Herbicides are chemical substances or cultured biological organisms that kill or suppress plant growth by affecting one or more of the processes that are vital to plant survival such as cell division, respiration, and enzyme activity. Herbicides applied at high rates kill all plants. At low rate, some herbicides kill some plants without damaging other plants. Herbicides with such ability are said to be selective. Selective herbicides are commonly used in weed control.
The human race has been farming for over 10,000 years. Weeds have been an unwelcome presence alongside the crops ever since the first framers saved and planted seeds in the region that is now present-day Turkey and the Middle East. The most obvious problem caused by weeds is the reduction of yield through direct competition for light, space, nutrients and water. Weeds can have many further effects on the use of land such as:
Reduce crop quality
Act as plant parasite
Delay harvesting
Interfere with animal feed
Cause poisoning
Reduce crop and animal health
The war on weeds in many countries was indefinitely prolonged by the advent of 2, 4-D, the first truly selective broad-leaf weed herbicide, and other selective herbicides.
Herbicide trials were initiated in 1929 by the arrival of the chlorate weed killers. Chlorate herbicides provided farmers with an effective, if severely limited, weapon for attacking hard-to-control perennial species. Tests showed that sodium or calcium chlorate sprays were quite effective in killing weeds such as leafy spurge (Euphorbia esula), hoary cress (Cardaria spp.) and some of the perennial thistles. However, because they were non-selective and quite persistent, chlorate weed killers also have the unfortunate effect of rendering the soil unsuitable for cropping for two or more years after the application at the high rates necessary to provide effective control of perennial weeds. In spite of their limitations, chlorates became the first chemical weed killers to gain a fair degree of acceptance.
By the late 1930, selective sprays such as sulfuric acid and the recently introduced French discovery, Sinox (sodium dinitro-ortho-cresylate), were become increasingly popular for the control of wild mustard, but their use was strictly limited to heavy weed infestations or to areas where intensive land use was practiced because they were expensive. Consequently, they never seriously challenged sodium chlorate's position as the most commonly used chemical weed killer.
Chlorate weed killers are significant as a harbinger of the developments that followed, for generating sustained herbicide research. This was revealed in 1945 with the commercial release of 2,4-dichlorophenoxyacetic acid, commonly known as 2,4-D. Discovered independently in England and United States in the early 1940s, 2,4-D was the product of plant hormone research rather than the agriculturists' random search for phytotoxic compounds and it proved to be far more potent than any herbicide previously developed. One to two ounces per acre are all that is required to kill a wide range of broad-leaf species. More importantly, 2, 4-D is also highly selective, for when it is applied at these rates, most grasses are unaffected. The potential use for such chemical in land dominated by cereal production and awash in broad leaf weeds is obvious and before long it was being credited signifying the arrival of a new era in farming, the hormone era. For many agriculturists, the arrival of 2, 4-D could not have been timelier, when science had suddenly delivered the ultimate anti-weed weapon just as farmers were on the verge of admitting defeat in their war for economic survival.
Between the years 1950 and 1970, the initial discovery and development of triazine herbicides took place. Their discovery and development were important scientific achievements that led to unprecedented success in crop weed management. Within a short time, the discovery of and screening for the herbicidal properties of dialkylamino-s-triazines led to selection of the most promising candidates for field development and eventual commercial use. For five decades, the triazines have provided weed control in more than 50 crops around the world. They are essential for high-yield sustainable agriculture. Also they are critical to integrated pest management (IPM) and conservation tillage practices in corn and other crops, reducing the devastating environmental impact of erosion, reducing fuel costs, and retaining moisture in soil.
Glyphosate, a phosphonomethyl derivative of glycine, was actually invented in 1950 as a pharmaceutical compound. Nevertheless, since the discovery of its herbicidal properties in 1970 and its commercialization in 1974, glyphosate has been used in crop lands and non-crop lands. Because of its lack of selectivity, glyphosate use was initially limited to preplant, post directed, and postharvest applications for weed control. With the introduction of glyphosate-resistant crops in the mid-1990s, glyphosate is now widely used for weed control in GR crops.
Types of Herbicides
There are several classifications for herbicides according to their use, activity or mechanism of action.
