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Biological methods of weed control

Biological methods of weed control are extermination measures based on the targeted use of various living organisms (bacteria, viruses, fungi, insects, fish, birds, rodents, plants) for the selective destruction of weeds. The action of biological methods is manifested at the level of agrophytocenosis.

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General information

The purpose of biological methods of weed control, as well as other control methods, is to bring the level of weed infestation on agricultural land to a level at which they do not lead to economic costs from crop losses. 

The advantage of the biological method is a long-lasting effect with relatively small primary costs. The disadvantages of biological methods are the narrow selectivity of action and the potential danger to other beneficial species created by organisms introduced into the agrophytocenosis.

Biological methods of weed control include the use of:

  • biological agents (insects, bacteria, viruses, fungi, etc.);
  • competitive relationships between species;
  • allelopathy;
  • crop rotation and other agricultural practices.

Any indirect influence manifested through the impact of any factor on the formation and condition of agrophytocenosis is called a phytocenotic measure. These influences may be manifested through:

  • soil conditions;
  • climatic factors (drought, lack of heat, hailstorms, etc.);
  • biogenic factors (development of diseases, bringing in of seeds by birds and animals, cattle grazing, etc.);
  • anthropogenic factors (soil treatment, fertilizers, pesticides, etc.).

The effect of phytocenotic measures is manifested through competitive relationships, allelopathy, crop rotation, agrotechnics, etc. For this reason, the biological methods of weed control also include liming, seeding rates, fertilizer system, etc.

Use of biological agents

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

Insects

Insects are one of the most common biological agents of weed control in world agriculture practice. In Russia, the use of insects in such a role has not yet become widespread.

In Russia, this method is used to control Orobanche by colonizing the Phytomyza fly (Phytomyza). Its ability to destroy Orobanche was known at the end of the last century. Feeding on the ovaries, tissues and immature seeds of Orobanche, which parasitizes sunflowers, hemp, tomato, tobacco and other crops, the Phytomyza fly damages 80-95% of flower stems in one summer, reducing seed production and causing the death of the weed. It produces 4 generations of insects in one season. Application of phytomiza reduces infestation of cucurbitaceous and vegetable crops by Zygogramma spp. in 3-4 years till economically insensible level; tobacco plantations – in 4-5 years.

The ragweed moth (Noctuidae) is used to control Ambrosia. Its caterpillars feed only on leaves of common ragweed (Ambrosia artemisiifolia) without damaging other plants. It was introduced to Russia from North America, from which the weed was introduced, and it is acclimatized to the European part of Russia.

Nematodes

A nematode species has been studied for the control of creeping mustard (Acroptilon repens). Its larvae get into leaf axils in spring and form large numbers of galls there, subsequently feeding on stem tissues. As a result, 50-60% of creeping mustard dies by the next year, and the harmfulness of the remaining weeds is reduced.

Fungi

Some pathogenic fungi can be used to control weeds, such as the fungi that infect the Canada thistle (Cirsium arvense) plant with rust. Its spores germinate on the plant, causing photosynthetic processes to slow down and killing the weed.

Fusarium fungi cause mass death of Orobanche in sunflower crops. 

To control Cuscuta in sugar beet, alfalfa, and kenaf crops, the Institute of Botany of the Academy of Sciences of Kyrgyzstan developed a method of destruction by Alternaria cuscutacidae spores. After spraying crops affected by dodder plants with an aqueous suspension of the fungus, dodder plants completely die in 12-20 days. The efficacy of the treatment is 90-95%.

Antibiotics

Some antibiotics are active against Orobanche. This direction is still at the research stage.

Competitive relationships

Cultivated plants are more competitive than weeds. More competitive are crops of continuous crops, as opposed to row crops.

Cultivated plants can be divided into three groups according to their competitive ability:

  • Crops with high competitiveness – winter rye, winter barley, winter wheat, winter rape, hemp, Jerusalem artichoke, and perennial grasses.
  • Crops with medium competitiveness – oats, barley, vetch-oat mixture, sunflower, corn, mustard, tobacco, fodder cabbage, lupine.
  • Crops with weak competitiveness – millet, sorghum, spring wheat, grain legumes, potatoes, sugar beets, flax.

The given grouping of crops is conditional, in view of the fact that the ability to suppress weeds depends on both biological characteristics and cultivation conditions. 

