Methods of Weed Control

Measures to Reduce Contamination of Organic Fertilizers

Weed seeds and fruits do not lose viability after passing through the gastrointestinal tract of animals and accumulate in manure. During storage of manure, seed viability decreases significantly but is not completely lost even at high temperatures. Organic fertilizers have a stimulating effect on the germination of old seeds in deep dormancy.

The content of viable seeds in 1 t of manure is from 43 to 56 thousand; in chicken litter, 120–412 thousand; in the silt and solid fractions of pig manure, almost a million; in peat, from 10 to 37 thousand seeds (VNII of Fodder named after V. R. Williams). The germination of seeds extracted from peat and manure reaches 25–84%.

Manure and peat without measures aimed at reducing contamination are a potential danger of soil contamination with weed seeds, which can lead to an increase in crop infestation by 10 or more times. Even high agronomic practice with proper preparation and storage of manure does not exclude an increase in infestation.

The greatest efficiency in sterilizing manure from weed seeds is achieved with the “hot” storage method. As a result of active microbiological processes, its decomposition temperature rises to 60–70°C, which causes the death of weed seeds. The technology of thermal treatment of chicken litter reduces seed viability by 90–100%.

The germination of weed seeds is directly dependent on the degree of manure decomposition: the more rotted the manure, the fewer viable seeds. Research data show that 1 kg of fresh manure contains 297 germinable seeds, rotted manure 22, and 1 kg of loose manure 4.

Cleaning Seed Material

Cleaning seed material — a technological process of separating weed seeds from crop seeds, based on differences in their physical-mechanical (size, weight) and aerodynamic properties.

The permissible content of weed seeds in seed material is regulated by normative documentation. For seeds of main crops (wheat, oats, barley, buckwheat, etc.), admixture per 1 kg is allowed: for the first class, 5 pcs.; second, 20; and third, 70 pcs.

The viability of weed seeds persists for 3–10 years or more when stored in grain warehouses.

Methods of seed cleaning are divided into:

preliminary (primary)

— removal of easily separable, large, and small impurities;

main

— deeper cleaning of seed material from weed seeds remaining after preliminary cleaning;

special

— cleaning of seed material from difficult-to-separate weed seeds, for example, seeds of perennial grasses obtained from contaminated seed plots. Even when agronomic control practices are carried out, 1 kg of perennial grass seeds included up to 20 thousand or more weed seeds (VNII of Fodder named after V. R. Williams).

For cleaning seeds, grain-cleaning units and flow lines are used. Sorting machines are used to obtain seeds of various classes. The following cleaning methods exist: hydroseparation, exposure to electric or electromagnetic fields, and pneumatic-gravity units.

Besides cleaning seeds, growing seed material on lightly infested plots is also important. Seeds of some weeds are difficult to separate from vegetable plants, primarily from the Brassicaceae and Apiaceae families. Therefore, contamination of seed fields of vegetable crops is not allowed.

Measures to Reduce Contamination During Irrigation

One pathway of weed seed spread is hydrochory, that is, by means of water, including irrigation systems and water sources.

In 1 m³ of irrigation water, from 2 to 6 thousand weed seeds may be contained. At an irrigation norm of 600 m³/ha, 120–360 seeds per 1 m² are introduced, including those not previously noted in the field. During floods, seed introduction is also great.

Under an irrigated farming system, a complex of measures should be provided that combines agronomic, biological, and chemical methods of weed control, as well as additional measures: cleaning irrigation water, mowing weeds before flowering on canals, and cleaning them from silt containing seeds and vegetative parts of weeds. Cleaning irrigation water involves the use of settling ponds, shields, booms, etc.

The combination of irrigations that cause mass emergence of weeds (provocative irrigations) with subsequent practices for their destruction is an effective control technique under irrigated land conditions. The optimal time for eradication measures is the period of germination and emergence before the beginning of stable weed growth. Large norms and early irrigation dates promote the regrowth of more weeds, which increases the effectiveness of control measures.

Presowing tillage, especially for late-sown crops, as well as pre-emergence and post-emergence harrowing, inter-row cultivation, and hilling are mandatory weed control practices under irrigated farming.

To destroy the most malicious weeds, special control methods are used: deep and half-fallow tillage, selection of competitive crops, allocation of “reclamation” fields in crop rotation, and the use of environmentally safe herbicides.

Measures During Harvest

Weed seeds whose growing season ends simultaneously with the crop enter the combine hopper together with the main mass. Direct combining of grain crops in short timeframes allows reducing the shedding of seeds and fruits. Harvesting machines are also equipped with seed catchers.

Leaving high stubble and uncut areas, due to the creation of favorable development conditions, allows low-growing weeds to continue vegetation.

Timely harvesting of tops and silage crops allows collecting the non-shed part of weed seeds together with the green mass.

The running parts of harvesting and tractor machinery and tillage implements should be cleaned of adhering soil, attached seeds, and rhizomes. Attention should also be paid to cleaning transport vehicles used for transporting agricultural products.

When using the technology of leaving chopped straw and chaff as organic fertilizer during harvest, the possibility of spreading weed seeds with it should be taken into account, especially difficult-to-thresh and immature seeds (scentless mayweed (Tripleurospermum inodorum), white goosefoot (Chenopodium album), common windgrass (Apera spica-venti), sow thistles (Sonchus spp.), etc.).

