Present trends in weed management - R. Labrada

INTRODUCTION

Weed Management for Developing Countries was published in the year 1994, and since that time developments in weed management have taken place mainly through the application of new approaches and methods, rather than the introduction of new herbicides.

Weeds still represent an important constraint to crop production in the world. Agricultural practice has demonstrated that the same philosophy of integrated management used for insect control also needs to be adopted for weed control. Because of the vast number of serious weed problems in all regions of the world, just one method alone can no longer be relied on.

Moreover, agriculture has promoted new methods, which also directly or indirectly affect weed control. These methods are original per se and demand a new approach for weed control. For example, organic agriculture does not permit the use of chemical herbicides, therefore cultural and biological control are the only possible means left to cope with weeds.

In this chapter, the author would like to give an overview on the present situation in weed management in general, identifying the major trends and problems still to solve.

PROBLEMATIC WEEDS IN VARIOUS CROPS

Several weed species are still difficult to control. Crops like rice, wheat, maize, sorghum, vegetables, sunflower, and several others are seriously and persistently affected either by one or more weed species. Some of the existing problems in these crops arise from the incidence of herbicide-resistant weeds. A summary of the situation in selected crops, such as rice and wheat is given below.

Rice

Rice is the major staple food in the developing world and its production is essential for many economically poor countries. For some of these, chemical weed control has become a normal practice, where herbicides such as propanil, thiobencarb, butachlor, oxadiazon, among others are currently used in pre- or post-emergence applications. Although such treatments have increased farmers’ productivity they also have brought about a proliferation of some difficult-to-control species, for which chemicals do not appear to be a solution in the long term.

The most significant problem weeds in rice are found in the Echinochloa complex and include several annual and perennial species, the most important ones being Echinochloa crus-galli, widespread almost everywhere, and E. colona in hot-climate countries. These species, in addition, have evolved resistance to the most commonly used herbicides (propanil, butachlor, and various ALS and ACCase inihibitors). At this stage there is a need to implement various control methods to reduce the incidence of these species. The methods proposed by the FAO Technical Meeting on Echinochloa Management (Beijing, 2000) were as follows:

(a) Preventive measures such as enforcement of quarantine laws and regulations and field hygiene may help to arrest the spread of Echinochloa seeds and propagules from the source of infestation to areas devoid of the menace.

(b) Since irrigation water and farm machines are two primary sources of infestation of Echinochloa, care should be taken to educate farmers on the importance of keeping farm machines free from Echinochloa seeds and propagules.

(c) Over-reliance on herbicides for Echinochloa control, e.g. propanil, fenoxaprop-ethyl and others, may lead to an incidence of herbicide-resistant biotypes.

(d) Over-reliance on herbicides for Echinochloa control, e.g. propanil, fenoxaprop-ethyl and others, may lead to an incidence of herbicide-resistant biotypes.

(e) Crop rotation is the key method for depleting Echinochloa seed banks.

(f) Monitoring of herbicide-resistant Echinochloa populations should be encouraged among weed scientists and extension specialists to help develop databases on such populations.

Echinochloa spp is not the only problem weed in rice. The fact is that direct-seeded areas have increased during the last ten years because of a labour shortage in many Asian countries. This has led to more use of herbicides in rice as well, and the major weed problem in these areas is the so-called weedy or red rice.

The global workshop organized by FAO in 1999, Varadero, Cuba, concluded and recommended that:

i) Only through an integrated control approach in the reduction of weedy/ red rice infestation can this be achieved effectively.

ii) The main sources of weedy/red rice infestation are rice seeds contaminated with weed seeds and weedy/red rice seed banks in soils. Therefore any control measure should be aimed at a reduction of infestations from these sources.

iii) Experience in the control of this weed from other countries that use advanced technologies shows that not even one single weedy/red rice seed should be tolerated in rice seeds.

iv) The reproduction of basic and foundation seed should be carried out in areas that are totally free of weedy/red rice infestation. Certified rice seeds should be free of weedy/red rice seeds.

v) Control measures against weedy/red rice in the field are diverse and their implementation will depend on specific site conditions. However, any control measure should be aimed at reduction of weedy/red rice seed banks in soils in the medium- or long-term future.

