Herbicide resistance, the acquired accommodation of a previously herbicide-susceptible edger citizenry to bear a herbicide and complete its action aeon aback the herbicide is acclimated at its accustomed dosage in an agricultural situation, has added steadily over the accomplished several years (Heap and LeBaron, 2001). Although the abundant majority of cases of herbicide attrition have occurred in the developed world, several important weeds accept acquired resistance in developing countries accepting an important bread-and-er appulse on specific crops. This abbreviate analysis focuses on the administration of herbicide-resistant weeds in developing countries, accordingly another is accustomed to cases acquired and practices acceptable for arresting with herbicide attrition in these countries.
Worldwide herbicides represented 47 percent of the ca US$28 billion agrochemical sales in 2001 (Agrow, 2002a). The developed economies (North America, Europe and Japan) anniversary for about 70 percent of the all-around agrochemical bazaar (Bryant, 1999). Sales in Latin America, which apparent the bigger increases in 1996 (Agrow, 1996, 1997a) were bottomward by almost 8 percent (to US$3.5 billion) mostly because of the collapse of the abridgement in Argentina and barter amount fluctuations in Brazil (Agrow, 2002a). Admitting a 3 percent sales access in 2000, the Japanese pesticide bazaar resumed its bottomward trend in 2001 as a aftereffect of bargain rice plantings, amount burden and low insect outbreaks (Agrow, 2002a). In 2001, the arch seven agrochemical companies (Syngenta, Monsanto, Aventis, BASF, Dow AgroSciences, DuPont and Bayer) arise sales of about US$22 billion ((Agrow, 2002a). The best widely- acclimated herbicide in the apple is glyphosate (Woodburn, 2000), which accounts for about 11 percent of the absolute amount of the agrochemical bazaar (Agrow, 2002b). Glyphosate represents 67 percent of the US$3.9 billion agrochemical bazaar share of Monsanto (Agrow 2001). There are additionally several companies accustomed producing all-encompassing glyphosate, abounding of them based in developing countries (Woodburn, 2000). Added broadly acclimated compounds are paraquat, brash the added largest-selling agrochemical, triazines and metolachlor. In developing countries, paraquat continues to be one of the herbicides of preference.
RESISTANCE CASES IN DEVELOPING COUNTRIES
The aboriginal case of herbicide attrition (to 2,4-D) was reported by Hilton (1957) but edger attrition to herbicides began to accept recognition abandoned afterwards the aboriginal case of triazine attrition in Senecio vulgaris was arise by Ryan (1970). For several years, attrition to triazines was most prominent. There are 64 breed that accept acquired attrition to triazines and added Photosystem II inhibitors (Heap, 2002). In contrast, abandoned four species accept been accustomed advancing to glyphosate alike admitting this herbicide has been broadly acclimated for added than 25 years. Currently and worldwide, there are 261 accustomed advancing biotypes that accord to 157 breed (95 dicotyledonous and 62 monocotyledonous) (Heap, 2002). Two modes of action (MOA) groups of more contempo commercialization, those of herbicides that arrest the enzyme acetolactate synthase (ALS), which accommodate the sulfonylureas (SFU), imidazolinones, triazolopyrimidines, pyridinilbenzoates and sulfonylaminocarbonyltriazolinones, and the inhibitors of acetyl CoA carboxylase (ACCase) comprised by the aryloxyphenoxy propanoates and cyclohexanediones, have contributed to the affliction of the herbicide attrition problem. There are 73 breed (104 biotypes) advancing to ALS herbicides and 28 breed (59 biotypes) advancing to ACCase herbicides (Heap, 2002).
Based on the database maintained by Heap (2002), developing countries accord 22 percent of the herbicide attrition incidences (a total of 465 arise up to September, 2002). There are differences in the relative administration of attrition cases based on MOA amid developed and developing countries (Figure 1). The three best important groups (Triazine, ALS and ACCase herbicides) anniversary for 74 percent and 65 percent of attrition cases in developed and developing countries, respectively. In both groups of countries, attrition to triazines charcoal best accustomed (on a per-biotype basis), but in the developed apple ALS attrition is proportionally alert as accustomed as ACCase resistance. In developing countries, affluence of attrition cases to these two MOAs is about identical. Bipyridilium, auxinic and urea/amide herbicides in admeasurement additionally accord added attrition cases in developing countries than in automated ones. A accessible account for this is the about college use of the herbicides paraquat, 2,4-D and propanil in developing countries.
EVOLUTION OF RESISTANT WEEDS
Weed populations become advancing because of the interaction of a few key factors. Plants, in general, and decidedly weeds, are variable. The genes appointment attrition are present artlessly in agrarian populations; attrition mutations are anticipation not to be induced by the herbicide (Jasieniuk et al. 1996). But these genes action in agrarian populations at a actual low affluence because, in absence of the herbicide, they about do not advise any adaptive advantage to those plants. The affluence of these resistance genes, however, is important in free how connected it would booty for attrition to become apparent already we alpha relying on a accurate herbicide. For example, the accelerated access in attrition to ALS herbicides is attributed in allotment to the aerial alteration affluence in the ambition armpit agitator and the existence of several mutations that can advise attrition (Chaleff and Day, 1984, Devine and Preston, 2000). Preston and Powles (2002) bent the affluence of individuals advancing to ALS-inhibiting herbicides in populations of Lolium rigidum never ahead apparent to these herbicides. The affluence of individuals advancing to sulfometuron-methyl and imazapyr assorted from 2.2 ×10-5 to 1.2 ×10-4 and from 1 ×10-5 to 5.8 × 10-5, respectively, depending on the population. These aerial affluence ethics advice explain the accelerated change of attrition already the populations are subjected to the another imposed by the ALS-herbicides. Unfortunately, the affluence of advancing individuals in a weed citizenry is not accustomed afore the accession of a new herbicide with a novel MOA as baseline abstracts are not acclimatized for artefact allotment and hardly are generated for approaching anatomy purposes (Moss, 2001).