Measures in Feed Preparation

Weed seeds and fruits that enter feed retain viability for a long time: in haylage and chopped hay throughout the entire storage period; in silage, despite the formation of acetic acid harmful to seeds, up to 30 days for amaranth (Amaranthus sp.), hemp-nettles (Galeopsis spp.), white goosefoot (Chenopodium album), and from 3 to 18 months for common barnyardgrass (Echinochloa crus-galli), thistles (Cirsium sp.), field bindweed (Convolvulus arvensis), sweet clover (Melilotus sp.).

Steaming grain feeds is one of the methods of destroying weed seeds that cannot be removed by grinding.

Compound feeds and fodder grain may contain a fairly large number of weed seeds. Considering that feeds may be imported from different countries or regions, they pose a potential threat of weed spread, including quarantine weeds. The use of feeds containing a large number of weed seeds requires measures to reduce their viability, including the use of herbicides.

Litter manure and solid fractions containing weed seeds, after spreading on the field, should be treated with herbicides at an increased application rate by 20–26%. The application technique in this case is similar to the application of herbicides under cultivation or over crops, after which the organic fertilizer is incorporated into the soil.

Non-agricultural areas and adjacent territories are potential foci of weed spread. Field-protective forest strips, especially in steppe and forest-steppe zones, largely protect agricultural lands from the spread of weed seeds.

In the System of Primary Tillage

When switching to regular no-moldboard tillage, for example, on eroded and erosion-prone soils, infestation of individual crops increases due to the accumulation of the main mass of weeds in the upper soil layer and deterioration of the nutrient regime, which naturally leads to a drop in yield. Barley, winter wheat, and perennial grasses are especially sensitive to this.

With moldboard tillage, the seed reserve decreases to a lesser extent, since deep burial creates a conservation effect for a long period. During subsequent tillage, once in the upper layers, they successfully emerge. Fall moldboard tillage is the main measure in the arsenal of agronomic weed control methods, including stubble disking and plowing.

On Eroded Soils

The introduction of soil-protective tillage technologies on eroded and potentially erosion-prone soils in modern agriculture has led to a change in the species composition of weeds. Agrophytocenoses of such lands differ greatly from flatland ones: weed numbers increase down the slope, and perennial weeds predominate in the lower part of the relief. With long-term use of flat-cut and minimum tillage, infestation increases by 60–80%. If in primary tillage systems based on plowing, spring annual weeds predominate, then with surface tillage, rhizomatous and root-suckering weeds predominate.

With no-plow tillage, the reserve of viable seeds in deep layers loses germination over time. Therefore, in this tillage system, the main task in weed control is to ensure the cleanliness of the upper 0–10 cm layer. This is achieved by periodic surface tillage in the summer-autumn period.

In a temperate climate with a moisture coefficient of 1–0.55, on eroded and erosion-prone soils, flat-cut and surface tillage practices are used. During crop harvesting, straw or green manure is spread over such fields, followed by their incorporation into the soil. Straw mulch is incorporated with harrows BIG-3, BM1P-15, UNS-5.7 to a depth of up to 8 cm. Green manure crops, for example, sweet clover, are incorporated with heavy disc harrows BDT-7 in 1–2 passes. In case of repeated regrowth of weeds, after 15–20 days an additional treatment with BMSH-15 and BDT-7 is carried out. This technique allows destroying weeds remaining in the stubble.

In Tillage Systems on Bare and Occupied Fallows

The greatest effect of weed eradication is observed in tillage systems on bare and occupied fallows. As a rule, the most infested fields are assigned to bare fallow.

The technology of bare fallow tillage should be soil-protective. It includes:

• post-harvest stubble disking;
• autumn fall plowing with a plow with a skimmer;
• from the spring of the next year — a system of spring-summer care.

Instead of mechanical tillage, herbicides may be used.

Depending on the type of infestation, the method of disking is chosen:

• with an abundance of annual weeds, single disking to a depth of 5–7 cm is carried out;
• with root-suckering weeds, double disking with disk harrows or flat-cut cultivators to a depth of 6–8 and 10–14 cm;
• with rhizomatous weeds, double longitudinal-transverse disking to a depth of 10–12–14 cm.

In spring, on the surface of a fallow field, seeds plowed in autumn germinate en masse. To destroy weed seedlings, share disking is most effective at a depth of 6–8 to 8–14 cm. Annual weeds are destroyed, and perennials are greatly depleted.

In the system of weed control on bare and occupied fallows, the features of winter and wintering weeds — the most dangerous for winter crops — should be taken into account. The fallow system does not ensure complete cleaning of fields from these weeds.

In the Presowing Tillage System

Eradication measures for weed control in the presowing tillage system are divided into:

• for sowing winter crops;
• for sowing spring crops;
• for sowing late-sown crops.

Efficiency depends on the intensity of germination and emergence.

Increased cultivation depth in the presowing tillage system substantially affects the infestation of fields by perennial weeds due to good germination and soil aeration. Tillage to the arable layer with ordinary cultivators is not carried out; chisel cultivators or chisel plows are used.

To accelerate the emergence of small-seeded crops, pre-sowing soil rolling is carried out, which promotes more uniform placement and faster emergence, and also enhances weed germination.

In the presowing tillage system, it is advisable to use combined units that improve the ecological conditions of crop growth and development, as well as reduce infestation and the number of machine passes.

Optimal conditions for the germination of winter and wintering crops are formed at a soil temperature of about 20°C and moisture of not less than 20–25%.

