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Harm caused by weeds

Weeds cause significant damage to agriculture, primarily by worsening the living conditions of cultivated plants. The damage caused by weeds is divided into:

  • direct – worsening the living conditions of plants and thereby reducing yields and product quality;
  • indirect – resulting in additional resource costs to counteract weed harm.
Weed competition for growth factors
Weed competition for growth factors using wheat and brome as examples

The negative impact of weeds on cultivated plants is the direct deterioration of plant conditions.

Weeds damage agriculture by:

  • creating a deficit of moisture and nutrients for cultivated plants;
  • shading of crops;
  • mechanical effects on cultivated plants;
  • suppressing the growth of cultivated plants through toxic effects (allelopathy, can also have a positive effect);
  • the spread of diseases and pests;
  • complication of production activities;
  • damage to livestock production;
  • deterioration of product quality;
  • reduction of crop yields.

Separately, in the absence of a proper system of weed control, conditions are created for their accumulation, which in subsequent periods exacerbates the harm.

Moisture deficiency

Weeds as well as cultivated plants consume soil moisture, which creates its deficit and leads to weakened growth and development of cultivated plants.

The root system of some weeds goes deep into the soil for several meters: Chenopodium album (Chenopodium album), Shirenitsa (Amaranthus), bristlewort (Setaria), oats (Avena fatua) – up to 2 m, common broom (Apera spica-venti), common hedgerow (Echinochloa crus-galli), chamomile non-scented (Tripleurospermum inodorum), marsh peaweed (Stachys palustris), field loach (Convolvulus arvensis) – up to 5 m, melilotus officinalis – up to 5.5 m, field thistle (Cirsium arvense) in the third year of life – up to 7-9 m, field horsetail (Equisetum arvense), creeping mustard (Rhaponticum repens) – up to 10 m. This property allows weeds to consume a huge amount of water.

Some weeds, such as wild radish (Raphanus raphanistrum), Galeopsis speciosa, Avena fatua, Ambrosia artemisiifolia, Centaurea cyanus, Tripleurospermum inodorum consume 1.5-2 times more water than cultivated plants during certain periods of vegetation. As a result, the moisture content in the root-containing layer of soil decreases by 2-5%. Even in zones with sufficient moisture, such as the Non-Chernozem zone, the value of the created moisture deficit becomes dangerous. This problem is especially acute during droughts, which occur in this zone once every 2-3 years.

Transpiration coefficients of many weeds are higher than those of cultivated plants. For example, corn consumes 250-400 liters of water from soil to create 1 kg of dry matter, millet – 200-300 liters, flax – 400-430 liters, wheat – 460-510 liters, oats – 600 liters. Whereas white vermilion (Chenopodium album), bogweed (Amaranthus) and thistle (Cirsium) – 800-1200 liters, couch grass (Elytrigia repens) – 1100-1200 liters, field mustard (Sinapis arvensis) 870-900 liters. 

Nutrient deficiencies

Weeds compete with cultivated plants not only for moisture, but also for nutrients. Nutrient uptake by weeds exceeds that of crops with high yields (table). For example, studies by the VIUA have shown that the nitrogen use factor of wild radish (Raphanus raphanistrum), lamb’s-quarters (Chenopodium album), and chamomile non-scented (Tripleurospermum inodorum) is 5-12% higher than that of wheat.[1]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 p..

Table. Nitrogen, phosphorus and potassium removal by cultivated and weed plants, kg/ha[2]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. - M.: Publishing house "Kolos", 2000. - 551 p. [3]Bazdyrev G.I. Weed plants and measures to combat them in modern agriculture: Textbook for universities. Moscow: MSKHA Publishing House, 1993. - с. 242.

Name of the plant
Yield, t/ha
Winter rye
Winter wheat
Spring wheat
Corn for silage
Centaurea cyanus

Therefore, it should be understood that the plant mass of weed vegetation is the mass of lost crop.

The degree of consumption and use depends on soil and climatic conditions, the species composition of weeds, their biological characteristics and quantity. Nutrients absorbed by weeds are partially alienated from the fields along with seeds during harvesting and stems, leaves during harvesting of straw or green mass. A large proportion is accumulated in the roots and rhizomes, seeds of perennial weeds and does not return to the soil for a long time.