Figure 1. Administration of cases of herbicide attrition in developing and developed countries according to access of action. (Based on abstracts aggregate by Heap, 2002)
Two added important characteristics of a edger in agreement of herbicide-resistance change are the admeasurement and action of the clay berry bank and the edger fitness. The clay berry coffer may act as a buffer, appropriately dabbling the change of resistance. This is because over the years the clay berry coffer has been accomplished by the berry allow by the absolute afflicted individuals. In some cases, individuals accustomed a alteration (such as those appointment resistance to herbicides) are penalized by actuality beneath acclimatized or beneath fit in absence of the herbicide. Bargain fettle is difficult to admeasurement but it can be accompanying to broken adeptness of key physiological processes such as photosynthesis or accomplished bulb characteristics such as decreased berry assembly or reduced advancing ability. About advancing biotypes, however, are no beneath fit than the normal, afflicted ones.
The best cogent agency that governs the change of herbicide attrition is the another burden imposed by the herbicide (Jasieniuk et al. 1996). College another burden is imposed aback we use herbicides at aerial doses, awful able and/or assiduous compounds and when we aerosol them too frequently. As the bloodshed amount increases so does the another burden we appoint with the herbicide. Appropriately advancing edger populations can be brash a case of accelerated adaptive change (Reznick and Cameron, 2001). The absolute change of attrition to a accurate herbicide within a breed above sites and times has been proposed as an archetype of recurrent change in acknowledgment to the aforementioned careful force (the herbicide) across populations (Levin, 2001). Edger populations additionally acknowledge to agricultural practices, including herbicides, by changes in their agreement and abundance. Edger accouterment are about associated with the connected use of a particular herbicide. Breed that are artlessly not afflicted by the herbicide become more accustomed (Hyvönen and Salonen, 2002), including those few that escape to non-selective herbicides such as glyphosate acclimated in glyphosate-resistant crops, as accurate in Argentina and in no-till assembly systems in Brazil (Merotto et al. 1999, Moreno 2001, Valverde 2002, Vita et al. 2001). These changes in the flora at a specific armpit should not be abashed with herbicide-resistance evolution.
Several mechanisms advise attrition to herbicides. The most accustomed and important are those accompanying to target-site aloofness and enhanced herbicide metaism or breakdown to abeyant products. Additionally, resistance can be attributed to herbicide aloofness (or abstention attributable to concrete or banausic breach of the herbicide from acute tissues or ambition sites), and bargain uptake (Devine and Preston, 2000). The aloofness apparatus has mostly been proposed for cases of paraquat resistance. For example, a paraquat-resistant biotype of the anniversary Asteraceae edger Crassocephalum crepidioides, was begin in 1990 in amazon fields a Tanah Rata, Malaysia, breadth paraquat had been activated alert a year for ten years (Ismail et al. 2001a). Physiological studies bent that paraquat was not metaized in the blade tissues of either the afflicted or advancing biotype. Plants of both biotypes captivated paraquat analogously and attrition appeared to be endowed by a aloofness apparatus that renders the paraquat abeyant (Ismail et al. 2001a). Recently, an added apparatus ahead articular in tissue adeptness selections, the overproduction of the ambition site, was proposed as the attrition apparatus to graminicides (ACCase inhibitors) in a johnsongrass (Sorghum halepense) biotype (Bradley et al. 2001).
There are cases in which added than one apparatus is involved in appointment attrition to herbicides in a distinct abandoned or a plant citizenry (multiple resistance). It is additionally accustomed that a edger that has evolved attrition to a specific herbicide additionally exhibits attrition to added herbicides in the aforementioned actinic or MOA ancestors because they allotment the aforementioned bounden site. Thus, a modification of this bounden armpit after-effects in target-site cross resistance. For example, populations of Ixophorus unisetus selected advancing by imazapyr (an ALS inhibitor) in Costa Rica were additionally cross-resistant to a accession of imidazolinone and SFU herbicides (Chaves et al. 1997). When attrition is able by accession apparatus such as added abasement of the herbicide, cross-resistance to herbicides of acclimatized MOA or chemistries can occur. For example, a citizenry of Digitaria sanguinalis from Australia that was called by, and advancing to, the ACCase herbicide fluazifop-p-butyl was begin to be cross-resistant to the ALS- inhibitor imazethapyr, admitting never actuality brash with any ALS-herbicide. Attrition to the ALS herbicide was not target-site based but was allegedly because of added herbicide metaism. But the agitator amenable for fluazifop acid detoxification in D. sanguinalis is acclimatized from the agitator that detoxifies imazethapyr (Hidayat and Preston, 2001). Best commonly, however, assorted attrition is conferred by the accession of two or added resistance mechanisms such as in populations of L. rigidum, Alopecurus myosuroides and Phalaris accessory (Preston and Mallory-Smith 2001). Extreme cases do occur. A biotype of L. rigidum from Australia selected by the abiding use of a cardinal of herbicides (diuron, chlorsulfuron and atrazine) and aloof two exposures to diclofop, exhibits (cross-) attrition to nine herbicide classes, apery bristles MOA categories (Burnet et al. 1994). Adeptness of both the MOA and the apparatus of attrition is important in designing and implementing herbicide blockage and administration practices. Groupings of herbicides according to their MOA accept been developed as a guidance in attrition management. The best able-bodied accustomed are those of the Herbicide-Resistance Action Committee (HRAC) and the Edger Science Society of America (WSSA), (Retzinger and Mallory-Smith, 1997; Schmidt, 1997).
In Australia, breadth attrition problems are of the greatest magnitude, it is now binding for herbicide labels to backpack a ample letter anecdotic the herbicide MOA (Powles, 1997). Another of herbicide products for mixtures, consecutive applications or rotations is facilitated by easily anecdotic the mode-of-action chic to which they belong.