Before sowing early spring crops, only 20% of weeds emerge from the total number; another 20–70% germinate after sowing, and the rest germinate throughout the season until frost. Depending on weather conditions, the intensity may increase or decrease.

The greatest effect of eradication measures is achieved in presowing tillage for late crops because most early spring weeds germinate by the time of sowing and can be easily destroyed by harrowing followed by cultivation.

With strong infestation of fields by perennial, rhizomatous, and root-suckering weeds, spring tillage begins with cultivation to a depth of 14–16 cm. As weeds regrow, a second and, if necessary, a third layer-by-layer cultivation to the sowing depth is carried out. This technique leads to the depletion of the root system of perennial weeds. The first and second cultivations should be carried out with as long a period as possible to allow more weeds to germinate. Immediately after the first cultivation, when the upper layer dries, it is effective to carry out rolling to create optimal conditions for seed germination, especially on soils that weakly retain moisture. Thanks to this technique, weed seedlings appear 4–6 days earlier, and their number is 2–3 times greater.

During Crop Care

The main tillage practices for crop care include: rolling, harrowing, inter-row loosening, and hilling.

The timing of practices is determined by the condition of cultivated and weed plants, the greatest sensitivity of weeds to mechanical impact, and the minimum reduction in yield from possible damage to crops.

The greatest harm to spring crops in the Non-Chernozem zone is caused by weeds growing in the spring-summer period and by perennials, while winter crops are harmed by winter and wintering weeds that emerged in autumn. Optimal germination conditions for the latter are created immediately after sowing, with the onset of cloudy days with precipitation and relatively favorable temperatures.

In the Post-Harvest Period

Agronomic methods in the post-harvest period are interrelated with eradication measures carried out in the system of primary tillage and also include disking, cultivation, and plowing.

The depth, timing, and types of disking implements are determined by soil-climatic conditions, the degree of infestation, and the species composition of weeds. In regions with a short growing season of the Non-Chernozem zone, early fall plowing is effective in weed control; 2–3 weeks after it, treatment with disk harrows or cultivators is carried out.

Additional surface tillage of fallow in autumn, of the half-fallow type, gives a positive result in the control of malicious rhizomatous and root-suckering weeds, especially couch grass (Elytrigia repens). It consists of treatment with spring-tooth cultivators in combination with harrows to a depth of more than 10 cm.

The system of improved fall tillage consists of two or three diskings depending on the crop to a depth of 6–7 or 8–10 cm and subsequent plowing to a depth of 20–22 or 25–27 cm. Moisture conservation and acceleration of weed germination are ensured by timely post-harvest and subsequent diskings. Thus, in crops of spring wheat, the number of weeds per 1 m² with simultaneous disking at the harvest of preceding winter crops was 35, and with a delay in disking by 10 days, 54. A delay in disking by 30 days increased infestation 3 times.

Agronomic control methods in the post-harvest period should be combined with the use of herbicides. Field experiments and production practice show that 80–90% of weeds die when the leaf surface of regrown weeds is treated. This technique includes disking, herbicide treatment, and deep plowing. For spraying, herbicides of the 2,4-D group are used at doses up to 2–3 kg per 1 ha. The greatest effect, especially against perennial weeds, is achieved at an air temperature of not less than 12–14°C. Lower temperatures reduce the entry of the herbicide into the root system, which reduces their effectiveness.

Biological Agents

Biological agents are of greatest interest in the control of weeds introduced from other places, since in new habitats their natural enemies are absent. The action of a biological agent (phytophage) usually manifests in reducing the role of the weed in the agrocenosis by damaging or suppressing it and suppressing the competitiveness of the weed, without causing its death.

Insects, nematodes, fungi, and antibiotics can be used as biological agents. One of the most common biological agents for weed control in world agricultural practice is insects.

Competitive Relationships

Cultivated plants have greater competitive ability compared with weeds. Continuous-sown crops are more competitive than row crops. With qualified use of this technique, crop infestation can be substantially reduced at very low costs.

By competitive ability, cultivated plants can be divided into three groups:

crops with high competitive ability

winter rye, winter barley, winter wheat, winter rapeseed, hemp, Jerusalem artichoke, perennial grasses.

crops with medium competitiveness

oats, barley, vetch-oat mixture, sunflower, maize, mustard, tobacco, fodder cabbage, lupine.

crops with weak competitive ability

millet, sorghum, spring wheat, grain legumes, potato, sugar beet, flax.

This grouping of crops is conditional, since the ability to suppress weeds depends both on biological features and on cultivation conditions.

Allelopathy

Allelopathy — biochemical interaction of plants, rhizosphere microorganisms, and their decomposition products after death through complex physiologically active substances. The effect of allelopathy can be positive, that is, stimulating the growth and development of other plants, or negative, acting suppressively.

Decomposition products of plant residues also show an allelopathic effect in crops. Stubble and roots of wheat incorporated into the soil have a suppressive effect on the germination of maize, oats, and wheat; residues of winter rye have a suppressive effect on wheat, red clover, and timothy grass.

Crop Rotation

A professionally designed crop rotation is the most important phytocenotic measure in weed control. The effectiveness of crop rotations as a biological (cultural) method of weed control has been confirmed by a number of scientific and practical uses. Errors in constructing a crop rotation lead to increased infestation of fields by specialized and malicious weeds. Crop infestation in a crop rotation is 2–5 times less than in continuous crops or when crop rotation is violated or not observed.

