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Weed science is a scientific discipline concerned with plants that may be considered weeds, their effects on human activities, and their management [1] "a branch of applied ecology that attempts to modify the environment against natural evolutionary trends.". [2]
Weeds have existed since humans began settled agriculture have existed since the advent of settled agriculture around 10,000 years ago it has been suggested that the most common characteristic of the ancestors of our presently dominant crop plants is their willingness—their tendency to be successful, to thrive, in disturbed habitats, mostly those around human dwellings. Farmers have likely always been aware of weeds in their crops, although the evidence for their awareness and concern is nearly all anecdotal.
Unlike other agricultural sciences like entomology or plant pathology, the emergence of weed science is comparatively recent, occurring largely within the 20th century and coinciding with the development of herbicides.
Weeds are controlled in much of the world by hand (roguing) or with crude hoes. The size of a farmer's holding and yield per unit area are limited by several things and paramount among them is the rapidity with which a family can weed its crops. More human labor may be expended to weed crops than on any other single human enterprise, and most of that labor is expended by women. Weed control in the Western world and other developed areas of the world is done by sophisticated machines and by substituting chemical energy (herbicides) for mechanical and human energy. There is a relationship between the way farmers control weeds and the ability of a nation to feed its people. Successful weed management is one of the essential ingredients to maintain and increase food production.
In 1923, Clark and Fletcher suggested that the "annual losses due to the occurrence of pernicious weeds on farm land in Canada, although acknowledged in a general way, are far greater than is realized." [3] They thought this was because "farmers gave little critical attention to the weeds growing among their crops." They did not deny that farmers were aware of the weeds only that they could do little about them. Many of the same weeds described by Clark and Fletcher are shown in most current weed identification books. In spite of continued research to mitigate weeds annually many of the same species continue to be problematic. The first U.S. Congressional appropriation for weed control was made in 1901 for work on control of johnsongrass, 23 years after Congress had appropriated funds for work on cotton insect pests. Petroleum-based herbicides were first used on California crop land in 1924 and soon became widely accepted in Southwestern states. In 1942, oils were used extensively for weed control in carrots and subsequently were used in forest nurseries. French scientists sprayed apple trees with dinitro dyes to control mosses, algae, and lichens. Some noticed that grasses that were wet from the spray did not die and that observation, or, more likely, a series of observations, led to the use of dinitros as herbicides for selective control of broadleaved weeds in cereals and flax. Sinox (sodium dinitro cresylate) was developed by Pastac in 1933. [4] It was the first selective organic herbicide introduced in the US. From the early 1930s until about 1945, it was used extensively in grains, clover hay, grass seed crops, peas, cane berries, onions, and lawns.
Timmons, writing in 1970 reported that "available literature indicates that relatively few agricultural leaders and farmers became interested in weeds as a problem before 1200 A.D. or even before 1500 A.D." [5] The “critical attention” Clark and Fletcher thought was absent increased slowly, primarily because the general attitude seemed to be that “weeds were a curse which must be endured, and about which little could be done except by methods which were incidental to crop production, and by laborious supplemental hand methods."
Jethro Tull, in 1731, appealed for greater attention to weeds:
It is needless to go about to compute the value of the damage weeds do, since all experienced husbandmen know it to be very great, and would unanimously agree to extirpate their whole race as entirely as in England they have done the wolves, though much more innocent and less rapacious than weeds.
Farmers however were bound by their inability to do much about weeds except by laborious hand methods.
Insects cause both human health and crop problems. Weeds, with a few exceptions, do not cause direct harm to humans. Those that do such as poison ivy and poison oak can be avoided. Poisonous weeds have never been widespread as a weed of crops nor of great concern to the majority of people. Many weeds aggravated human allergies but many did not and other common plants are also allergenic. Insects and insecticides were respectively causes of and solutions to human disease problems. Weeds and herbicides were not and less attention was paid to them. Weeds and herbicides were agricultural problems. They were not of general societal concern. There were a few scientists interested in the study of weeds and in developing techniques to reduce the crop losses caused by weeds. There were only three full-time weed experts in 1934 and only a few part-time ones.