HERBICIDE RESISTANCE PREVENTION AND MANAGEMENT
Herbicide attrition is an adumbration of overdependency of herbicides in a accurate assembly system. Developing countries are not afar from the accustomed trend of herbicide dependence; these chemicals are broadly acclimated in awful adult crop assembly systems as able-bodied as by poor adeptness farmers. Indeed some of the best alarming cases of herbicide attrition in developing countries accept occurred in areas and in crops where both all-embracing bartering farms as able-bodied as baby farmers accept accustomed a distinct herbicide or MOA as the capital apparatus for eliminating a key weed. This is the case of propanil attrition in junglerice (Echinochloa colona) in Central America, Mexico and genitalia of South America (Valverde et al. 2000) and that of isoproturon attrition to graminicides in Phalaris accessory in India (Malik and Singh, 1995). It is additionally the case of one of the few arise cases of attrition to glyphosate. Goosegrass (Eleusine indica) acquired resistance to glyphosate in several regions in the Malaysian Peninsular in orchards, vegetable areas, nurseries and oil access plantations (Ismail et al. 2001b). At a guava acreage in Teluk Intan, afterwards abortion of glyphosate to control goosegrass at the recommended dosage of 540 g a.e. ha-1 (Lee and Ngim, 2000), beginning appliance of the herbicide at 4.32 kg a.e. ha-1only provided 25 percent ascendancy of the advancing biotype. Attrition acquired in a abbreviate aeon (about three years) of accelerated glyphosate use (6-7 sprays per year at accretion doses). Added areas breadth attrition has been accustomed were apparent to added acute regimes (up to ten times per year for bristles years). Characterization of some biotypes indicates that at optimum temperature seeds of both advancing and afflicted biotypes germinate in the aforementioned manner, but the advancing biotypes appeared to be added alive and advantageous than the afflicted ones (Ismail et al. 2001b). The mechanism of attrition to glyphosate in goosegrass has afresh been elucidated by the assignment of Baerson and co-workers (2002). Basal or glyphosate-induced activity levels of the agitator targeted by glyphosate, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), amid a advancing and a afflicted biotype were similar, advertence that attrition was not accompanying to ambition agitator overexpression in the advancing biotype. The two biotypes additionally accommodate identical EPSPS gene copy numbers. Attrition was accustomed to be the aftereffect of an acclimatized EPSPS enzyme that exhibits bargain acuteness to glyphosate. In the acclimatized EPSPS, the amino acerbic proline is commissioned by serine at position 106. This apparatus differs from that empiric in a afresh detected citizenry of L. rigidum that additionally acquired attrition to glyphosate in Australia, in which an over-production of the EPSPS agitator was associated with the empiric attrition (Gruys et al. 1999).
Most herbicide-resistance administration practices discussed here accredit to abundant agronomics systems and another has been accustomed to examples from developing countries to allegorize their development and implementation. There are, however, cases of attrition in acreage crops and pastures which may deserve adapting specific practices to cope with herbicide resistance. An all-embracing accumulation of herbicide attrition administration practices in apple grain crops has afresh become accessible (Powles and Shaner, 2001).
Prevention of herbicide resistance
In bounded areas or specific farms breadth herbicide attrition has not yet appeared, efforts should be fabricated to accommodate weed ascendancy access that would abstain or adjournment the another of resistant populations. Aback another burden is the alive force for resistance evolution, access that abatement the another burden imposed on the citizenry should be implemented. Herbicide dose, adeptness and affluence of appliance abundantly actuate another pressure. Monoculture, intensive herbicide use accepting the aforementioned MOA and bargain agronomics about characterize systems breadth attrition has acquired (Mortensen et al. 2000). Highly able herbicides, acclimated persistently, appoint a aerial another burden that can aftereffect in herbicide-resistant populations evolving in aloof a few generations. This has been able-bodied accurate in developing countries with the ALS and ACCase herbicides that are brash abnormally decumbent for resistance evolution. Repeated herbicide appliance in the aforementioned agronomics division increases another pressure, abnormally if a edger germinates and emerges in different flushes and completes added than one bearing per season. Such herbicide regimes are amenable for propanil attrition in E. colona in Central America (Valverde et al. 2000) and for cases of paraquat attrition in Malaysia (Itoh et al. 1992), two acquaintance herbicides with no soil persistence.
If farmers accept to await on herbicides to ascendancy weeds, a advantageous access to adjournment or anticipate herbicide attrition is to use mixtures or to circle herbicides. Conventionally, herbicides are accumulated with the aim of adorning the edger ascendancy spectrum, about blurred the doses of the products in mixture. For attrition management, however, there are added requirements: both herbicides in the admixture charge be at abounding dosage and able on the target edger species, and acquire agnate alternation but acclimatized mechanisms of action and/or abasement pathways in the bulb (Wrubel and Gressel, 1994). Abounding rice farmers in Central America amalgamate propanil with pendimethalin for E. colona control. Pendimethalin is an accomplished accomplice for propanil in attrition blockage aback it meets best of these requirements. Appropriately resistance to propanil has been delayed or has not occurred in rice fields breadth propanil is consistently alloyed with pendimethalin, a herbicide additionally advantageous as an alternative artefact aback propanil attrition has already acquired (Garita et al. 1995; Garro et al. 1991; Riches et al. 1996, 1997).
By the aforementioned rationale, herbicide circling helps in delaying the another of herbicide-resistant populations. According to archetypal predictions with absorption to two herbicides which accept acclimatized MOA but are appropriately effective adjoin a ambition weed, mixtures are bigger than anniversary rotations in dabbling the actualization of herbicide attrition (Powles et al. 1997). In developing countries and abnormally with all-encompassing herbicides, it is accustomed to acquisition a variety of bartering articles that accommodate the aforementioned alive ingredient. Farmers not appropriately brash sometimes circle with or alike tank-mix formulated products absolute the aforementioned alive ingredient, but awash beneath a acclimatized cast name. Farmers are additionally frequently addled by the accession of new articles that are no added than associates of the aforementioned actinic or MOA group.
Use of certified seed, weed-free seeds and abstention of attenuated acreage accessories should advice in preventing the accession of advancing actual to new areas or fields. Unfortunately, seed-saving from antecedent harvests is accustomed in abounding developing countries. In Vietnam, for example, beneath than 5 percent of the absolute rice berry acclimated by farmers is certified and the self-supplied berry is of poor affection and heavily attenuated with weed seeds (Chin, 2001). Breakdown of herbicide advancing weeds through contaminated crop berry is ailing accurate although there are cases suggesting such a movement (Thill and Mallory-Smith, 1997). The accent of thorough machinery-cleaning in preventing the advance of advancing weeds was illustrated by Itoh et al. (1997a, 1997b). A agriculturalist in Japan called a resistant citizenry of Lindernia dubia var. above afterwards bristles years of using bensulfuron added mefenacet in a rice field. Movement of acreage machinery (transplanting accoutrement and a amalgamate harvester) infested a absolutely separated uproad acreage with advancing individuals in a pattern, afterward the movement of accessories from the acreage access onwards.
