crucial for accurate application as is sprayer calibration. If you don't know the exact output of your sprayer, you're only guessing at how much chemical to put in it.

Soil preparation is also very important for effective weed control. Pre-plant incorporated and pre-emergence herbicides must be applied to a well worked, moist and smooth, but fairly firm seedbed, free of clods and trash to obtain uniform chemical distribution and to encourage good seedling establishment. If incorporating volatile products such as Eptam, Treflan and Eradicane, the soil should be warm and moist, but dry enough on the surface to obtain good mixing. Speed of incorporation is also critical and this will depend on the herbicide and incorporation machinery used. Generally, disc implement should be run at 7 to 10 km per hour; field cultivators at 10 to 13 km per hour.

Lastly, timing as determined in your plan must be strictly adhered to. All too often, producers delay attempts at control until weeds are tall and smothering the crop. By this stage, the competitive damage to the crop is done and the weeds are too far advanced for effective chemical weed control. EARLY diagnosis and EARLY treatment are the keys to success.

Step 4. Monitoring Success/Failure

It is essential to review records to determine what worked and what didn't so that improvements can be made in subsequent programs. Monitoring should point to changes that may be necessary in crop or habitat management to prevent weeds from invading.

Steps to Reduce Herbicide Use

• use herbicides only when weeds are in the susceptible stage
• use herbicides only when weather and soil conditions are appropriate for effective control
• use Wipe-on technology where appropriate for weeds growing above the crop
• use band treatments over the row and cultivation between rows
• selective flaming or steam treatment may be appropriate in some situations (more research needed)
• use winter cover crops to reduce winter annual weed seed production
• use rotary hoe and precision cultivators when possible
• properly maintain application equipment and accurately calibrate
• maximize competition from beneficial plants
• think weed PREVENTION!
• READ THE HERBICIDE LABEL BEFORE USE