Weeds

Conceptual clarification of the term

The meaning of the term “weed” depends on subjective human perception. It is incorrect to call some plant species weeds, since the same species can be a weed, a crop, a grass, an indicator plant, etc. For example, sweet clover is used as a green manure, and couch grass for hay. A number of weed plants are used in veterinary medicine and medicine: deadly nightshade or belladonna (Atropa bella-donna), valerian (Valeriana officinalis), marsh cudweed (Gnaphalium uliginosum), broadleaf plantain (Plantago major), etc.

V. R. Williams considered any plant that did not correspond to the purposes of a given crop to be a weed from the farmer’s point of view. A. I. Maltsev argued that a weed-field plant should be considered a wild or semi-cultivated plant that grows in arable fields not by the farmer’s will, having adapted to arable conditions and growing together with cultivated plants.

According to GOST 16265-89 “Agriculture. Terms and Definitions,” weeds are wild plants occurring on agricultural land and reducing the quantity and quality of produce.

A plant becomes a weed only when it is perceived as “troublesome, undesirable.” It does not matter whether the weed is a herbaceous or woody species.

In forestry, the term “associated growth” is often used, since both negative and positive associated effects can be expected.

In ecological agriculture, the term “weed” means not only a harmful plant but also an integral part of the ecosystem.

Plants are called weeds if:

• they compete with cultivated plants for growth factors;
• harm may be caused to crop quality;
• there is mass spread creating a threat to weed-free lands;
• they disrupt human aesthetic perception, for example, in ornamental gardens, parks, lawns, or areas without vegetation;
• they create a toxic effect that renders land unsuitable;
• displacement of local plants (by neophytes) from their habitats occurs.

The term “weed” is also applied to invasive plants, that is, those spreading as a result of human activity outside their natural habitat. These include species bred by humans for various economic purposes, for example, Sosnowsky’s hogweed (Heracleum sosnowskyi), Mantegazzi’s hogweed (Heracleum mantegazzianum), or introduced from other places, for example, in Europe these include: Japanese quince (Chaenomeles japonica), staghorn sumac (Rhus typhina), Himalayan balsam (Impatiens glandulifera), black cherry (Prunus serotina).

Origin of weeds

Through systematic selection and cultivation over hundreds of years, certain plant species possessing broad ecological plasticity, high productivity, and a number of properties useful for food and economy were introduced into cultivation. Along with the appearance of the first cultivated plants, weeds also appeared. Proof of the origin of weeds simultaneously with the emergence of agriculture is archaeologists’ finds of seeds in plant remains of such weeds as white goosefoot (Chenopodium album), rye brome (Bromus secalinus), wild oat (Avena fatua), common corncockle (Agrostemma githago), common barnyardgrass (Echinochloa crus-galli), cleavers (Galium aparine), etc. These finds date to the Stone and Bronze Ages.

It is assumed that weeds, in the sense of the term — fast-developing plants exploiting environments disturbed by humans — arose in response to the Neolithic Agricultural Revolution about 12,000 years ago. However, evidence found at Ohalo II — an archaeological excavation site in Israel — indicates that “proto-weeds” existed long before that — at least 23,000 years ago.

Seeds of some weed species were found in the European tundra during the Ice Age; other species originated from natural species in frequently disturbed ecosystems (river banks, rocky slopes, dunes, marshes).footnote:[Jauzein, P., 2001. Biodiversité des champs cultivés: L’enrichissement floristique. Dossier de L’environnement de l’INRA 21, 43–64.]footnote:[Baker, H.G., 1974. The evolution of weeds. Annual Review of Ecology and Systematics 5, 1–24. doi:10.1146/annurev.es.05.110174.000245.]

Agriculture probably arose in the Fertile Crescent1 and then appeared in Central Europe around 5000 BC. Weeds spread along with the spread of agriculture. Already adapted to agricultural practice, they quickly adapted to the agroclimatic conditions of the new environment. During archaeological research it was established that the species composition of weeds from 4000 BC (Neolithic) to 1250 BC (Bronze Age) remained practically unchanged and was represented by the following species in decreasing order of frequency: garden orache (Atriplex hortensis), buckwheat (Fagopyrum esculentum), common nipplewort (Lapsana communis), barren brome (Bromus sterilis), cleavers (Galium aparine), common sorrel (Rumex acetosa), rough bluegrass (Poa trivialis), redshank (Persicaria maculosa), and various vetches (Vicia spp.).

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.The Fertile Crescent.

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The weed problem is as old as agriculture itself. In the Bible they are mentioned as God’s punishment for the Fall.