Weed seeds are characterized for accepting adaptations to facilitate their breakdown and about there are no differences in berry size or weight amid those produced by advancing and afflicted plants. Appropriately weed seeds of advancing biotypes can artlessly biking abbreviate and connected distances aided by these adaptations and the agnate disseminating agents. For example, it has been acclimatized that afoot birds may accept played an important role in overextension seeds of Solanum nigrum advancing to triazines in Europe (Stankiewicz et al. 2001). Russian arrow (Salsola iberica) is a actual abounding bulb acclimatized for wind dispersal. Mature plants breach from the clay and tumble with the wind, address their berry forth their aisle of travel. SFU-resistant Russian arrow plants abound new areas over absolute regions in this address (Stallins et al. 1994).
While botheration weeds in general, and herbicide advancing ones in particular, can be dispersed, it is important to accent that selection burden is the capital alive force in the actualization of advancing populations at a acreage or specific field. Indeed, studies appliance atomic markers in Lindernia micrantha accustomed that attrition to ALS-herbicides evolved as assorted contest and that it was not the aftereffect of a architect citizenry being advance over several locations, either pollen or bulb introductions in attenuated accessories (Shibaike et al. 1999).
Managing edger populations already advancing to herbicides
The best accustomed bearings faced by the agriculturalist is to control weeds that already accept become advancing to herbicides. Although the immediate acknowledgment is to about-face to a acclimatized herbicide still alive on the weed population, abiding administration of attrition can abandoned be accomplished by affiliation of acclimatized access based on an able adeptness of the biology and anatomy of the edger and of the herbicide MOA and attrition mechanism. Very hardly is advice complete in all these respects, abnormally for developing-country agronomical systems. But compassionate the fundamentals of herbicide attrition evolution, benefiting from the acquaintance acquired elsewhere with agnate cases and abiding to acceptable agronomical husbandry allows for the architecture and accomplishing of acceptable administration programs.
Several agronomic access can advice in attached the local spread, anatomy access and appulse of advancing populations. It should be emphasized that administration practices should be directed to abbreviating the admeasurement of berry of advancing individuals in the clay berry bank, especially for those breed whose berry has bound constancy and clay persistence. Thus practices that anticipate berry ambience and address by plants that accept survived all ascendancy practices during the agronomics aeon would advice in the abatement of infestations with advancing individuals. In Australia, for example, some farmers use adapted accessories or accept acclimatized their harvesters to absorb seeds of L. rigidum at harvest, a convenance admired as an important basal for managing populations that display assorted attrition (Powles 1997, Gill and Holmes, 1997). Stubble-burning or assimilation can additionally abort edger seeds or anticipate berry assembly of plants that are still crumbling at crop harvest. In Taiwan and China, farmers bake rice harbinger afterwards autumn to ascendancy balance weeds and to anticipate the accretion of clay berry banks (De Datta and Baltazar, 1996). Stubble incorporation, however, had no aftereffect on E. colona anatomy in the afterward agronomics division or aural the amount of a alternation of acreage trials conducted for up to three years in Costa Rica (Valverde et al. 2001a). To abatement the berry set of L. rigidum in Australia, some farmers additionally spray paraquat at low doses actual backward in the growing season, a convenance called crop topping (Powles, 1997).
Soil alertness affects edger berry dynamics and seedling densities at burying and appropriately can accord to administer herbicide resistant populations (Buhler et al 1997). Success of an anniversary edger is largely abased on time of emergence, which determines whether a bulb competes auspiciously with its neighbours, is captivated by herbivores, adulterated with diseases, and whether it flowers, reproduces, and matures appropriately by the end of the growing division (Forcella et al. 2000). Agronomics systems affect weed actualization by modifying the composition, vertical administration and anatomy of edger berry banks. Edger breed whose seeds can germinate at or a the soil apparent and become accustomed accept the greatest abeyant to breed under absorption agronomics systems (Buhler et al 1997).
In accustomed tillage, disking or ploughing at intervals afore crop sowing achieves ascendancy of antecedent edger populations that otherwise would arise with the seeded crop. A three-week adjournment in aureate sowing enables ascendancy of multiple-resistant L. rigidum by non-selective herbicides still able on the edger or by agronomics afore seeding. This practice, however, may be bound by the abridgement of the division accessible for crop growth, which reduces the crop abeyant of the crop (Gill and Holmes, 1997). Delayed burying to acquiesce weeds to arise and be abandoned with herbicides is broadly acclimated in some Latin American countries for angular rice (Oryza spp.) and E. colona management, abnormally in rainfed rice (Fischer, 1996) and has accurate advantageous for authoritative propanil-resistant E. colona (Valverde et al. 2001a). In Italy, attrition to ALS herbicides was not empiric in rice fields breadth contagion with red rice afflicted farmers to use dried seed bed alertness and to administer oxadiazon afore burying (Sattin et al. 1999).
If advancing and afflicted weeds accept cogwheel emergence patterns in the acreage because of differences in berry dormancy, ascendancy tactics can be acclimatized to annihilate best of the advancing individuals afore the crop is active (Dyer et al. 1993, Alcocer-Ruthking et al. 1992). Seeds of SFU-resistant kochia (Kochia scoparia) germinate beforehand than those of afflicted plants authoritative it accessible to ascendancy them mechanically or with a herbicide accepting a acclimatized MOA afore burying a crop such as aureate (Thompson et al. 1994). Similarly, Solanum nigrum seedlings advancing to triazine herbicides emerged faster and in greater admeasurement than triazine-susceptible seedlings, because of the alternation amid soil temperature and cogwheel minimum formation requirements of seeds between the biotypes (Kremer and Lotz, 1998).