According to their use:
Pre-emergence herbicides:
They are applied before the weed emerges and prevent the germinating seedlings from developing once the germination process begins. For pre-emergence herbicides to work properly, they must be applied before weed emergence and need approximately one-half inch of rainfall or overhead irrigation within one week after application. Pre-emergence herbicides are most effective against annual weeds like crabgrass, and generally provide poor control of most perennial weeds like dandelion.
Post-emergence herbicides:
They are used to control weeds that have emerged and are actively growing. Post-emergence herbicides can either be classified as selective or nonselective. A selective herbicide controls certain weeds without injuring the desired plants (i.e., fruits and vegetables, ornamental plants or turf) it may contact. Selective post-emergence herbicides are available for control of annual and perennial broadleaf weeds, grass weeds and sedges. Nonselective post-emergence herbicides will severely injure or kill all weeds as well as desirable plants. Nonselective herbicides (products containing glyphosate, glufosinate, diquat or pelargonic acid, just to name a few) can be used for spot applications around desired plants as long as the user is extremely careful to avoid contact of the herbicide with the desired plant.
According to their activity:
Contact Herbicides:
The word "Contact" means it kills the parts of the plant it touches. Generally, that means everything above the soil line. For some plants, that is all that is needed to kill them. However, for others, it is not enough. Plants - that spread by stolons, rhizomes, tubers, or if they can regrow from root fragments - will continue to be a problem.
A benefit of contact herbicides is their so-called "quick kill" effect. Popular advertisements talk of its power to "knockdown in one day". However, for most weeds, systemic products are the better choice. Some herbicides will contain a mixture of the two, both Contact and Systemic, for better control.
Systematic herbicides:
The word "Systemic" means the plant absorbs the chemical and transports it internally throughout the plant. It usually doesn't have the quick "knockdown" effect of the contact varieties, but the entire weed will be killed, roots and all. The speed of chemical movement in the plant is largely dependent on soil, air temperature and the "mode of action" of the chemical (how the chemical works inside the plant).
According to their mechanism of action (MOA):
This classification indicates the first enzyme, protein, or biochemical step affected in the plant following application. The main mechanisms of action are:
ACCase inhibitors: are compounds that kill grasses. Acetyl coenzyme A carboxylase (ACCase) is part of the first step of lipid synthesis. Thus, ACCase inhibitors affect cell membrane production in the meristems of the grass plant. The ACCases of grasses are sensitive to these herbicides, whereas the ACCases of dicot plants are not.
ALS inhibitors: the acetolactate synthase (ALS) enzyme is the first step in the synthesis of the branched-chain amino acids (valine, leucine, and isoleucine). These herbicides slowly starve affected plants of these amino acids which eventually lead to inhibition of DNA synthesis. They affect grasses and dicots alike. ALS is a biological pathway that exists only in plants and not in animals thus making the ALS-inhibitors among the safest herbicides.
EPSPS inhibitors: The enolpyruvylshikimate 3-phosphate synthase enzyme (EPSPS) is used in the synthesis of the amino acids tryptophan, phenylalanine and tyrosine. They affect grasses and dicots alike. Glyphosate (Roundup) is a systemic EPSPS inhibitor but inactivated by soil contact.
Synthetic auxin inaugurated the era of organic herbicides. They were discovered in the 1940s after a long study of the plant growth regulator auxin. Synthetic auxins mimic this plant hormone. They have several points of action on the cell membrane, and are effective in the control of dicot plants. 2,4-D is a synthetic auxin herbicide.
Photosystem II inhibitors reduce electron flow from water to NADPH2+ at the photochemical step in photosynthesis. They bind to the Qb site on the D1 protein, and prevent quinone from binding to this site. Therefore, this group of compounds causes electrons to accumulate on chlorophyll molecules. As a consequence, oxidation reactions in excess of those normally tolerated by the cell occur, and the plant dies.
Photosystem I inhibitors steal electrons from the normal pathway through FeS - Fdx - NADP leading to direct discharge of electrons on Oxygen. As result ROS (reactive oxygen species) are produced and oxidation reactions in excess of those normally tolerated by the cell occur leading to plant death.
Environmental Effects of Herbicides Use:
Some substantial benefits can be gained through the use of herbicides to manage unwanted vegetation. Compared with alternative means of weed control, such as;
Mechanically weeding by hand or machine.