Changing ecological conditions of crop growth in crop rotation links cause a suppressive effect on weeds. For example, alternation of continuous-sown crops with different sowing and harvesting dates with row crops, in which more thorough care is carried out; annual crops with perennial grasses. Tillering and fast-growing winter rye and winter wheat suppress weeds more strongly than weakly tillering spring wheat and millet. The latter has slow growth in the first 2–3 weeks after emergence, which causes low competitiveness. Bare fallows and early-harvested crops (oat-legume mixtures, winter crops for green fodder) play a large role in weed control, allowing weeds to be destroyed before seeding.

Seeding Rate and Method

The density of the stand of cultivated plants has a direct suppressive effect on weeds. The seeding rate is determined by the specific zone. Its reduction leads to a sharp increase in crop infestation. Cross sowing creates a more uniform distribution of seeds and allows cultivated plants to use growth factors more effectively and facilitates crop care, exerting a suppressive effect on weeds. Narrow-row and wide-row sowing with inter-row cultivation during care of vegetative plants also allows reducing infestation. For example, narrow-row sowing of cereals reduces infestation by 20% compared with the row method.

Fertilizers

The application of fertilizers can increase infestation by 1.5–2 times, and total biomass by 2–4 times. Losses on fertilized fields from weeds for cereals are up to 40–55%, for vegetables up to 80–90%. Conversely, high agronomic practice in fertilizer application can be an effective biological method of suppressing weeds. Violation of technology leads to an increase in the number and mass of weeds, accumulation of their propagation organs in the soil, and also requires the use of improved control technologies.

The response of weeds to fertilizer types differs. This property is due to differences in the use of nutrients by weeds. Unbalanced application of fertilizers can promote the accumulation of one weed species. Taking into account the reaction of weeds to individual fertilizers allows building a fertilizer system in such a way as not to give weeds an advantage in spread and subsequently to build an integrated system of control measures.

Long-term experiments of MSKhA named after K. A. Timiryazev showed the influence of soil fertility on crop infestation: in crop rotations, improved plant nutrition restrained infestation, whereas continuous cultivation of crops led to its growth.3

Soil Liming

Liming of soils reduces the number of weeds by 2 times, but increases the species composition by 1.5–2 times, which creates a potential danger of increased infestation in subsequent periods.

The reaction of weeds to liming is different. Liming changes not only soil acidity, but also the conditions of mineral nutrition of plants, as well as air, water, and thermal regimes. Consequently, liming affects the species composition and number of weeds.

History

Herbicides were first used at the end of the 19th century. They were mainly inorganic substances: copper nitrate, ferrous sulfate heptahydrate (iron vitriol), ammonium sulfate, sulfuric acid, sodium nitrate, sodium arsenite, kainite, calcium cyanamide, and others. Due to significant drawbacks, until the 1940s herbicides were not widely used. Only one third of arable lands and perennial plantings in Russia were treated with herbicides.

After the discovery in 1941 of 2,4-dichlorophenoxyacetic acid (2,4-D), a synthetic growth regulator, herbicides became widespread in weed control. 2,4-D, its salts, and esters at concentrations of 0.01% cause the death of dicotyledonous plants without causing the death of monocotyledonous plants. Subsequent studies showed that other substances have similar properties, for example, derivatives of 2-methyl-4-chlorophenoxyacetic acid (2M-4X) and trichloroacetic acid (TCA).

Current State

Herbicides form an integral part of modern agriculture, combining comprehensive chemicalization of production with the use of energy-saving and soil-protective tillage technologies.

Chemical methods of weed control have a number of disadvantages:

• emergence of herbicide-resistant weeds (superweeds);
• insufficient selectivity;
• long inactivation;
• absence of necessary preparations;
• pollution of the environment and agricultural products.

Research in the field of developing new herbicides is conducted in a number of countries. Promising directions of this research are the elimination or minimization of negative effects of application. The use of herbicides in cereal crops in Russia gives an additional 0.2–0.25 t/ha, maize for grain 0.5 t/ha, for silage 5 t of green mass per 1 ha, sugar beet 2 t of roots per 1 ha, rice 0.8–1 t/ha (propanid) (VNII of Plant Protection). Under irrigated farming conditions, due to more favorable growth conditions for weeds, the use of herbicides is of great importance.

Classification of Herbicides

Herbicides are classified by a number of features: chemical composition, methods of application, nature of action, degree of danger to humans and warm-blooded animals, and effect on products and the environment.

By chemical composition, herbicides are divided into:

• organic;
• inorganic.

By nature of action:

• continuous (general eradication);
• systemic (selective):

Systemic herbicides are divided into:

• growth-regulating;
• without growth-regulating action;
• with broad selectivity;
• with narrow selectivity.

By site of action:

• leaf;
• leaf with movement through the plant;
• soil through the root system;
• leaf and soil.

By application time:

• before sowing or planting;
• simultaneously with sowing or planting;
• after sowing or planting, before emergence;
• after crop emergence;
• during the period of mass regrowth of weeds.

By degree of toxicity:

• strong toxic substances LD₅₀ < 50 mg/kg;
• highly toxic LD₅₀ = 50–200 mg/kg;
• medium toxic LD₅₀ = 200–1000 mg/kg;
• low toxic LD₅₀ > 1000 mg/kg.

By phytotoxicity:

• sensitive;
• medium sensitive;
• resistant.