With skilled use of this technique you can significantly reduce the infestation of crops at a very low cost.

Allelopathy

Allelopathy is a biochemical interaction between plants, rhizosphere microorganisms, and their decay 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 in a depressing way.

Examples. Active substances secreted by rhizomes of couch grass (Elytrigia repens) into the soil reduce the growth of corn, winter rye, and oats by 1.5-2.0 times, and reduce stem density in rye fields by 2-3 times.

Corn spurrey (Spergula arvensis), sorrel (Persicaria lapathifolia), and linseed rag (Camelina alyssum) reduce the growth of fiber flax by 1.5-2.0 times and the stand haulm density by 5-20 times.

Corn spurrey (Spergula arvensis), sorrel (Persicaria lapathifolia), and double-cut pickleweed (Galeopsis bifida) have a negative impact on barley crops. Winter wheat crops are strongly depressed by secretions of chamomile (Tripleurospermum inodorum), bachelor’s button (Centaurea cyanus), common broomrape (Apera spica-venti), and common sorrel (Rumex acetosella). Excreta of bristlegrass (Setaria pumila), barnyard grass (Echinochloa crus-galli), wild radish (Raphanus raphanistrum), wild mustard (Sinapis arvensis), and lamb’s-quarters (Chenopodium album) inhibit corn growth.

Inhibitory effect of weeds is manifested at all stages of vegetation of cultivated plants, including the germination phase. Extracts of lamb’s-quarters (Chenopodium album) and Corn spurrey (Spergula arvensis) reduce germination and germination energy of winter rye, oat, long-fibered flax and meadow clover seeds by 1.3-4.0 times and more. Water extracts from root extracts and leaves of perennial sow thistle (Sonchus arvensis) reduce germination of barley, millet, and corn.

Products of decomposition of plant residues also show an allelopathy effect on crops. Residues of stubble and wheat roots buried in the soil have a depressing effect on the germination of corn, oats, and wheat; residues of winter rye on wheat, meadow clover, and meadow thymothea.

Crop rotation as a biological factor in the management of phytosanitary status of crops and soil

Professionally constructed crop rotation, in addition to solving the main tasks of scientifically based crop rotation, is the most important phytocenotic measure in weed control. The effectiveness of crop rotations as a biological method of weed control has been confirmed by a number of scientific studies and practical use.

Mistakes in building crop rotations entail strengthening the weed infestation of fields by specialized and malevolent weeds. Weed infestation of crops in the crop rotation is 2-5 times less than that of the crops in a permanent crop or in the case of violation and non-observance of crop rotation.

Repeated sowing, as well as their prolonged cultivation in the same place leads to the spread of field broom (Apera spica-venti), chamomile odoriferous (Tripleurospermum inodorum), rye brome (Bromus secalinus); in spring crops – lamb’s-quarters (Chenopodium album), pickleweed (Galeopsis), thornbush (Spergula), cleavers (Galium aparine), hilltails (Polygonum), etc. For example, if winter crops were sown after winter crops, the infestation of chamomile (Tripleurospermum inodorum) was 650 pcs/m2, after vicia-oat mixture – 127, after barley – 40, after clover – 25, after – bare fallow – 5 pcs/m2 (table).

Number of weeds depending on the number of crops in the rotation
Number of weeds depending on the number of crops in the rotation

Table. Crop infestation and crop yields depending on the conditions of cultivation[1] Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. - M.: Publishing house "Kolos", 2000. - 551 p.

Weeds, pcs/m2
Weed weight, g/m2
Yield, t/ha
total
including perennials
Winter wheat
Permanent
605
22
560.3
2.8
Fruit-changing (simple) crop rotation
182
15
186.1
3.9
Cereal-row crop rotation
153
10
143.7
3.7
Barley
Permanent
459
87
487.3
2.5
Fruit-changing (simple) crop rotation
108
30
297.4
2.9
Cereal-row crop rotation
115
23
206.9
3.2
Specialized cereal crop rotation
279
45
306.5
2.6
Картофель
Permanent
66
8
897.4
11.5
Fruit-changing (simple) crop rotation
159
16
537.5
18.6
Cereal-row crop rotation
135
12
497.6
17.9
Specialized cereal crop rotation
187
15
520.4
19.3

Table. Contamination of winter wheat with chamomile (Tripleurospermum inodorum) depending on the predecessor[2]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. - M.: Publishing house "Kolos", 2000. - 551 p.