The VIUA experiments with labeled nitrogen showed that some species of weed plants absorb nitrogen more intensively than cultivated plants. For example, while flax, spring wheat, and millet have nitrogen utilization factor of 35-56%, the nitrogen utilization factor of chamomile (Tripleurospermum inodorum), field broom (Apera spica-venti), lamb’s-quarters (Chenopodium album), wild mustard (Sinapis arvensis), cleavers (Galium aparine), and ubiquitous mountain knot (Persicaria lapathifolia) varies from 56 to 70%.

According to the results of chemical analyses of cultivated and weed plants that grew together, the amount of nutrients in weeds increases with their numbers, while in cultivated plants it decreases. Thus, the removal of nutrients in the joint growth of corn and weeds without weeding was 157.3 and 106.8 kg/ha, with two weeding – 273.6 and 33.6 kg/ha, with four weeding – 318.6 and 0.23 kg/ha respectively.[4]Weed control in the cultivation of crops. Edited by G.S. Gruzdev. – M.: Agropromizdat, 1988.

Similar conclusions follow from the experiments of the Moscow Agricultural Academy, delivered on crops of winter wheat in the Moscow region. In the tillering phase nutrient removal by wheat was 70.8 kg/ha, by weeds 7.2; in the flowering phase – 183.6 and 115.4 kg/ha; in the ripening phase – 137.3 and 154.7 kg/ha, respectively. Application of control measures reduced weed nutrient removal to 33.2 kg/ha, while winter wheat nutrient removal increased to 199.3 kg/ha and yield was 4.52 t/ha, while the control yield was 1.97 t/ha.

Table. Relative removal of nutrients by cultivated plants and weeds, %[5]Bazdyrev G.I. Weeds and measures to control them in modern agriculture: Textbook for universities. M.: Publishing house of Moscow Agricultural Academy, 1993. - с. 242.

Name of the plant
Canada thistle (Cirsium arvense)
Wild buckwheat (Fallopia convolvulus)
(Persicaria lapathifolia)
Lamb’s-quarters (Chenopodium album)
Common chickweed (Stellaria media)
Perennial sow thistle (Sonchus arvensis)
(Tripleurospermum inodorum)
Field horsetail (Equisetum arvense)


Due to outstripping growth, many weeds outstrip cultivated plants in the initial stages of growth, which leads to strong shading of cultivated plants. For example, the illumination of barley and potato crops decreases by 17.7 and 23.0%, respectively, compared with the crops without weeds. It especially affects the crops of flax, millet, corn, sugar beet, which develop slowly in the initial periods of growth.

Strong shading of crops by weeds weakens the strength of ground stems of cultivated plants, resulting in lodging of cereals, millet, corn, etc.

Shading affects soil temperature, reducing it by 1-4 °С. This reduces the biochemical processes in plants and microbiological activity of soil, which generally worsens the living conditions of cultivated plants.

Mechanical impact

The mechanical impact of weeds is manifested by physical pressure on the vegetative organs of cultivated plants. For example, climbing and thin-stemmed weeds such as wild buckwheat (Fallopia convolvulus), cleavers (Galium aparine), field bindweed (Convolvulus arvensis), interweave the plant and due to their own mass lead to lodging of crops. Chenopodium album, Centaurea cyanus, Artemisia absinthium, Tussilago and others build up a strong aboveground part, which suppresses growth and leads to crop thinning.

Parasitic and semi-parasitic weeds, primarily Cuscuta and Orobanche, attach to plant stems or roots with their haustoria (suction cups) and extract water, minerals, and plastic substances from them. Their action is shown on more than 100 plant species.

Disease and pest accumulators

Weed plants play a significant role in reserving viruses, pathogenic fungi, bacteria and insects, contributing to their persistence, development and then mass distribution.

For example:

  • Weeds in the cruciferous family – wild mustard (Sinapis arvensis), wild radish (Raphanus raphanistrum), shepherd’s purse (Capsella), yellow rocket (Barbarea vulgaris) – are reservoirs of fungal diseases (clubroot of cabbage, white mold, powdery mildew) of cruciferous crops.
  • Foxtail (Alopecurus), creeping wheatgrass (Elytrigia repens), wild oat (Avena fatua) contribute to the growth of brown rust spores in grain.
  • Blue broom (Setaria verticillata), lamb’s-quarters (Chenopodium album), black nightshade (Solanum nigrum), bachelor’s button (Centaurea cyanus), canada thistle (Cirsium arvense) are reservoirs of cereal mosaic, wheat root rot, viral diseases of potatoes.
  • The sources of infestation of potato stem nematodes are often chamomile (Tripleurospermum inodorum), black nightshade (Solanum nigrum), redroot pigweed (Amaranthus retroflexus), fritillary (Atriplex patula), common dandelion (Taraxacum officinale), etc.
  • Couch grass (Elytrigia repens), meadow-grass (Poa pratensis), awnless bromegrass (Bromus inermis) serve as pest Eurygaster reservoirs; winter bollworm – Chenopodium album, Convolvulus arvensis, Solanum nigrum; beet weevil – Cirsium arvense, Carduus crispus, Fallopia convolvulus, etc.
  • Couch grass (Elytrigia repens) contributes to the maintenance of the wireworm population. 
  • Sorghum halepense contributes to the dwarf virus that causes corn mosaic and leaf chlorosis. 