By late 1951, 46 state agricultural experiment stations had active weed research programs and most of them were working on weed control with herbicides. Now all colleges of agriculture in land-grant universities have weed scientists and a well-developed weed management program.
Weed science has been strongly influenced by herbicides and mechanical technology developed by supporting industries, by research by weed scientists, and, ultimately, used by farmers. Herbicides greatly expanded the opportunities and range of methods for vegetation management and weed control. The definition accepted by the Weed Science Society of America (WSSA) is “A chemical substance or cultured biological organism used to kill or suppress the growth of plants.”
Weed scientists have tended to focus on results and progress. Modern agriculture in the world's developed nations has addressed but not eliminated most weed problems through extensive use of herbicides and the more recent development of herbicide resistant crops through genetic modification. These methods while undeniably successful for their intended purpose also have created environmental, non-target species, and human health problems. Farmers in the world's developing nations use some herbicides but newer herbicides and the necessary application technology are often unavailable or too expensive. Weeds are always present in these farmer's fields but often the most available, affordable control methods are mechanical weeding, usually with animal power, or by hand, and most of the labor is provided by women. Neither the hypothesis that more energy is expended for the weeding man's crops than for any other single human task nor the corollary hypothesis that women do most of the world's weeding has been verified, but they are widely accepted.
Weed science no longer focuses exclusively on agriculture, with applications in industrial activities like maintaining railroad rights-of-way, controlling invasive species (including aquatic weeds) in natural areas and sports/park/home lawn care
While any plant can be a weed, approximately 250 plant species are sufficiently troublesome, cosmopolitan and economically injurious to warrant targeted research into their biology to assist in their management and control. [6] Examples of some of these troublesome weed species in North America are Palmer amaranth (Amaranthus palmeri S. Watson), common lambsquarter (Chenopodium album L.), horseweed (Erigeron canadensis L.), morningglory (Ipomoea spp.), waterhemp (Amaranthus tuberculatus (Moq.) J. D. Sauer) and common ragweed (Ambrosia artemisiifolia L.). [7] Some weed science researchers provide extension resources for farmers and land managers by trialing a variety of weed control tools and tactics on a specific weed, publishing the results and providing recommendations for their future management. Other researchers may study the biology of weed seeds in order to determine how long weed seeds can remain viable in a soil. Much of this research is conducted at public land-grant universities.
Another aspect of weed science research is concerned with generating knowledge about the active ingredients of herbicides. Specifically, evaluating the response of weeds and crops to different combinations of herbicides at varying rates, droplet sizes, and environmental conditions within different cropping systems containing different weed communities. Some herbicides become more effective when mixed together (syngerism), while other herbicide combinations reduce the overall control (antagonism). [6] Some weed science researchers trial a variety of herbicides applications and combinations on weeds to evaluate their impact on the weeds and crops. Most of this research is conducted by private companies.
Another aspect of weed science is concerned with how herbicides move in the environment after they are applied. Some herbicides degrade very quickly in sunlight and can be made inactive before entering the plant while others can persist for years in the soil after they are applied, causing problems for future crops. Registering herbicides can cost hundreds of millions of dollars in order to demonstrate how the chemical moves and degrades within the environment it is applied. [6] Most of this research is conducted by private companies in order to be able to register their products for sale.
Many professional societies exist for weed scientists to publish research findings and support future research. The Weed Science Society of America (WSSA) is a non-profit professional society that hosts annual conferences and regional conferences in the United States as well as provides guidance on the use of herbicides. For example, the WSSA created a categorization system of herbicides by mode of action that is adopted by all herbicide manufacturers to clearly communicate the way in which each herbicide product impacts plant physiology. By encouraging herbicide applicators to use different modes of action, this reduces the likelihood of herbicide resistant populations of weeds from occurring, thereby stewarding the long-term ability of these weed control products.
Many other professional societies exist for weed science that support the current research challenges of a given country or region. For example, the Indian Society of Weed Science, the Weed Science Society of Japan, and more.