By duration of action:

• long-acting;
• short-acting.

Continuous and Systemic Herbicides

Continuous (general eradication) herbicides are active against almost all plant species and are used to destroy vegetation on areas without crops: on roadsides of railways and highways, power lines, drainage and irrigation canals, sports grounds, livestock farms, around grain factories, in foci of quarantine weeds, etc. The modern level of research on selective herbicides allows their use in crops of most cultivated plants.

The selective properties of herbicides depend on a number of factors:

• anatomical-morphological (leaf shape, their arrangement, pubescence, wax coating, etc.) and physiological features of plants;
• chemical composition of the herbicide;
• physicochemical properties;
• physiological activity;
• differences in the properties of cell protoplasm.

Selective properties depend on the application rates of herbicides. For example, 2,4-D and 2M-4X are effective against many dicotyledonous weeds in cereal crops. Exceeding the established doses makes them continuous-action herbicides, causing the death of all plants. Broad-spectrum selective herbicides affect many weeds without damaging crops. For example, atrazine destroys a wide range of dicotyledonous and monocotyledonous weeds in maize crops.

Narrow-spectrum selective herbicides affect several species or only one species of weeds. For example, avadex is used against wild oat (Avena fatua) in crops of wheat, barley, and pea, acting on it for a limited time in the 1–2 leaf phase. Propanid is used in the control of common barnyardgrass (Echinochloa crus-galli) in rice crops, while its effect on other weeds is limited.

Systemic herbicides are divided into:

• with typical growth-regulating action — cause disruption of growth and cell division, deformation of stems and leaves, tissue proliferation, and formation of aerial roots. When used in recommended doses, they show high selectivity to dicotyledonous plants and do not act on grasses.
• without typical growth-regulating action — affect photosynthetic and other processes of plant vital activity. A change in leaf color is often manifested; plants wilt and die.

Contact substances — cause burns of leaves at the sites of contact, destruction of chlorophyll, and death of plants (nitrafen, reglon, etc.).

The first two groups include: 2,4-D, 2M-4X, simazine, atrazine, piramine, chlor-IFK, eptam. They are especially effective in the control of perennial rhizomatous and root-suckering weeds.

Herbicides by Site of Action

Leaf herbicides act mainly at the sites of their contact with the plant or, moving through the plant, exert a systemic effect on tissues.

Soil herbicides are absorbed by the root system and transported to aboveground organs. They act in the roots or in the aboveground part of the plant.

Also, herbicides by site of action can be classified as:

contact,

that is, herbicides acting directly on those parts of plants to which they were applied;

translocated,

that is, herbicides that enter some parts of plants but act on other parts.

The selectivity of contact herbicides in foliar application partly depends on the differential retention of the herbicide by the crop and weeds. For example, pea leaves are covered with a large amount of epicuticular wax, which repels aerosol droplets. The wax of leaves of white goosefoot (Chenopodium album), for example, is not so water-repellent and is damaged due to greater retention of the contact herbicide. Other post-emergence herbicides have a contact effect on the crop, and if the amount of wax on the leaves is low or it is damaged by wind, hail, frost, flying sand particles, or mechanical damage of any kind, the effectiveness of the wax cuticle decreases and serious burn may occur.

If post-emergence preparations are planned to be used, the amount of wax on leaves can be checked before spraying by taking a plant sample and immersing it in a container with a 1% methyl violet dye solution. Then the plant is removed, and excess dye is carefully shaken off. The amount of retained dye indicates the areas where the wax cuticle is damaged or incomplete. This level is assessed, and the decision to use the herbicide is made upon reaching a satisfactory level of dye-repellent tissue. In some cases, several days may be required for wax accumulation before spraying is safe, especially after a period of bad weather.

Translocated herbicides are absorbed and transported in the vascular system of the plant. Selectivity depends on the resistance of the crop and the susceptibility of the weed to the herbicide. The crop may have a mechanism for breaking down the herbicide to a non-toxic derivative, or, conversely, a susceptible weed may metabolize the toxic derivative through a biochemical process that the crop lacks. An example is non-toxic MCPB (4-(4-chloro-2-methylphenoxy)butanoic acid), which is converted to toxic phenoxyacetic MCPA (2-methyl-4-chlorophenoxyacetic acid) by β-oxidation in susceptible broadleaf weeds, while legumes lack this metabolic process. The selective use of bentazone may be associated with differential retention and absorption, as well as the ability to metabolize bentazone, and this material is widely used on broad beans (Vicia faba) and beans (Phaseolus sp.).

Herbicides of Different Application Times

Herbicides applied before sowing seeds or planting seedlings, before weed emergence, are usually called pre-sowing herbicides. This group includes both soil-acting preparations used before emergence and leaf-acting preparations used after emergence.

Herbicides applied simultaneously with sowing are usually called at-sowing herbicides. They are applied in bands in the row zone and in the protective zone between rows (not treated by a cultivator), as well as by broadcast spraying. They are used mainly in row crops.

Pre-sowing and at-sowing herbicides are often incorporated into the seedbed by cultivation. Cultivation, necessary for their effective mixing in the sowing layer, can lead to the formation of too thin a layer, which negatively affects the crop. On soils with a denser structure, sowing may be delayed until soil conditions become suitable for the use of incorporation methods. Pre-sowing herbicides are usually used where populations of perennial grasses are present.