Crops usually accept a archetypal edger flora associated with them. Thus, crop circling modifies the breed agreement of edger communities (Hyvönen and Salonen, 2002). Accelerated assembly in monoculture selects for edger floras awful accordant with the accurate agronomical system; these weeds usually are additionally actual advancing and adamantine to kill. Crop rotations brings about changes in burying patters, agronomics practices, action cycles, competitive characteristics, and edger administration that agitate about-face niches of weed breed and prevents the accretion of those awful acclimatized (Buhler, 2002). In India, breadth the rice-wheat arrangement was disconnected by the circling with other crops, the accident of isoproturon-resistant P. accessory was substantially lower (Malik and Singh, 1995). A. myosuroides is an anniversary grass weed that has acquired attrition to several herbicides in Europe (Heap, 2002). A contempo abstraction in France (Chauvel et al. 2001) evaluated the aftereffect of called administration practices, including crop rotation, on an A. myosuroides citizenry advancing to both fenoxaprop and clodinafop. This citizenry apparent cross-resistance to the SFU-herbicide flupyrsulfuron, which had never been acclimated in that field. The allotment of the ACCase-resistant plants did not change during the three-year circling alike admitting herbicides with this MOA were not used, allegedly because the advancing plants do not backpack a fitness amends compared to the afflicted ones. Crop rotation, however, decreased blackgrass density, abnormally aback bounce crops were alien into the circling scheme.
Crop advancing advantage
There are added agronomic practices that can provide advancing advantage to the crop, abbreviation the appulse of the weeds associated with them and abbreviating the charge for actinic control. Added absorption is actuality paid to ancestry and anecdotic crop varieties with adeptness to suppress weeds, abnormally in atom crops such as aureate and rice (Coleman et al 2001, Fasoula and Fasoula, 1997). Crop varieties should be evaluated according to the bounded agronomics systems and their botheration weeds. Lemerle et al. (2001) begin basal advancing grain-yield advantage of aureate varieties growing with L. rigidum in Australia authoritative it all-important to advance other access to access the competitiveness of the crop. Garriti et al. (1992) evaluated 25 rice cultivars for their advancing adeptness adjoin weeds beneath low- and high-weed densities. Tall cultivars suppressed weeds better, compared with average and semi-dwarf cultivars. The best competitive cultivar suppressed edger dry weight by up to 75 percent. Tall cultivars, however, are added afflicted to lodging, accept a lower tillering accommodation and a about ample blade breadth base that after-effects in alternating concealment of the leaves. In Colombia, Fischer et al. (1997) begin a rice cultivar, which, under astringent edger pressure, produced acceptable atom and was able to aish E. colona. Competitiveness of the rice cultivars activated was activated with added blade breadth index, agriculturalist cardinal and awning ablaze interception. An important anatomy of abstruse and activated acquaintance has additionally been acquired in anecdotic ancestry and testing rice cultivars that are allelopathic to weeds (see Olofsdotter et al. 2002).
The advancing advantage of the crop can additionally be added by accretion its density. In Australia, as a aftereffect of abundant infestations with herbicide advancing L. rigidum, farmers accept broadly adopted the practice of accretion aureate agronomics ante by 20-40 percent to aish the edger (Powles, 1997). Ante should be adapted according to the accustomed advancing edger to be suppressed. College aureate densities supplemented by careful herbicides were acclimatized to ascendancy Avena ludoviciana than to ascendancy Phalaris paradoxa (Walker et al. 2002).
Rice, with its adapted agronomics systems, offers additional alternatives to accomplish the crop added competitive. These accommodate transplanting, puddling the clay and administration of the baptize table in abounding or irridated rice (Valverde and Itoh, 2001). Added practices that access edger flora composition and herbicide amount are intercropping and the use of awning crops.
Alternative actinic control
Once attrition to a herbicide has become axiomatic to the farmer, usually afterwards a re-application of the aforementioned artefact at a best dose fails afresh to ascendancy the weed, the archetypal band-aid is to about-face to another herbicide. Another of another herbicides charge be accurate aback it has been already accurate that herbicide use patterns accept been such that resistance has evolved. To the abruptness of farmers and advisors, sometimes the selected advancing citizenry additionally aftereffect advancing to a herbicide that has not been activated afore or that has been acclimated to a actual bound extent. In accession to antecedent examples, added cases are illustrative. Some populations of A. myosuroides are advancing to the PS II herbicide chlortoluron because of added alloyed action oxydase action that after-effects in accelerated metaism of both chlortoluron and pendimethalin (James et al. 1995). Thus, for this population, pendimethalin is not a applicable another herbicide. In a few cases, to the abruptness of scientists, farmers abide appliance the aforementioned herbicide to which the key edger in the assembly arrangement has become resistant. This occurs aback the herbicide still controls added important weeds and is actual inexpensive. Such is the case of the connected use of bensulfuron in rice in California and Australia, breadth important weeds acquired attrition to this ALS-herbicide (Valverde and Itoh 2001). Similarly, in Costa Rica, some farmers abide using propanil, admitting accepting called propanil-resistant junglerice, as propanil still selectively controls several dicotyledonous weeds (Valverde et al. 2000).
An archetype of the use of herbicides with another MOAs to cope with a attrition botheration in developing countries is illustrated by the administration of advancing broadleaf weeds in soybean in Brazil and Argentina, which are the added and third bigger soybean producers in the world, respectively. According to FAO statistics, in 2001 Brazil active 13.9 million ha and Argentina, 10.3 actor ha. For the year 2002, the breadth active in Brazil added by an estimated 16 percent to a absolute of 16.3 actor ha (CONAB 2002). In Brazil, the best important producer- states are amid in the south and midwest of the country. Paraná and Rio Grande do Sul in the south and Mato Grosso in the midwest bulb added that 3 actor ha each. The added two above producers, both in the midwest, are Goiás and Mato Grosso do Sul. No transgenic, herbicide-resistant soybean is active accurately in Brazil. On the added hand, Argentina plants glyphosate-resistant varieties about entirely (James, 2001). The capital soybean-producing ambit in Argentina are Cordoba, Santa Fe and Buenos Aires. In Argentina, of the US$600 actor agrochemicals bazaar in 2001, two-thirds corresponded to herbicides. As expected, glyphosate was the best broadly acclimated herbicide but no cases of attrition to this herbicide accept been arise in Argentina yet.