Increasing complexity of production activities

Weeds complicate the production activities of agricultural enterprises, causing indirect damage.

The main manifestations are:

  • Complication of harvesting. 
  • Difficult work of harvesting equipment, clogging of separating mechanisms, breakdown of machines.
  • Contamination of agricultural products with seeds, fruits, parts of stems and leaves of weeds, which get into the threshed grain, increases its clogging up to 20-30% and additional moisture, which leads to high costs for cleaning and drying of grain, increasing the cost of production.
  • Additional tillage, deterioration of agronomic practices, increased fuel consumption.

As a consequence, the complication of production activities leads to an increase in labor and material resources, increasing the cost of agricultural production. Depending on the level of weed infestation of crops, the cost of additional tillage in the Non-Black Soil Zone can increase by 30-50%.

Harm to livestock production

Harm to livestock caused by weeds is manifested primarily in the reduction of the quality of livestock products and feed poisoning of animals. Feed poisoning of animals can cause poisoning of people who have eaten products from diseased animals.

For example, grazing on pastures littered with large celandine (Chelidonium majus), poison hemlock (Conium maculatum), Hypericum perforatum, Stellaria graminea, Ranunculus acris, Equisetum arvense, Cuscuta causes illness or even animal death. When feeding cows with feed containing absinthe (Artemisia absinthium), common tansy (Tanacetum vulgare), stinkweed (Thlaspi arvense), round onion (Allium rotundum), cloverberry (Lepidium), mustard (Sinapis), melilot (Melilotus), milk and meat with a specific, unpleasant taste are obtained.

Among the weeds there are a large number of poisonous plants, many of which are found in pastures: Pickleweed (Galeopsis), intoxicating spittlebush (Lolium temulentum), cereal stellaria (Stellaria graminea), black squirrel (Hyoscyamus niger), common datura (Datura stramonium), field horsetail (Equisetum arvense), dodder (Cuscuta), milkweed (Euphorbia), poisonous echos (Cicuta virosa), Aconitum, Delphinium elatum, Veratrum lobelianum, Chelidonium majus, Conium maculatum, Ranunculus, Caltha palustris, Rhaponticum repens and others. Toxic effects of poisonous plants are caused by the accumulation of toxins in various parts of the plant in the form of poisonous decay products (catabolites), alkaloids, glycosides, resins, saponins, tannins, organic acids. These chemicals, when ingested by animals, can cause metabolic disorders, feed poisoning or death. 

Some weed plants, such as poisonous echinacea (Cicuta virosa), black squirrel (Hyosyamus niger), intoxicating tares (Lolium temulentum), etc., retain their toxicity after drying and ensilaging.

Some weeds may be toxic to some species and harmless to others. For example, larkspur (Delphinium) is toxic to cows in large quantities and relatively harmless to sheep and horses. In contrast, amsinckia (Amsinckia spp.) is toxic to horses, while other livestock are tolerant of it.

In addition to direct poisoning, animals may experience other problems associated with eating certain varieties of weeds. For example, Hypericum, Eriogonum longifolium, and Alchemilla arvensis contain toxins that cause photosensitization in animals, i.e., increased sensitivity to sunlight. Other plants contain teratogenic substances that lead to embryonic malformations. For example, ugly lambs can appear if a sheep eats California hellebore (Veratrum californicum) during the two-week gestation period. The common eagle weed (Pteridium aquilinum) causes a disease of cows called “red water” because of the blood-colored urine that is its sign. This weed causes bladder cancer if consumed frequently in sufficient quantities.