Herbicides are less expensive, often safer (especially in forestry), faster.
Sometimes Herbicides are more selective.
However, if herbicides are not used properly, damage may be caused to crop plants, especially if too large a dose is used, or if spraying occurs during a time when the crop species is sensitive to the herbicide. Unintended but economically important damage to crop plants is sometimes a consequence of the inappropriate use of herbicides.
In addition, some important environmental effects are associated with the use of herbicides. These include;
Unintended damage occurring both on the sprayed site, and offsite. For example, by changing the vegetation of treated sites.
Herbicide use also changes the habitat of animals such as mammals and birds. This is especially true of herbicides use in forestry, because biodiverse, semi-natural habitats are involved. This is an indirect effect of herbicide use, because it does not involve toxicity caused to the animal by the herbicide. Nevertheless, the effects can be severe for some species.
In addition, not all of the herbicide sprayed by a tractor or aircraft deposits onto the intended spray area. Often there is drift of herbicide beyond the intended spray site, and unintended, offsite damages may be caused to vegetation.
There are also concerns about the toxicity of some herbicides, which may affect people using these chemicals during the course of their occupation (i.e., when spraying pesticides), people indirectly exposed through drift or residues on food, and wildlife.
For these and other reasons, there are many negative opinions about the broadcast spraying of herbicides and other pesticides, and this practice is highly controversial.
The intention of any herbicide treatment is to reduce the abundance of weeds to below some economically acceptable threshold, judged on the basis of the amount of damage that can be tolerated to crops. Sometimes, this objective can be attained without causing significant damage to non-target plants. For example, some herbicides can be applied using spot applicators or injectors, which minimize the exposure to non-pest plants and animals. Usually, however, the typical method of herbicide application is some sort of broadcast application, in which a large area is treated all at once, generally by an aircraft or a tractor-drawn apparatus.
An important problem with broadcast applications is that they are non-selective -they affect many plants and animals that are not weeds - the intended target of the treatment. This is especially true of herbicides, because they are toxic to a wide variety of plant species, and not just the weeds. Therefore, the broadcast spraying of herbicides results in broad exposures of non-pest species, which can cause an unintended but substantial mortality of non-target plants. For example, only a few species of plants in any agricultural field or forestry plantation are abundant enough to significantly interfere with the productivity of crop plants. Only these competitive plants are weeds, and these are the only target of an herbicide application. However, there are many other, non-pest species of plants in the field or plantation that do not interfere with the growth of the crop plants, and these are also affected by the herbicide, but not to any benefit in terms of vegetation management. In fact, especially in forestry, the non-target plants may be beneficial, by providing food and habitat for animals, and helping to prevent erosion and leaching of nutrients.
This common non-target effect of broadcast sprays of herbicides and other pesticides is an unfortunate consequence of the use of this non-selective technology to deal with pest problems. So far, effective alternatives to the broadcast use of herbicides have not been discovered for the great majority of weed management problems. However, there are a few examples that demonstrate how research could discover pest-specific methods of controlling weeds that cause little non-target damage. These mostly involve weeds introduced from foreign countries, and that became economically important pests in their new habitats. Several weed species have been successfully controlled biologically, by introducing native herbivores of invasive weeds. For example;
The klamath weed (Hypericum perforatum) is a European plant that became a serious pasture weed in North America, but it was specifically controlled by the introduction of two species of herbivorous leaf beetles from its native range.
In another case, the prickly pear cactus (Opuntia spp.) became an important weed in Australia after it was introduced there from North America, but it has been successfully controlled by the introduction of a moth whose larvae feed on the cactus.
Unfortunately, few weed problems can now be dealt with in these specific ways, and until better methods of control are discovered, herbicides will continue to be used in agriculture, forestry, and for other reasons.
Most herbicides are specifically plant poisons, and are not very toxic to animals. (There are exceptions, however, as is the case with the herbicide paraquat.) However, by inducing large changes in vegetation, herbicides can indirectly affect populations of birds, mammals, insects, and other animals through changes in the nature of their habitat.
For example, studies in Britain suggest that since the 1950s, there have been large changes in the populations of some birds that breed on agricultural land. These changes may be partly caused by the extensive use of herbicides, a practice that has changed the species and abundance of non-crop plants in agroecosystems. This affects:
The structure of habitats.