Herbicides used after sowing seeds, before the emergence of cultivated and weed plants, are mainly soil-acting. They are often called pre-emergence herbicides. Some of them act on emerged weeds by contact, others destroy weeds by absorption from the soil, and others work in a combination of both; most of them disrupt photosynthesis. Materials whose action is based on absorption from the soil are classified as residual herbicides. They have low solubility and remain in the surface layers of the soil, but to be effective, most of them require moisture soon after application so that they penetrate into the soil within reach of weed seeds and roots. The selectivity of pre-emergence herbicides is achieved by planting the crop below the location and availability of the herbicide in the soil, and in some cases by the innate tolerance of the crop to the herbicide.

The effectiveness of residual herbicides is affected by the amount of clay and organic matter in the soil. Clay particles can adsorb the chemical on their surface, thereby blocking a certain proportion and reducing effectiveness. Organic matter acts in a similar way. Residual herbicides are degraded by ultraviolet light if they remain on the surface, and bacteria complete the process in the soil. Pre-emergence contact herbicides are aimed exclusively at destroying weeds that appear before sowing. Since weed control and crop emergence usually coincide, the possibility of their use on some crops is limited.

Herbicides applied after emergence are applied by broadcast or band spraying of the soil at the base of the stem, preventing the working solution from contacting vegetative parts. They are also called post-emergence herbicides. Post-emergence herbicides act on leaves by translocation only or by contact and translocation, and in some cases they also have a residual effect. The selectivity of such preparations may be explained by the absence of contact with crop leaves due to the wax cuticle of the leaves (wax coating) or by the absence of a negative effect on the crop when absorbed by leaves.

Sensitive and Resistant Plants to Herbicides

Depending on the reaction to herbicide application, cultivated and weed plants are divided into:

• sensitive — completely destroyed;
• medium sensitive — partially die or are suppressed;
• resistant.

The development phase affects sensitivity to herbicides. Cultivated and weed plants show the greatest sensitivity at an early age, especially during germination. The presence of a stronger root system positively affects plant resistance. For this reason, perennial weeds are more resistant than annual weeds. For example, the root system of such weeds as creeping thistle (Cirsium arvense), field sow thistle (Sonchus arvensis), Russian knapweed (Rhaponticum repens), field bindweed (Convolvulus arvensis), and field horsetail (Equisetum arvense), lying at a depth of more than 10 cm, is resistant to atrazine, which does not penetrate to a great depth. This property allows the use of this preparation in orchards and forest nurseries, destroying weeds whose root system lies in the upper soil layers, without affecting the roots of trees and seedlings.

Short- and Long-Acting Herbicides

Long-acting preparations persisting for more than one year include atrazine, propazine, simazine, and diuron. Short-acting ones include 2,4-D, 2M-4X, reglon, piramine, prometryn, tillam, etc.

The aftereffect of herbicides should be taken into account when alternating crops in crop rotation.

Timing of Herbicide Treatment

Depending on the timing of treatment, herbicides are distinguished as:

• pre-sowing application to the soil with incorporation by harrows or cultivators;
• post-sowing application to the soil with simultaneous incorporation or without it;
• pre-emergence spraying of the field before crop emergence;
• post-emergence spraying;
• post-harvest treatment during periods of mass regrowth of weeds.

Pre-emergence treatment with herbicides is limited to several days — from sowing to the moment of emergence, and if herbicides cannot be incorporated by tillage, non-volatile preparations must be used.

Pre-sowing and pre-emergence treatments are effective because they suppress weeds in the most vulnerable early phases of growth, when crop sensitivity to infestation is greatest.

Methods of Herbicide Application

Herbicide treatment is divided into:

• broadcast — treatment is carried out uniformly over the entire area, used in crops and unoccupied fields;
• row — used more often in row crops;
• band — used in the same way as the row method;
• spot — local destruction of weeds, more often used in the control of especially malicious and quarantine weeds with continuous-action herbicides.

Post-emergence treatments are carried out by spraying. In this case, the timing of treatment and application rates play a large role in order to prevent damage to cultivated plants.

In row crops, post-emergence directed spraying is used with the help of special sprayers into inter-rows or rows.

Technologies for the use of granular herbicides are being developed. Application is carried out by special machines to a specified depth in rows or inter-rows, or by scattering on the soil surface. Granules have a number of advantages: longer action in the soil, and decomposition processes under the action of microorganisms proceed more slowly.

In a number of countries, herbicides are applied together with irrigation water. This technique is called herbigation.

On erosion-prone soils, the use of herbicides in fallow fields allows reducing the number of tillage operations.

Formulations of Herbicides

Herbicide formulations can be:

• powdery, water-soluble and water-insoluble (suspensions), containing up to 50–80% active ingredient;
• aqueous solutions and water-soluble concentrates;
• emulsion concentrates;
• granular, containing up to 5–10% active ingredient;
• mixed.

The formulation of the preparation affects the effectiveness of the herbicide.

Surfactants (sulfanol, OP-7, OP-10, etc.) may be added to the preparations to increase the stability of the emulsion or suspension, as well as for better wettability and adhesion to treated surfaces.

Herbicide Application Rates

Optimal application rates, timing, and methods of application have been established for all herbicides. The exact application rate of herbicides is determined taking into account the following factors:

• species composition of weeds;
• soil texture and organic matter content;
• degree of infestation;
• weather conditions during treatment;
• residual effect on subsequent crops.