Weeds with seeds covered with prickles, thorns harm livestock, clogging the wool of sheep (seeds of lippula (Lappula), black root (Cynoglossum officinale), succession (Bidens tripartita), thistle (Carduus)), damaging the skin, stomach and intestines of animals, causing inflammation. When eaten by animals, mature ears of bristlegrass, down of thistles and thistles, and cotton-grass sometimes roll up in the gastrointestinal tract into spherical lumps (phytobezoars), preventing the movement of food through the intestine, up to the death of animals.

Harm to agricultural products

Weeds cause significant damage to the quality of agricultural products. Grain from weedy fields has increased moisture, which complicates its cleaning and storage. The ingress of seeds, fruits, parts of weeds in the grain increases its weediness up to 20-30%. When grinding such grain into flour its quality sharply deteriorates and in some cases makes it unfit for consumption because of the content of impurities harmful to the human body and animals. Among the most harmful weeds in this regard are common bromegrass (Agrostemma githago), louse spike (Lepidium ruderale), black calamine (Hyosyamus niger), bitterroot pink (Rhaponticum repens), chaff intoxicating (Lolium temulentum) and others.

Seeds of weeds of rye bromegrass (Bromus secalinus) and Tartar’s mustard, getting into rye flour, cause rapid hardening of bread. In general, the quality of flour from contaminated grain has poor baking qualities: grain nature (bulk density) decreases by 8-11 grams, vitreousness – by 0.5-3.3%, protein content – by 0.6-2.0% compared to flour made from grain of herbicide-treated fields. Grain harvested from contaminated fields has lower protein content, oil crops have lower oil content in seeds, and sugar beet root crops have lower sugar content. 

Potatoes, beets, carrots in contaminated fields produce small tubers and roots; their dry matter content is 0.1-0.6% lower, ascorbic acid content – 0.3-0.5 mg per 100 g of raw materials.

The yield of alfalfa hay is the highest in the first year after sowing, but it is also the most weed-infested, which can reduce the protein content by half.

In some cropping systems, weed seeds and cultivated plant seeds are similar in weight and shape, making it seriously difficult to separate them, such as alfalfa and dodder seeds, soybean seeds and nightshade fruits.

Lower yields

The main harm caused by weeds is mainly in the reduction of crop yields.

Weeds cause significant damage to agricultural production. In 1996 it was estimated that weeds reduce yields by 20-40%.[6]Erich-Christian Oerke, Ulrike Steiner: Ertragsverluste und Pflanzenschutz. Die Anbausituation für die wirtschaftlich wichtigsten Kulturpflanzen. In: Schriftenreihe der Deutschen Phytomedizinischen … Continue reading Globally, by 10%. In Russia they are on average 26%.[7]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 p. It is believed that 10-15% of the total market value of agricultural products in the United States is lost due to weeds. This loss amounts to approximately $8 to $10 billion per year.

Crop losses globally from weeds and pests are: cereals – 500 to 510 million tons, potatoes – 125 to 135 million tons, sugar beets – 65 to 75 million tons, vegetables – 78 to 79 million tons. Total damage is estimated at $75 billion (1980).[8]Actual Questions of Weed Control. – Moscow: Kolos, 1980.

Table. Potential losses of agricultural production in the world in 2001-2003[9]Herbes sauvages des champs, bonnes ou mauvaises? INRA Dijon/Jardin des sciences. (Translation by UniversityAgro.ru)

World production, million tons, 2001-2003
Potential losses, %
from viral diseases
from diseases
from pests
from weeds
Corn on grain

On average over 1991-1995 in Russia yield losses from weeds of grain crops amounted to 16.6%, long-fiber flax – 21.8%, sugar beet – 22.4%, oilseeds – 16.1%, potatoes – 15%, fodder crops – 15.8%. In recent years, the decrease in yields is not only not reduced, but there is a positive trend.[10]Fundamentals of agricultural production technology. Farming and crop production. Ed. by V.S. Niklyaev. – Moscow: “Bylina”, 2000. – 555 p.

According to the Central Institute of Agrochemical Service of Agriculture, in Russia the average and severe weed infestation of grain crops is 61% of the area, sugar beet – 66, potato – 51, forage grass – 49%. With systematic and effective organization of crop protection from weed vegetation, and bringing the number of weeds to an acceptable level (the critical threshold of harmfulness) increase of grain crops yield could be 9-18%, technical – 10-17%, potato and vegetable – 10-15%, forage – 10-15%.[11]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 p.