The availability of nest sites.
The food available to granivorous birds, which mostly eat weed seeds.
The food available for birds that eat arthropods, which rely mainly on non-crop plants for nourishment and habitat.
During the time that herbicide use was increasing in Britain, there were also other changes in agricultural practices. These include:
The elimination of hedgerows from many landscapes.
Changes in cultivation methodologies.
Introducing of new crop species.
Increases in the use of insecticides and fungicides.
Improved methods of seed cleaning, resulting in fewer weed seeds being sown with crop seed.
Still, a common opinion of ecologists studying the large declines of birds, such as the gray partridge (Perdix perdix), is that herbicide use has played a central but indirect role by causing habitat changes, especially by decreasing the abundance of weed seeds and arthropods available as food for the birds.
Similarly, the herbicides most commonly used in forestry are not particularly toxic to animals. Their use does however, because large changes in the habitat available on clear-cuts and plantations, and these might be expected to diminish the suitability of sprayed sites for the many species of song birds, mammals, and other animals that utilize those habitats.
Modern, intensively managed agricultural and forestry systems have an intrinsic reliance on the use of herbicides and other pesticides. Unfortunately, the use of herbicides and other pesticides carries risks to humans through exposure to these potentially toxic chemicals, and to ecosystems through direct toxicity caused to non-target species, and through changes in habitat. Nevertheless, until newer and more pest-specific solutions to weed-management problems are developed, there will be a continued reliance on herbicides in agriculture, forestry, and for other purposes, such as lawn care.
Biosafety of Herbicide Resistant Crops (HRCs):
For HRCs, risks can be considered qualitative estimates which combine the likelihood and severity of both immediate and delayed adverse effects to human health, the environment and the farmer's economy. The likelihood and severity of each unwanted effect associated with HRCs depends on the crop, the HR trait, the local weed flora, climatic conditions and farm management practices and can only be estimated on a case-by-case basis.
In glyphosate-resistant crops, optimal weed control often requires sequential applications with glyphosate, and the timing relative to weed emergence is important, When glyphosate is sprayed 2-3 times annually at high rates it imposes a high selection pressure on the weed flora. In 5-8 years this may cause shifts in weed composition towards species that naturally tolerate glyphosate and other herbicides may be needed to control these weeds.
Gene-flow from crops to other crops or related species is another route to the development of resistant weed populations in the field. Once the resistance gene is present in crop volunteers or related weed species then it is expected that the same weed control practices (consistent sprayings with herbicides having the same mode of action), which cause herbicide resistance to occur in naturally tolerant/resistant weed biotypes, will lead to a rapid build-up of HR-weeds and volunteers.
Increased herbicide use is considered a risk in some parts of the world although the effects on human health or the environment are seldom specified in details, but derived effects from pesticide-use such as ground-water pollution and pesticide residues in vegetables, for example, have caused public concern. There seem to be two major explanations why herbicide use in HRCs may increase. One reason is that a high level of crop tolerance may enable the farmer to increase doses to achieve an improved weed control without harming the crop. The other reason is problems with tolerant/resistant weeds and volunteers, which require farmers to increase dose or mix herbicides with different modes of action to maintain an acceptable level of weed control.
Biodiversity within the field may be influenced if the herbicide, to which the HRC is resistant, is used at a higher level of efficacy than hitherto in order to achieve an improved weed control. Furthermore, weed species respond differently to different herbicides or other weed-control measures and a shift in prevailing species is very likely. If the growing of an HRC is taking place at the centre of genetic origin, then changes in the diversity of the indigenous species and risks of diminishing the genetic diversity of these species is a hazard. It is, however, very unlikely that HR crops will cause erosion of genetic diversity of wild species outside the cultivated land, because the trait is associated with the use of herbicides which are not being applied in the wild, and a HR trait does not confer selective advantage unless the herbicide is used. Therefore, there is a low risk of erosion of the genetic diversity of wild species growing in natural environments.
Despite these concerns, some uses of GM crops, e.g. herbicide resistant sugar beet, appear to be safe so far as ecological risks are concerned, when these are judged by ordinary scientific standards (Madsen and Sandøe, 2001).