Application rates are most often calculated in kilograms of active ingredient per 1 ha D or in kilograms of preparation (technical product) per 1 ha:

where

d

– application rate of active ingredient, kg/ha;

А

– content of active ingredient in the technical preparation, %.

It is also possible to calculate the herbicide application rate given the preparation rates:

The herbicide application rate for the band method is calculated by the formula:

where

D _л

– herbicide application rate for band application, kg/ha;

D _с

– herbicide application rate for broadcast application, kg/ha;

S

– width of the sprayed band, cm, М – width of inter-rows, cm.

When using herbicides in liquid form, the liquid application rate is calculated taking into account the machines and equipment used. For contact soil-acting preparations, liquid application rates are higher. In aerial treatments, application rates are lower than when using mounted or trailed sprayers in combination with tractors.

Approximate application rates for ground sprayers: when using contact herbicides, 300–600 l/ha; systemic, 150–300; soil-acting, 300–400 l/ha.

The liquid application rate in the case of aerial sprayers with low-volume spraying is 25 l/ha in cereal crops; at relative air humidity less than 50%, 50 l/ha.

The concentration of the working solution K, %, depending on the herbicide application rate by active ingredient D, kg a.i./ha and the liquid application rate Q, l/ha, is calculated by the formula:

Herbicide doses are adjusted taking into account the sensitivity of cultivated and weed plants. For the control of perennial weeds, the application rate is increased by 15–25%.

Conditions for Herbicide Application

Wind. Gusts of wind lead to uneven distribution and increased evaporation of the herbicide solution during spraying. At a wind speed of 3.6 m/s, droplets with a diameter of less than 100 µm are completely carried away by gusts even at a distance of 38 cm from the sprayers to the soil surface. Drift decreases with increasing droplet size, and at a diameter of more than 325 µm it stops. Therefore, optimal conditions for spraying with ground boom sprayers are formed in calm weather; for aerosol generators of aerial spraying, wind speed should be not more than 2 m/s. To protect crops sensitive to the preparation in neighboring territories, a protective strip 5–10 m wide is left; in windy weather its width is increased, or spraying is not carried out at all. For aerosol generators, the protective strip is 100 m, and in the case of aerial spraying with wind directed toward a neighboring field, it is increased to 2 km.

Precipitation. Spraying is carried out in the morning and evening in the absence of precipitation. In daytime, ascending air currents lift small droplets of the solution upward and can carry them over long distances. The effectiveness of herbicides decreases if precipitation falls during spraying or in the first 6 h after it.

Temperature. The optimal air temperature during treatment is 16–22°C. Low temperatures reduce the effectiveness of preparations due to reduced growth activity in weeds; high temperatures increase evaporation of the solution and create a threat of damage to crops in neighboring fields sensitive to the herbicide.

Soil moisture. Low soil moisture reduces the effectiveness of a number of soil herbicides; detoxification proceeds more slowly, which leads to a long aftereffect on subsequent sensitive crops of the crop rotation. High moisture promotes the penetration of herbicides deep into the soil, where they are quickly inactivated, without creating the proper effect in the upper layers.

Sunlight. Herbicides that are not resistant to solar radiation (triallate, treflan, tillam, etc.) should be incorporated into the upper soil layers.

Soil composition. Soils rich in humus or silt fractions have increased adsorption to herbicides, which reduces their effectiveness. Therefore, on such soils, application rates are increased. Conversely, on poor sandy and sandy loam soils, doses are reduced.

Characteristics of Main Herbicides

Salts and esters of 2,4-D and 2M-4X have become widespread in the control of dicotyledonous weeds. The recommended rate is up to 0.5–1.6 kg/ha.

Against individual malicious weeds, narrow-action or specially synthesized herbicides are used. Preparations of continuous action based on glyphosate 1 have become widespread in the AIC and private subsidiary farms. In different doses from 2–3 to 6–8, they have shown their effectiveness in the control of many malicious weeds in crops of various cultures (provided the crop is protected), vineyards, fruit plantings, nurseries, hayfields and pastures, and bare fallows.

Improving Herbicide Application Technology

One-sided application of the chemical method of weed control leads to the actualization of a number of problems:

• insufficient data on the influence of conditions under which the phytotoxicity of herbicides manifests fully;
• violation of agronomic practices, leading not only to a decrease in the effectiveness of the herbicide but also of the farming system used;
• the use of one type of herbicide for a long time leads to the suppression of sensitive weeds and the accumulation of resistant ones.

To prevent negative consequences and obtain the maximum effect, it is necessary to take into account agroclimatic conditions, the state of weeds and cultivated plants, field infestation, and comply with the application regulations. To solve the problem of the spread of resistant weeds, broad-spectrum selective herbicides should be used with alternation by years.

Chemical methods of weed control should be part of an integrated system of eradication measures. Agronomic methods are priority, while chemical methods are additional.

The introduction of energy-saving soil-protective technologies has also exacerbated the problem of controlling resistant weeds. Research is being conducted on ways to increase the effectiveness of herbicides in the following directions:

• changing treatment timing;
• increasing the effectiveness of action with mixtures of herbicides, growth regulators, and fungicides;
• research on new promising preparations;
• development and mastering of herbicide systems.

Herbicide System

Herbicide system — a complex of measures aimed at the systematic implementation of eradication and preventive measures for controlling the infestation of agricultural lands and adjacent territories, and which is an integral part of the farming system. The herbicide system is inextricably linked with the farming system, including the ecological and environmental safety of systems for protecting agrophytocenoses from weeds, as well as high economic and biological efficiency.