Herbicides, also commonly known as weed killers, are substances used to control undesired plants, also known as weeds. Selective herbicides control specific weed species while leaving the desired crop relatively unharmed, while non-selective herbicides kill plants indiscriminately. The combined effects of herbicides, nitrogen fertilizer, and improved cultivars has increased yields of major crops by 3x to 6x from 1900 to 2000.
Pesticide resistance describes the decreased susceptibility of a pest population to a pesticide that was previously effective at controlling the pest. Pest species evolve pesticide resistance via natural selection: the most resistant specimens survive and pass on their acquired heritable changes traits to their offspring. If a pest has resistance then that will reduce the pesticide's efficacy – efficacy and resistance are inversely related.
Weed control is a type of pest control, which attempts to stop or reduce growth of weeds, especially noxious weeds, with the aim of reducing their competition with desired flora and fauna including domesticated plants and livestock, and in natural settings preventing non native species competing with native species.
Glyphosate is a broad-spectrum systemic herbicide and crop desiccant. It is an organophosphorus compound, specifically a phosphonate, which acts by inhibiting the plant enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSP). It is used to kill weeds, especially annual broadleaf weeds and grasses that compete with crops. Its herbicidal effectiveness was discovered by Monsanto chemist John E. Franz in 1970. Monsanto brought it to market for agricultural use in 1974 under the trade name Roundup. Monsanto's last commercially relevant United States patent expired in 2000.
In agriculture, cover crops are plants that are planted to cover the soil rather than for the purpose of being harvested. Cover crops manage soil erosion, soil fertility, soil quality, water, weeds, pests, diseases, biodiversity and wildlife in an agroecosystem—an ecological system managed and shaped by humans. Cover crops can increase microbial activity in the soil, which has a positive effect on nitrogen availability, nitrogen uptake in target crops, and crop yields. Cover crops reduce water pollution risks and remove CO2 from the atmosphere. Cover crops may be an off-season crop planted after harvesting the cash crop. Cover crops are nurse crops in that they increase the survival of the main crop being harvested, and are often grown over the winter. In the United States, cover cropping may cost as much as $35 per acre.
Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial or detrimental effects on the target organisms and the community. Allelopathy is often used narrowly to describe chemically-mediated competition between plants; however, it is sometimes defined more broadly as chemically-mediated competition between any type of organisms. The original concept developed by Hans Molisch in 1937 seemed focused only on interactions between plants, between microorganisms and between microorganisms and plants. Allelochemicals are a subset of secondary metabolites, which are not directly required for metabolism of the allelopathic organism.
Genetically modified crops are plants used in agriculture, the DNA of which has been modified using genetic engineering methods. Plant genomes can be engineered by physical methods or by use of Agrobacterium for the delivery of sequences hosted in T-DNA binary vectors. In most cases, the aim is to introduce a new trait to the plant which does not occur naturally in the species. Examples in food crops include resistance to certain pests, diseases, environmental conditions, reduction of spoilage, resistance to chemical treatments, or improving the nutrient profile of the crop. Examples in non-food crops include production of pharmaceutical agents, biofuels, and other industrially useful goods, as well as for bioremediation.
Striga, commonly known as witchweed, is a genus of parasitic plants that occur naturally in parts of Africa, Asia, and Australia. It is currently classified in the family Orobanchaceae, although older classifications place it in the Scrophulariaceae. Some species are serious pathogens of cereal crops, with the greatest effects being in savanna agriculture in Africa. It also causes considerable crop losses in other regions, including other tropical and subtropical crops in its native range and in the Americas. The generic name derives from Latin strī̆ga, "witch".
A noxious weed, harmful weed or injurious weed is a weed that has been designated by an agricultural or other governing authority as a plant that is harmful to agricultural or horticultural crops, natural habitats or ecosystems, or humans or livestock. Most noxious weeds have been introduced into an ecosystem by ignorance, mismanagement, or accident. Some noxious weeds are native, though many localities define them as necessarily being non-native. Typically they are plants that grow aggressively, multiply quickly without natural controls, and display adverse effects through contact or ingestion. Noxious weeds are a large problem in many parts of the world, greatly affecting areas of agriculture, forest management, nature reserves, parks and other open space.