Predecessor
Weeds, pcs/m2
Percentage of chamomile from the total weed population
total
including chamomile
Winter wheat
681
650
95
Vicia-oat mixture
202
127
63
Clover
60
25
42
Barley
52
40
77
Bare fallow
15
5
33

Bare and seeded fallows reduce the number of viable seeds of weeds by 2-3 times. Segment of crop rotation with row crops in terms of impact on weeds is comparable in effectiveness with pure fallow.

Intermediate crops have a versatile impact on agrophytocenosis. Their use allows to reduce the weediness in the following segments by 30-50%, and the damage by root rot – by 1.5-2 times. The positive effect of intermediate crops is caused by the dense stand, which suppresses weed vegetation, and the plant residues plowed into the soil increase the activity of soil biota suppressing root rot causative agents and weed seeds.

Changing ecological conditions of crops growth in the segments of the crop rotation cause suppressive effect on weeds. For example, the alternation of crops of continuous crops with different terms of sowing and harvesting with row crops, in which more careful care is taken; annual crops with perennial grasses. Bushy and fast-growing winter rye and winter wheat are stronger suppressors of weeds than weakly bushy spring wheat and millet. The latter have a slow growth in the first 2-3 weeks after sprouting, which causes low competitiveness. A great role in weed control is played by clean fallows and early harvested crops (oat-bean mixtures, winter green fodder), which allow destroying weeds before they are inseminated.

Crop rotation is also important in controlling dodder (Cuscuta). Alternation of susceptible crops (alfalfa, potatoes, clover, beets, tobacco, grain legumes) with resistant ones (barley, wheat, rice, oats, millet, and perennial cereals) reduces the infestation of fields by this weed.

In control of Orobanche, its selectivity with cultivated plants should be taken into account; they should be returned to the same place not earlier than in 7-8 years. Provocative sowing of host plants followed by harvesting before seed formation in Orobanche is effective.

Norm and method of seeding

The density of stems of cultivated plants has a direct suppressive effect on weeds. Seeding rate is determined by the specific climatic zone. Its reduction leads to a sharp increase in the weediness of crops.

Cross-row sowing creates a more uniform distribution of seeds and allows cultivated plants to use growth factors more effectively and makes it easier to care for crops, having a suppressive effect on weeds. 

Narrow-row and wide-row sowing with intercropping cultivation when taking care of vegetative plants also helps to reduce weed infestation. For example, narrow-row sowing of cereals reduces weed infestation by 20% compared with the row method.

Fertilizer

Main article: Agrochemistry: Fertilizers

Application of fertilizers, as a factor of plant nutrition, creates favorable growth conditions for weeds as well. Fertilizer application can cause an increase in weediness 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%, in vegetable crops – up to 80-90%.

On the contrary, high agricultural technology of 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, the accumulation of their breeding organs in the soil, and also requires the use of improved control technologies.

Responsiveness of weeds to types of fertilizers is different. This property is caused by differences in the use of nutrients by weeds.

Positive response to nitrogen fertilizers shows: lamb’s-quarters (Chenopodium album), multiseed (Chenopodium polyspermum), sprawling quinoa (Atriplex patula), wild radish (Raphanus raphanistrum), wild mustard (Sinapis arvensis), rugged mountain knotweed (Persicaria lapathifolia), spotted knotweed (Galeopsis speciosa), double-split knotweed (Galeopsis bifida), barnyard grass (Echinochloa crus-galli), annual bluegrass (Poa annua), black nightshade (Solanum nigrum), small sorrel (Rumex acetosella), Canada thistle (Cirsium arvense).

For phosphorus fertilizers – groundsel (Senecio vulgaris), dog nettle (Urtica urens), field violet (Viola arvensis), red broom (Spergularia rubra), corn spurrey (Spergula arvensis), apothecary woodruff (Fumaria officinalis), henbit (Lamium amplexicaule), rugged mountain knot (Persicaria lapathifolia), perennial sow thistle (Sonchus arvensis).

To potassium fertilizers responded positively to the lamb’s-quarters (Chenopodium album), cleavers (Galium aparine), stinkweed (Thlaspi arvense), perennial sow thistle (Sonchus arvensis), chamomile non-scented (Tripleurospermum inodorum), etc.