Three broadleaf weeds (Bidens pilosa, Bidens subalternans and Euphorbia heterophylla) accept acquired attrition to ALS herbicides in Brazil. Amaranthus quitensis additionally became advancing to this accession of herbicides in the ambit of Cordoba and Tucuman in Argentina. In addition, in Brazil, Brachiaria plantaginea was accustomed as resistant to ACCase herbicides (Christoffoleti et al. 2001, Gazziero et al. 2000, Vidal and Fleck 1997), attrition allegedly actuality able by a target-site alteration (Cortez et al. 2000). Several studies accept been conducted to accept the attributes of resistance, actuate cross-resistance patterns and annotate the attrition mechanism. These studies analyze biotypes doubtable or accustomed to be advancing to ALS herbicides from acclimatized locations, all called by imazethapyr and chlorimuron ethyl or both, to a respective afflicted one. Patterns of cross-resistance arise to be absolutely consistent amid biotypes.
Initial letters declared that B. pilosa had become advancing to ALS inhibitors (Ponchio et al. 1997) but afterwards it was accustomed that this breed grows in actual aing affiliation to B. subalternans. Appropriately both breed are now accustomed advancing to ALS herbicides (Gelmini et al. 2001, Christoffoleti 2002). A B. pilosa biotype from Mato Grosso do Sul, field-treated with ALS herbicides for at least eight years, exhibits attrition indices, RI (based on accomplished bulb bioassays) of 40 for chlorimuron-ethyl, 60 for metsulfuron-methyl and imazethapyr, and 175 for nicosulfuron (Christoffoleti, 2002). The RI is afflicted as the arrangement between the herbicide doses that inhibits advance by 50 percent (GR50) in the citizenry of absorption over the GR50 amount of the susceptible, advertence population. This cross-resistance arrangement is broadly accurate for both Bidens spp. (Gelmini et al. 2002, Monqueiro et al. 2000, Monqueiro and Christoffoleti 2001b). Beneath Brazilian conditions, the seed of B. pilosa survives in the clay for 3-4 years (Voll et al. 2001). In a berry burying experiment, it was additionally approved that germination and adulteration of B. pilosa seeds was added acute on the clay surface. About 80 percent of the berry on the clay apparent was absent as a aftereffect of germination and adulteration aural the aboriginal two months (rainy season); the actual 20 percent maintained its action until agreement achievement (one year). Aback active at 10 cm, berry losses were about 50 percent (Carmona and Villas-Bôas, 2001). Thus, the use of pre-emergence and early-post actualization herbicides in no-till soybean assembly systems could rapidly abatement the clay berry coffer and advice to ascendancy the body up of ALS- advancing populations. Advancing biotypes of these breed can be controlled with lactofen, fomesafen, bentazon, glufosinate and glyphosate (Christoffoleti, 2002, Gelmini et al. 2002). A afflicted and ALS-resistant biotype of B. pilosa showed agnate advance as individuals in a pot abstraction (Christoffoleti, 2001).
Tuesca and Nisensohn, (2001) evaluated the acknowledgment to herbicides of three populations of A. quitensis from no-till agricultural systems in Argentina that were allegedly advancing to ALS-herbicides. Two of these populations came from fields breadth imazethapyr had been acclimated in soybeans for the accomplished 4-5 years; the third citizenry came from a acreage breadth imazethapyr had been rotated with nicosulfuron (in maize) and chlorimuron-ethyl during the aforementioned period. The reference, afflicted citizenry was calm in a field breadth accustomed agronomics was practised and soybean was rotated with maize, actuality apparent to an ALS-herbicide (imazethapyr) abandoned once, bristles years afore the berry was collected. The two populations called by imazethapyr were resistant to this herbicide but not to chlorimuron-ethyl. The citizenry apparent to both the imidazolinone and SFU herbicides was advancing to both imazethapyr and chlorimuron-ethyl. Bloodshed of the afflicted biotype was 95 percent at half the recommended dosage of either herbicide and absolute at the abounding bartering dose. Lactofen, fomesafen and bentazon controlled ALS-resistant A. quitensis (Monqueiro and Christoffoleti 2001b). Attrition to ALS herbicides in Bidens spp. and in A. quitensis is conferred by an aloof ambition enzyme (Monqueiro and Christoffoleti, 2001a).
E. heterophylla advancing to ALS-herbicides has also been accustomed in soybean fields in the states of Paraná, Rio Grande do Sul, Sao Paulo and, alongside in an adeptness acreage study, in Mato Grosso do Sul (Gazziero et al. 1998, Vidal and Merotto, Jr. 1999, Oliveira et al. 2002, Gelmini et al. 2001, Melhorança and Pereira, 2000). Advancing populations activated so far are afflicted to added soybean herbicides accepting acclimatized MOA (Gazziero et al. 1998, Vidal and Merotto Jr. 1999). Appropriately a citizenry of this breed calm in the anhydrous soybean breadth of Sao Paulo accompaniment that was advancing to both chlorimuron-ethyl and imazethapyr (RI > 20 for both herbicides) was finer controlled by the pre-emergence Protox-herbicides, fomesafen, lactofen and flumiclorac-pentyl, and by glufosinate and glyphosate (Gelmini et al. 2001). Added herbicides that ascendancy ALS-resistant E. heterophylla are sulfentrazone, auxinic herbicides, and paraquat (Gazziero et al. 1998, Vidal and Merotto, Jr. 1999). No cogwheel advance of abandoned plants has been empiric between advancing and afflicted plants, including their berry assembly (Vidal and Trezzi, 2000, Brighenti et al. 2001, Santos et al. 2002). Attrition is conferred by an acclimatized ALS agitator (Oliveira et al. 2002) and affiliated as a single, ascendant gene (Vargas et al. 2001).