Upland rice is rice grown in dry-land environments. The term describes varieties of rice developed for rain-fed or less-intensely irrigated soil instead of flooded rice paddy fields or rice grown outside of paddies.
A weed is a plant considered undesirable in a particular situation, growing where it conflicts with human preferences, needs, or goals. Plants with characteristics that make them hazardous, aesthetically unappealing, difficult to control in managed environments, or otherwise unwanted in farm land, orchards, gardens, lawns, parks, recreational spaces, residential and industrial areas, may all be considered weeds. The concept of weeds is particularly significant in agriculture, where the presence of weeds in fields used to grow crops may cause major losses in yields. Invasive species, plants introduced to an environment where their presence negatively impacts the overall functioning and biodiversity of the ecosystem, may also sometimes be considered weeds.
Striga hermonthica, commonly known as purple witchweed or giant witchweed, is a hemiparasitic plant that belongs to the family Orobanchaceae. It is devastating to major crops such as sorghum and rice. In sub-Saharan Africa, apart from sorghum and rice, it also infests maize, pearl millet, and sugar cane.
2,4-Dichlorophenoxyacetic acid is an organic compound with the chemical formula Cl2C6H3OCH2CO2H. It is usually referred to by its ISO common name 2,4-D. It is a systemic herbicide that kills most broadleaf weeds by causing uncontrolled growth, but most grasses such as cereals, lawn turf, and grassland are relatively unaffected.
4-Hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors are a class of herbicides that prevent growth in plants by blocking 4-Hydroxyphenylpyruvate dioxygenase, an enzyme in plants that breaks down the amino acid tyrosine into molecules that are then used by plants to create other molecules that plants need. This process of breakdown, or catabolism, and making new molecules from the results, or biosynthesis, is something all living things do. HPPD inhibitors were first brought to market in 1980, although their mechanism of action was not understood until the late 1990s. They were originally used primarily in Japan in rice production, but since the late 1990s have been used in Europe and North America for corn, soybeans, and cereals, and since the 2000s have become more important as weeds have become resistant to glyphosate and other herbicides. Genetically modified crops are under development that include resistance to HPPD inhibitors. There is a pharmaceutical drug on the market, nitisinone, that was originally under development as an herbicide as a member of this class and is used to treat an orphan disease, type I tyrosinemia.
Karen Bailey is a retired research scientist who specialized in plant pathology and biopesticide development at Agriculture and Agri-Food Canada. Her research focused on developing alternatives to synthetic pesticides and improving plant health through integrated pest management strategies. She is internationally recognized for her expertise on soil-borne pathogens and biological control, and she has more than 250 publications, 23 patents, and 7 inventions disclosures in progress.
The Weed Science Society of America (WSSA) is a nonprofit, learned society focused on weed science. It was founded in 1956. The organization promotes research, education, and extension outreach, provides science-based information to the public and policy makers, and fosters awareness of weeds and their impact on both managed and natural ecosystems. President for 2018-19 is Scott Senseman from the University of Tennessee.
Jonathan Gressel is an Israeli agricultural scientist and Professor Emeritus at the Weizmann Institute of Science in Rehovot, Israel. Gressel is a "strong proponent of using modern genetic techniques to improve agriculture" especially in third world and developing countries such as Africa. In 2010, Gressel received Israel's highest civilian award, the Israel Prize, for his work in agriculture.
Indaziflam is a preemergent herbicide especially for grass control in tree and bush crops.
Aclonifen is a diphenyl ether herbicide which has been used in agriculture since the 1980s. Its mode of action has been uncertain, with evidence suggesting it might interfere with carotenoid biosynthesis or inhibit the enzyme protoporphyrinogen oxidase (PPO). Both mechanisms could result in the observed whole-plant effect of bleaching and the compound includes chemical features that are known to result in PPO effects, as seen with acifluorfen, for example. In 2020, further research revealed that aclonifen has a different and novel mode of action, targeting solanesyl diphosphate synthase which would also cause bleaching.
Tribenuron in the form of tribenuron-methyl is a sulfonylurea herbicide. Its mode of action is the inhibition of acetolactate synthase, group 2 of the Herbicide Resistance Action Committee's classification scheme.
weed science.