Many scientific studies prove a significant impact of weeds on reducing crop yields. Which leads primarily to a decrease in economic efficiency of agricultural activity. For example, when there are 11 sprouts of Creeping Thistle (Rhaponticum repens) per 1m2 of winter wheat, its yield is reduced by 28-30%, when 26 – by 48-50% and when 60-70 – by 70-75%. Under severe weed infestation of corn with field thistle (Cirsium arvense) yield of culture decreases by 50-72%, under moderate – by 35-40% and weak – by 20-30%. Cuscuta weed infestation reduces alfalfa hay yield by 20-30% and seed yield by 80-95%; gross output of cotton – by 8-12%, root crops of sugar beet – by 14-18%, vegetables – by 30-50%.[12]Fundamentals of agricultural production technology. Farming and crop production. Ed. by V.S. Niklyaev. – M.: “Bylina”, 2000. – 555 p.

In the early stages of growth, the presence of weeds does not yet mean a loss of yield, as both cultivated and weed plants can develop without interfering with each other. However, as they grow, their need for water and nutrients increases. The period until competition becomes significant is about three weeks. Studies have shown that once competition begins, the final onion yield decreases by almost 4% per day.[13]Bleasdale, J. K. A.; Salter, Peter John (January 1, 1991). The Complete Know and Grow Vegetables. Oxford University Press. ISBN 978-0-19-286114-6.

Harm caused by weeds unrelated to agriculture

Weeds affect many human activities that are difficult to quantify in monetary terms. The presence of weeds can reduce property prices due to the unkempt and unsightly appearance of property. Dense, moisture-retaining weed growth contributes to the deterioration of wood and metal structures and machinery. Dried weed vegetation in arid areas is a serious fire hazard and fire spreading hazard. It is not uncommon for weeds to damage recreational areas.

American scientists Ross and Lembi (1985) provide an interesting example of how weeds affect transportation costs. They point out that in 1969 and 1970, 487,000 tons of Avena fatua were inadvertently transported from Canada to the United States along with 16 million tons of grain. The transportation costs of the shipment of the chaff were estimated at $2 million, not including the cost of the subsequent cleaning of the grain at $2 million.[14]Radosevich, Steven R. Ecology of weeds and invasive plants: relationship to agriculture and natural resource management / Steven R. Radosevich, Jodie S. Holt, Claudio M. Ghersa.-3rd ed. p. cm. Rev. … Continue reading

Large economic costs are caused by the destruction of weeds along roads, railroads and power lines, and airport areas.

Toxic substances and allergens from weeds can cause serious health problems, discomfort or illness, including hay fever (pollinosis or seasonal allergic rhinoconjunctivitis), dermatitis and direct poisoning. Hay fever affects millions of people each year. It is caused by the adverse effects of the proteins associated with the pollen of certain plants on the respiratory system of susceptible individuals. A number of plants produce the toxic substance urushiol, which causes intense itching and rashes on skin contact, called “urushiol contact dermatitis.” A more obvious example, Sosnovsky’s hogweed. The transparent watery sap of the hogweed contains photosensitive substances from the furanocoumarin group. When exposed to ultraviolet light, they are converted to an active form which can cause serious skin damage requiring long periods of treatment and may not always go away.[15]The Poisoned Weed: Plants Toxic to Skin. Department of Environmental Toxicology University of California-Davis Donald G. Crosby Professor Emeritus – 2004After contact with a plant, especially on sunny days, you may get a severe burn on your skin. Many plants contain toxic substances that can cause poisoning or death if ingested. Toxic substances found in weeds include alkaloids, glycosides, oxalates, resins and rubberoids, essential oils, phytotoxins (toxalbumin), and minerals.


Fundamentals of agricultural production technology. Farming and crop production. Edited by V.S. Niklyaev. — Moscow: Bylina, 2000. — 555 с.

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

Bazdyrev G.I. Weed plants and measures to combat them in modern agriculture: a textbook for universities. M.: Publishing house of the Moscow Agricultural Academy, 1993. – с. 242. ISBN 5-7230-0196-5.

Fundamentals of agronomy: textbook / Yu.V. Evtefeev, G.M. Kazantsev. – M.: FORUM, 2013. – 368 p.: ill.

Radosevich, Steven R. Ecology of weeds and invasive plants: relationship to agriculture and natural resource management / Steven R. Radosevich, Jodie S. Holt, Claudio M. Ghersa.—3rd ed. p. cm. Rev. ed. of: Weed ecology / Steven Radosevich, Jodie Holt, Claudio Ghersa. 1997. Includes index. ISBN 978-0-471-76779-4 (cloth)