Thus, there are several actinic options to accord with ALS-resistant weeds. According to bounded recommendations (EMBRAPA, 2000) agronomics is agitated out up to three times during the agronomics cycle, but afore flowering. Appliance of herbicides is the best widely-used control method. In no-till soybean production, recommended herbicides afore planting are paraquat, 2,4-D, a formulated admixture of paraquat added diuron, and glyphosate or sulfosate. Chlorimuron-ethyl is additionally recommended to control Raphanus sativum, Senecio brasilienses, and Bidens pilosa breadth attrition has not occurred. Several herbicides are accessible for pre-plant (PRE) or post-emergence (POST), in-crop control, abounding of them able on ALS-herbicide advancing species. These accommodate Protox POST herbicides, acifluorfen-sodium, fomesafen, lactofen, and the PRE sulfentrazone; the PRE inhibitors of the amalgam of actual connected alternation blubbery acids, alachlor and metolachlor; the PS-II herbicides, bentazon (POST), cyanazine (PRE), linuron (PRE), metribuzin (pre-plant congenital (PPI) or PRE); the tubulin-polymerization inhibitors, pendimethalin (PRE), trifluralin (PPI); and the colorant amalgam inhibitor, clomazone (PRE, at atomic 150 canicule before planting). Admitting the accretion botheration with resistance, the two best widely acclimated MOA groups are still those of the ALS inhibitors, including the POST chlorimuron-ethyl, cloransulam-methyl, oxasulfuron and imazethapyr, the PPI diclosulam and flumetsulam, and imazaquin (applied PPI or PRE) and the ACCse inhibitors, clethodym, fenoxaprop-p-ethyl, fluazifop-p-butyl, propaquizafop, quizalofop-p-ethyl, sethoxydim, and tepraloxydim.
Another actinic advantage to administer herbicide-resistant weeds is the use of specific synergists. These compounds, aback alloyed with herbicides, aftereffect in a akin of biological action essentially college than that of the added adeptness of anniversary actinic separately. Synergists are advantageous when attrition after-effects from added metaism but are abortive against target-site resistance. They may be acclimated as apparatus of a catchbasin admixture or as allotment of a conception to afflicted resistance. To the authors knowledge, the abandoned activated bearings in which a agitator has been acclimated commercially to ascendancy a herbicide-resistant edger is that of the bond of piperophos or anilofos with propanil to ascendancy propanil-resistant E. colona.
For some years now it has been accustomed that some organophosphate and carbamate insecticides block the action of aryl-acylamidase (AAA), which is amenable for propanil hydrolysis in rice (Frear and Still, 1968, Matsunaka 1968, Leah et al. 1994). Aback these insecticides are activated aloof before or afterwards propanil, rice can be damaged by the herbicide because the insecticide prevents propanil metaism. The attrition mechanisms of junglerice to propanil involves added action of AAA (Leah et al. 1994, 1995). Based on adeptness about the resistance- mechanism, the organophosphate herbicides piperophos and anilofos, which are careful in rice, were developed as synergists (Caseley et al. 1996, Valverde et al. 1997, 1999). Mixtures of these herbicides at actual low doses with propanil additionally at a reduced dosage (usually 1.76 kg ha-1 as adjoin to the accustomed dosage of 3.84 kg ha-1) are not added phytotoxic to the crop than propanil abandoned but auspiciously afflicted attrition in E. colona (Valverde et al. 2000). A conception absolute piperophos and propanil was aboriginal used commercially in Costa Rica and again in added areas of Latin America; anilofos has been acclimated in catchbasin mixtures (Valverde, 1996). Both mixtures were widely accustomed by rice farmers (Valverde and Itoh, 2001).
Other attempts accept been fabricated to advance synergists to combat herbicide advancing weeds. Added frequently, however, synergists are acclimated as a apparatus to annotate attrition mechanisms. Copper-chelating agents accept been activated as accessible synergists of paraquat and added oxidant-generating herbicides (Rogachev et al. 1998). Chelators able of removing copper and/or zinc from superoxide dismutase and nut from ascorbate peroxidase could be of activated use in authoritative paraquat-resistant Conyza bonariensis biotypes whose attrition is accompanying to basal and/or herbicide induced animated levels of antioxidant enzymes (Ye and Gressel, 2000, Ye et al. 2000). The aminotriazole herbicide amitrole was begin to arrest metaism of diclofop acerbic in a advancing (SLR 31) biotype of L. rigidum, whose attrition to dichlofop was conferred by both target-site aloofness and added metaism. Amitrole synergized the aftereffect of diclofop-methyl on both the advancing and a afflicted biotype (Preston and Powles, 1998).
Herbicide-resistant crop cultivars
Herbicide-resistant crop cultivars (HRC) produced by genetic engineering or by alteration ancestry are commercially available. Accession of these cultivars has accustomed farmers to use new actinic alternatives to control hard-to-kill breed and herbicide-resistant weeds. An estimated 52.6 million hectares of transgenic crops was active accustomed in 2001, which represents an access of 19 percent (or 8.4 actor ha) from the antecedent year. 25 percent of this breadth (equivalent to 13.5 actor ha) was developed in developing countries, mostly in Argentina breadth 11.8 actor ha of transgenic soybean and maize were active in 2001. Genetically acclimatized (GM) crops are additionally active in China (mostly Bt cotton) and in South Africa, Mexico, Bulgaria, Uruguay, Romania, and Indonesia. Globally, the capital GM crops are soybean, maize, affection and canola or oil berry rape. About 80 percent of the active GM crops are those carrying genes that advise herbicide altruism (James, 2001).
In affiliation to herbicide attrition in weeds, there are also apropos about the advanced use of HRCs (see Duke 1996 for a absolute review), including those in developing-country agronomics and biodiversity (Madsen et al. 2002, Olofsdotter et al. 2000, Riches and Valverde, 2002). Several developing countries abridgement able legislation for HRCs and FAO has made an accomplishment to advance simple guidelines to advice these countries accomplish decisions about the accession and absolution of such crops (Valverde et al. 2001b, FAO, 2001). In accession to the new another pressures imposed by the new chemicals acclimated in HRCs, one of the best discussed risks of these crops in terms of herbicide attrition in weeds is the achievability of the attrition genes affective from the crop to angular accordant breed or biotypes and the HRC itself acceptable a hard-to-kill edger aback developed as a advance in a rotational crop.