Core issues to be addressed when assessing risks from HRCs
The first step is to determine which unwanted effects are relevant in the particular scenario. In an earlier publication we proposed the use of decision-keys for identification of hazards/unwanted effects. These keys were developed to assess the likelihood that a new type of arable weed will be produced by gene flow between the transgenic crop and its relatives; the likelihood that the transgenic crop will become a volunteer problem on arable land or wild areas and the likelihood of a build-up of HR-resistant weeds.
Gene flow; transfer of genes from one population to another may lead to unwanted effects for weed management and the environment. Gene flow may enable the resistance genes to move between HR and non-HR varieties and thus pollute a crop which is considered GM-free. Or HR-genes may be stacked from years of cross-pollination of HRCs, which may result in problems for the farmer in controlling volunteer crops in the field. Multiple herbicide-resistant volunteer oilseed rape has been observed in Canada where oilseed rape with resistance to different herbicides was grown on neighboring fields. Gene flow between related species e.g. the crop and certain weeds in the field may, furthermore, result in the development of HR weeds.
If gene flow is a relevant process to consider for the particular HRC then the next step is often to quantify the level of gene flow within and between species in time and space. Over the past 10- 15 years there has been a range of studies on this issue, many of them focusing on hybridization within and between crops and wild relatives. Other studies have aimed at determining the distance of gene flow, and some have evaluated the ability of the hybrids to survive and reproduce seed over a number of generations,
Competitive ability; competition experiments have shown that herbicide-resistant biotypes of both crops and weeds may have similar competitive abilities as the non-resistant biotypes when they are not sprayed with the herbicide to which they are resistant. It is, therefore, unlikely that fitness is increased by the mere presence of a HR-gene. Reduced fitness has, however, been observed, e.g. the triazine-resistant oilseed rape variety OAC Triton yielded significantly less than non-resistant varieties. When the HR weed biotype is sprayed with the herbicide then not only is it undamaged by the spray, it is, furthermore, released from competition with all the non-resistant weeds and volunteers, which perish. This is to the advantage of the HR biotype, which rapidly builds up vegetative biomass in the field and produces large numbers of seed or propagules that enter into the soil seed bank. There are several experimental designs available to assess competitive ability over one cropping season. However, in experimentation it is complicated to assess the long-term effects on population dynamics from an altered competitive ability.
Simulation models; many of the concerns over HRC can best be addressed experimentally by multiyear experiments, but these experiments are costly. Furthermore these crops are already being grown on large areas, which mean that management strategies to prevent/delay problems must be developed soon to be effective. Combining simulations with selected mid- and long-term experiments may provide a better understanding of the suitability of available weed management strategies that may prevent or delay the selection of HR-weeds. Furthermore, development of a model system would reveal where relevant information is lacking or scarce. Simulation may thus be able to anticipate problems with injudicious herbicide use such as development of resistant weeds and volunteers, and/or major shifts in the weed flora. Of course the pre-requisites for the model must be clearly stated to arrive at a 'sound judgement' of the predictions.
Madsen et al. (2002) developed a simulation model of growing HR varieties of rice in a rainfed Central American production system to investigate the following question: Is there a risk of increased weed problems, which is derived from gene flow and rice volunteers? Simulation with glufosinate resistant rice enabled a prediction of potential long-term effects and allowed for testing of different scenarios including contrasting weed management practices, hybridization levels between the commercial HR cultivated and weedy rice, and seed predation rates. Because risks may only become conspicuous after long-term cultivation of HR rice, simulations were run for a 10-year period. In a cropping system relying on glufosinate-resistant rice for weed control, the model predicted that resistance to glufosinate would occur after 3-8 years of monoculture. Increasing the hybridisation level from 1-5 percent decreased the time for resistance to occur by 1-3 years. Increasing annual rate of weedy rice seed predation at the soil surface delayed development of resistance. Tillage as a weed control tactic also delayed the occurrence of resistance compared to a no-till situation. It must, however, be emphasized that the model was a first attempt to simulate a production system with HR rice and many of its parameters were highly uncertain. Furthermore, the presented model had not been validated with field data, which is a prerequisite to make reliable predictions about long-term consequences of growing HR rice.