Thus, unbalanced application of fertilizers can contribute to the accumulation of weeds of the same species.

Despite the conventionality of the given grouping, it allows you to build a fertilizer system so as not to give weeds an advantage in spreading and subsequently build a comprehensive system of control measures.

Weeds have an effect on fertilizer efficiency. Increased fertilizer rates can have both positive and negative effects on the competitive ability of a crop.

Table. Influence of weed plants on the efficiency of mineral fertilizers[3]Bazdyrev G.I., Smirnov B.A. Weed plants and the struggle against them. - Moscow: Moscow worker, 1986.

Crop
Decrease in yield increase from 1 ton of fertilizers depending on weediness, %
weak
medium
high
Cereals
3.4
6.4
12.0
Flax fiber
0.96
1.8
3.4
Root vegetables
14.6
29.2
58.4
Potatoes
13.3
26.6
53.2
Vegetables
21.3
42.6
85.2
Fruits and berries
4.2
8.4
16.8

Studies conducted in the Leningrad region of Russia showed a 10% reduction in barley yield from the planned yield of 3.5-4.5 t/ha with weeds of 100 weeds per 1 m2, with weeds of 300 weeds per 1 m2 the reduction was 28%. However, on the background of fertilizers, with a planned yield of 5.2-6.4 t/ha, these levels of weeds had no effect on the yield. Only when the number of weeds 700 pcs. per 1 m2, barley yield losses were 12.6%, and at 1000 plants – by 31.6%.[4]Actual issues of weed control. – Moscow: Kolos, 1980.

Similar studies were conducted in the Tver region: with the planned yields of oats and barley 2.0 and 4.0 t/ha on the background of fertilizers. At the increased background of fertilizers due to biological suppression the number of weeds in the barley crops decreased by 30-35%, in oats – by 23-27% in comparison with the low background of fertilizers.[5]Zakharenko V.A. Herbicides. – Moscow: Agropromizdat, 1990.

Long experiments of the Moscow Agricultural Academy have shown the influence of soil fertility on the weediness of crops: in crop rotations improved plant nutrition restrained the growth of weediness, whereas, permanent cultivation of crops led to its growth.[6]Bezuglov V.G. Application of herbicides in intensive farming. – Moscow: Rosagropromizdat, 1988.

Soil liming

Liming of soils reduces the number of weeds by 2 times, but also increases the species composition by 1.5-2 times, which creates the potential danger of increasing weed infestation in subsequent periods.

The reaction of weeds to lime treatment is different. Soils with a strong to weakly acidic reaction have a positive effect on the growth of common sorrel (Rumex acetosella), corn spurrey (Spergula arvensis), red broom (Spergularia rubra), common broom (Apera spica-venti), chamomile odorless (Tripleurospermum inodorum), small muskrat (Myosurus minimus), wild radish (Raphanus raphanistrum), fasting kachima (Psammophiliella muralis), etc.

In soils with a soil reaction close to neutral (slightly acidic to slightly alkaline) wild oat (Avena fatua), sprawling swan (Atriplex patula), wormseed mustard (Erysimum cheiranthoides), thistle (Euphorbia virgata), black henbane (Hyoscyamus niger), toadflax (Linaria vulgaris), gooseberry lupus (Potentilla anserina), perennial sow thistle (Sonchus arvensis), marsh hedgenettle (Stachys palustris), stinkweed (Thlaspi arvense).

Weeds that respond little to the reaction of the soil environment: lamb’s-quarters (Chenopodium album), shepherd’s purse (Capsella bursa-pastoris), purple cockle (Agrostemma githago), horseweed (Erigeron canadensis), Galeopsis species, field scabious (Knautia arvensis), Achillea.

Soil liming changes not only soil acidity, but also changes the conditions of mineral nutrition of plants, as well as air, water and heat regimes. Consequently, liming of soils affects the species composition and number of weeds in a set of changing factors of plant conditions.

Sources

Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 с.

Fundamentals of Agricultural Production Technology. Farming and crop production. Ed. by V.S. Niklyaev. – Moscow: “Bylina”, 2000. – 555 с.

Bazdyrev G.I. Weed plants and measures to control them in modern agriculture: Textbook for universities. M.: Publishing house of the Moscow Agricultural Academy, 1993. – с. 242.