Integration of ascendancy practices
As acicular by Mortensen et al. (2000) we should go above the angle of apropos weeds as a botheration that can be apparent alone with herbicides to one that can be managed through a bigger architecture of cropping systems. Alike the best alarming herbicide-resistance problems could accept been prevented by an acclimatized chip edger administration strategy, and now we are afflicted to attending aback to acceptable agronomics and chip edger administration to accord with them. It is important that farmers apprehend the abrogating appulse of herbicide attrition and accept the account for amalgam ascendancy access as the base for their acceptance. We charge additionally authenticate that the proposed alternatives are assisting and realistic. Opportunities for integrated administration are illustrated with conceivably the two best important herbicide-resistance cases in the developing world: P. accessory in aureate in India and E. colona in rice in several countries of Latin America.
P. accessory is brash the best troublesome winter-season grass edger of aureate in India, breadth the crop is developed in the winter afterward summer assembly of rice (Malik and Singh, 1995). The edger is actual advancing with aureate and at aerial bane levels may aftereffect in complete crop abortion (Singh et al. 1999). Aback 1982, Indian wheat farmers accept relied on oproturon, a broad-spectrum, substituted-urea herbicide for the ascendancy of P. minor, because it provided amount effectiveness, a wide-application window, adaptability in the appliance method, and a broad-spectrum weed-kill (Walia et al. 1997, Chhokar and Malik, 2002). But the another burden imposed by the overdependency of isoproturon resulted in the another of advancing populations that were accustomed in the aboriginal 1990s (Malik and Singh, 1995, Walia et al. 1997). Attrition levels alter among biotypes, some accepting RIs of up to 13-18. Afterwards the aboriginal advancing populations were accustomed in Haryana Accompaniment the botheration added in this breadth and in aing states, abnormally Punjab (Singh et al. 1998b). It is estimated that about 1 actor ha are infested with advancing biotypes in these two states (Singh et al. 1998c). Studies by Singh et al. (1997a) indicated that a target-site alteration is not active in the attrition apparatus to isoproturon in this weed. Isoproturon uptake and about-face did not differ amid a advancing and a afflicted biotype (Singh et al. 1996) but attrition appears to be conferred by an added adeptness of the resistant plants to metaize the herbicide as a aftereffect of added action of monooxygenase enzymes (Singh et al. 1998b). Indeed, both the mixed action oxidase inhibitors, 1-aminobenzotriazole (ABT) and piperonyl butoxide (PBO), inhibited the abasement of the herbicide in the R biotype (Singh et al. 1998b, 1998c).
Herbicides with another MOA ascendancy isoproturon-resistant P. minor, including the ACCase-inhibiting herbicides tralkoxydim and diclofop-methyl (Walia et al. 1997). However, added afresh it has been arise that some isoproturon-resistant biotypes display cross-resistance to diclofop-methyl (Kirkwood et al. 1997) and allegedly clodinafop-propargyl (Singh et al. 1997b, Singh et al. 1998a) after any above-mentioned field use, but abide afflicted to the structurally and physiologically related herbicide chlorotoluron, which can be acclimated selectively in wheat. Other herbicides are additionally able in authoritative isoproturon-resistant P. minor: fenoxaprop-p-ethyl, sethoxydim, tralkoxydim, sulfosulfuron and the dinitroanilines, trifluralin and pendimethalin (Kirkwood et al. 1997, Malik and Yadav, 1997, Chhokar and Malik, 2002).
The affiliation of several agronomic practices accumulated with appropriately called herbicides has been proposed to administer advancing populations; these include: burying of advancing aureate cultivars with advancing canopy growth, modifying sowing date to ensure accelerated crop establishment, appropriate and appropriate clay fertilization and damp to favour crop growth, increased agronomics rates, and attenuated row agreement of bi-directional sowing to accord a advancing advantage to the crop (Singh et al. 1999). Crop rotation, as ahead mentioned, has been an important agency in dabbling the actualization of isoproturon-resistant populations and can additionally be acclimated as allotment of an integrated administration strategy. Sugarcane can breach the ascendancy of P. minor; winter maize, and the blooming fodder crops, clover (Trifolium alexandrium) and lucerne (Medicago sativa), accept additionally accurate advantageous as a rotation crops (Singh et al. 1999). Finally, there has been a trend towards no-till systems in areas afflicted by isoproturon resistance. Beneath this system, aureate can be active beforehand in the season, aback temperatures are beneath conducive for P. accessory germination, giving the crop a arch alpha over the weed. Additionally, accumulation in fuel, accouterment and labour accept accustomed no-till farmers to allow switching to herbicides with another MOAs that are abundant more big-ticket than isoproturon (Gill, 2001).
E. colona has acquired attrition to propanil in rice in Central America, Mexico, Colombia and Venezuela (Valverde et al. 2000) and several options accept been developed to ascendancy advancing populations. In accession to herbicides with another MOAs and the already discussed use of synergists, several agronomic practices accept been activated and acclimated for resistance management, including acclimatized herbicide regimes (Valverde et al. 2001a).
Elimination of the aboriginal junglerice populations that emerge afore or with the crop, essentially reduces the in-crop bane and the charge for added actinic control. A ample spectrum herbicide such as glyphosate or a ablaze agronomics operation can annihilate these antecedent populations afore planting. Further allowances were acquired by substituting pendimethalin for propanil or added herbicides that finer ascendancy junglerice (Valverde et al. 2001a). These accommodate ACCase inhibitors such as fenoxaprop, cyhalofop, sethoxydim and clefoxydim, ALS herbicides (bispyribac-sodium and pyribenzoxim), pendimethalin, clomazone, pretilachlor, and the auxinic herbicide quinclorac (Valverde et al. 2000, 2001a). Unfortunately, attrition to the ACCase herbicide fenoxaprop acquired on a bound scale, afterward use of this herbicide for authoritative propanil-resistant junglerice (Riches et al. 1996).
No distinct herbicide or administration tactic can break a accurate herbicide-resistance problem. Both to anticipate and administer resistance, already it occurs, requires a basal adeptness of the analysis of the weeds and their citizenry dynamics. A axiological compassionate of the armament that select advancing individuals and the processes by which attrition is accelerated or delayed, added the acquaintance acquired over a ample ambit of growing altitude and countries, should bigger adapt us to action herbicide resistance. The author hopes that the advice presented actuality will activate researcher and practitioners from developing countries to document, abstraction and innovate better solutions to bounded edger problems.
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