In Europe, agriculture spread both along the Mediterranean coast and up the Danube valley. These two routes are marked by differences in weed genotypes. For example, sterile oat (Avena sterilis) is found in Northern Europe, and wild oat (Avena fatua) in Southeastern Europe.

Species that arrived in Europe before 1500 BC are called archaeophytes. Weeds introduced after 1500 BC are neophytes.

In Roman times, many weed species characteristic of cereal crops were introduced with seeds from Mediterranean countries.

The discovery of America and the development of trade with other continents caused an influx of new weed species into Europe, mainly from North America. The rate of appearance was about 3 species per year until the 19th century, and about 30 species per year in the 20th century. For climatic reasons, they spread mainly in Southeastern Europe. In northern Europe their frequency decreases. Among species introduced from America are: redroot pigweed (Amaranthus retroflexus), common ragweed (Ambrosia artemisiifolia), jimsonweed (Datura stramonium), etc.

Scented chamomile was introduced to Europe from the Far East; narrow-leaved ragwort (Senecio inaequidens) and Bermuda buttercup (Oxalis pes-caprae) from South Africa.

In the Middle Ages, most fields were cultivated according to the three-field system (winter cereals — spring crops — fallow). During the fallow period, livestock were driven onto the fields. Livestock were even driven over young cereal shoots, since there were significantly more weeds, and the harm to the grain was insignificant. At the same time, this improved tillering. This period is characterized by the spread of pasture and meadow weeds. Targeted weed control was carried out by weeding.

In the 18th century, an improved three-field system appeared (winter cereals — spring crops — root crops), which contributed to the spread of weeds typical of root crop and row crop sowings.

Changes in agricultural practice lead to a decline in weed diversity and abundance, which begins at the end of the 18th century and then intensifies after 1950. This loss of biodiversity was caused by improvements in agricultural practices: seed sorting, mechanical weeding of crops, more complex crop rotations, and then, after 1945, the use of herbicides. From the 1950s, mechanized weeding, new crop rotation systems, and better environmental control (liming, application of fertilizers, drainage) were added, leading to soil homogenization. For example, such weeds as field cudweed (Filago arvensis) and field nigella (Nigella arvensis) have not been observed in France since 1920. In 2001, a decline in the population of 300 species was established, and 100 were threatened with extinction. The average number of weed species in Europe decreased by 20% between 1945 and 2000 and by 42% between 1970 and 2000 in France.

Since 1980, an increase in the number of weed species has been observed, probably due to the development of organic farming and reduced use of herbicides. These species are mainly species preferring nutrient-rich soils, neophytes, or monocotyledons. At the same time, the abundance of rare and endangered weed species does not increase.

Herbology

Herbology — the field of science dealing with weeds, weed biology and ecology, control methods, and related environmental impacts. Herbology has developed in recent decades and covers the following issues:

• competitiveness of weeds in relation to arable crops;
• harmfulness of weeds;
• positive effects of weeds;
• determination of thresholds of harmfulness of weed vegetation;
• assessment of biological efficacy of herbicides and their mixtures depending on weather conditions, soil condition, and degree of infestation;
• chemical, physical, mechanical, biological, and agronomic weed control methods;
• testing varietal sensitivity of agricultural plants to herbicides;
• immunization of weeds;
• environmental consequences of herbicide application;
• subsequent treatment with herbicides;
• herbicide residues in the environment and plants;
• development of an integrated weed control method;
• compilation of phytosociological maps;
• zoning of weeds.

Contaminant crops

Contaminant crops — cultivated plants occurring in sowings of another cultivated crop or another variety and being undesirable. For example, winter rye plants occurring in sowings of winter wheat reduce the quality of the resulting grain. The presence in sowings of spring wheat of plants of this crop of another variety makes it impossible to use the resulting grain for seed production purposes. Sowings of spring crops are often heavily infested in spring with volunteer sunflower shoots formed from achenes that overwintered in the soil.

List of sources

1. Bazdyrev G. I., Loshakov V. G., Puponin A. I., et al. Agriculture. Textbook for universities. Moscow: Kolos Publishing House. 2000.

2. Bazdyrev G. I. Weeds and measures of control in modern agriculture: Textbook for universities. Moscow: MSKhA Publishing House. 1993.

3. Evtefeev Yu. V., Kazantsev G. M. Fundamentals of agronomy: textbook. Moscow: FORUM. 2013.

4. Niklyaev V. S. Fundamentals of agricultural production technology. Agriculture and crop production. Moscow: Bylina. 2000.