Herbicide systems allow:

• more fully using the potential of crop rotation;
• enhancing the action of agronomic practices and fertilizers as regulating factors of infestation;
• systematically taking into account the action and aftereffect of each herbicide;
• reducing application rates and the number of treatments;
• replacing, where possible, chemical methods of weed control with non-chemical ones.

Herbicide systems in crop rotations are determined by the composition of cultivated plants, agroclimatic conditions, and soil type. For example, in crop rotations on sod-podzolic soils (1 — occupied fallow, 2 — winter wheat, 3 — barley + perennial grasses, 4–5 — perennial grasses, 6 — winter wheat, 7 — potato, 8 — barley), herbicides were used only in crops of winter wheat, potato, and barley. At the same time, the reduction in infestation was 80–95%, and the productivity of the crop rotation increased from rotation to rotation.

Safety and Labor Protection When Using Herbicides

All herbicides are toxic substances. Toxicity is measured by the value of LD₅₀ (mg/kg body weight), that is, the dose that, when entering the stomach, causes the death of 50% of warm-blooded animals. By degree of toxicity, herbicides are divided into:

strong

— LD₅₀ < 50 mg/kg;

highly toxic

— LD₅₀ = 50–200 mg/kg;

medium toxic

— LD₅₀ = 200–1000 mg/kg;

low toxic

— LD₅₀ > 1000 mg/kg.

Most approved herbicides are low toxic.

Federal Law of the Russian Federation No. 109-FZ of June 24, 1997, “On the Safe Handling of Pesticides and Agrochemicals” defines the legal support for the use of agrochemicals, including herbicides. A number of normative documents establish requirements and norms for working with agrochemicals, their storage and transportation. These rules are mandatory for familiarization and compliance by persons engaged in work with agrochemical plant protection agents and weed control.

Persons who have familiarized themselves with the safety rules for the use of herbicides are allowed to work with herbicides. The rules should include information about the properties of the preparations used, their effect on humans, precautions during work, and first aid in case of poisoning. Persons under 18 years of age, pregnant women, and nursing women are not allowed to work with herbicides. Persons allowed to work with herbicides must undergo periodic medical examinations establishing professional suitability and the possibility of developing occupational diseases. According to legal requirements, medical examinations are carried out if, at workplaces, based on the results of a special assessment of working conditions (SOUT), hazard class 3.1 or higher is established. Also, if class 3.1 for the chemical factor is established, workers are given milk.

According to the Standard Norms for the Issuance of Special Clothing, workers are provided with special clothing. When working with solutions, respirator filters should be changed 2–3 times a day. At the end of work, special clothing and personal protective equipment (PPE) are washed with warm soapy water and dried. Respirators are washed and treated with a 0.5% solution of potassium permanganate or alcohol, rinsed with clean water, and dried. Special clothing and PPE are stored in a specially designated place.

Persons guilty of violating the rules for the safe handling of agrochemicals bear responsibility in accordance with the legislation of the Russian Federation.

Containers intended for the transportation and storage of herbicides must be sealed, easy to clean, and disinfect. Transportation and storage of preparations with food products, feed, and agricultural products are not allowed. Special warehouses located at least 200 m from economic and residential buildings are used for storing herbicides. They must be equipped with ventilation and lighting, without access by unauthorized persons. They are designed with sloping floors without cracks and without a basement, a shower and washbasin, and a room for storing special clothing and a first aid kit. Agrochemicals are issued from the warehouse by written order of the head of the enterprise or the chief agronomist. Bulk storage of herbicides is not allowed. Vehicles, containers, and inventory after use are disinfected: metal objects are treated with kerosene or 3–5% sodium carbonate, wooden ones with bleach; containers from under agrochemicals are burned, and residues of the solution are disposed of according to the instructions.

Work with herbicides is carried out under the supervision of a specialist. Before treatments, residents of nearby settlements are notified. This measure is necessary to prevent accidental poisoning of animals and bees by chemicals during and after treatment. Beehive entrances are closed during treatment. Treated plants are unsuitable for animal feed for two weeks. Livestock driving routes must be at least 200 m from the place of preparation and pouring of solutions.

Fields and plots located at a distance of 300 m from water bodies, economic and residential buildings may be treated with low-toxic herbicides by ground boom sprayers with the permission of the plant protection station.

Refueling points are located at least 200 m from residential and economic buildings. Operations for preparing solutions and pouring must be mechanized. At the end of work, the place of the refueling point is treated with bleach and plowed.

During work, it is prohibited to eat, drink, or smoke. At the end of work, hands and face are thoroughly washed with soap or a shower is taken.

Common signs of poisoning by chemical preparations are dizziness, convulsions, chills, nausea, sore throat, cough, etc. When the first signs of poisoning appear, the worker is immediately taken away from the work site to fresh air and a doctor is called. The doctor is given the name of the active ingredient of the agrochemical and the label or instruction for the preparation, which indicates first aid measures in case of poisoning. If a chemical agent enters the stomach, the victim is induced to vomit, after first giving several glasses of warm water and several tablets of activated charcoal. If a chemical substance gets on exposed parts of the body, it should be immediately washed off with plenty of water, then washed with soap. If it gets into the eyes, they are washed with plenty of water, then treated with a 2% soda solution.