vi) To date the, most efficient control measures have been those based on the combination of wet soil preparation to bring about the emergence of weedy/red rice seeds, followed by the application of herbicide over the weed stand and before rice seeding, and water management before and after seeding.

vii) Under upland and irrigation conditions, it is advisable to implement, whenever possible minimum tillage or zero tillage combined with the use of non-selective herbicides. This practice is cheap and sustainable by rice farmers.

viii) Within the context of integrated management it is necessary to conduct regular studies of the behaviour of available rice cultivars in terms of their ability to compete with weedy/red rice, life-cycle and tolerance to submersion during flooding.

Therefore, as can be seen for both weed complexes, the solution lies in integrated weed management, where the use of clean seeds, suitable crop rotation and land preparation play an important role, as well as a rational use of herbicides, thus avoiding problems of herbicide resistance.

Wheat

The major problems in wheat fields are associated with herbicide-resistant weeds. In this context, one of the most serious incidences is the one associated with isoproturon-resistant grass Phalaris minor in North India. The entire rice-wheat area of 10 million ha is affected by the resistant biotype of Phalaris in the Indo-Gangetic plains. The problem is more serious in a 5 million ha area. (Malik, R.K. personal communication, 2002).

To overcome this problem, Indian specialists have adopted an integrated approach for cropping wheat in rotation with direct-seeded rice. There are already more than 10 000 ha of wheat planted with the no zero till system in India, and it has brought about several benefits, such as improving water-use efficiency, preventing soil erosion and compaction, reducing the need for applying herbicides, and increasing wheat yields. The studies on long-term trials on zero till have shown that Phalaris minor stand decreased over the three-year period because of the combined effect of the herbicide clodinafop and zero tillage (Rice-Wheat Consortium and CIMMYT, 2003).

The most widely adopted cropping system is the rice/wheat system, with an area of 13.5 million hectares in South Asia and 10 million hectares in China. While rice cultures are of different natures (rainfed or irrigated) and duration, depending on farming situations, wheat is mostly irrigated. Because of a decrease in the total factor productivity, the returns from growing these two cereals are either low or negative, especially in the high-productivity zones of India, China or Pakistan. In the Eastern Sector of India and large parts of Nepal and Bangladesh, the productivity of both crops is low because of late plantings and the use of relatively less than the recommended rates of inputs. The intensification of cropping systems has led to a stress on natural resources, including land and water resources. Weed flora has become simple, with a dominance of annual grass weeds in both rice and wheat. The dominance of a few annual grasses with very high populations has led to the use of herbicides, which have not proved to be a sustainable proposition in the long term. Herbicide resistance has already proved that the economic risks and benefits of herbicides are not evenly matched. In addition, resource-poor farmers are unable to make a choice from a limited number of herbicides available to them, and often their continuous use may also lead to a contamination of the food chain. Therefore, farmers in these countries not only need technologies that reduce their cost of cultivation but also solve sustainability issues related to natural resources, risks associated with herbicides and environmental protection. Special attention has to be paid to the problem of herbicide resistance. Phalaris minor has evolved resistance to isopropturon, and this author, in recent visits to these areas, has seen that clodinafop is not so effective in several sites where this compound has been used during the last 3-4 years. Worst of all, no monitoring of herbicide resistance is in place, and farmers lack competent advice. There is a need to train extension workers and farmers in topics related to herbicide resistance to enable them to detect resistant species on time, and to replace herbicide use by other control methods, if they exist, using alternative modes of action.

Parasitic Striga in cereals and Orobanche weeds in various dicot. crops

The situation in parasitic weed control has not changed significantly during the past ten years. There have been some attempts to conform a single control method based on seed dressing with a herbicide and a safener for the crop seeds. This method is really effective in the control of Striga in maize (Kanampiu and Friessen, 2002). Unfortunately, there are some constraints on spreading the use of such a method among farmers. On the one hand there is a need to have some facilities for treating the seed - which farmers do not have - added to the fact that farmers will have to pay to dress their seeds and for the chemicals used. In Kenya, this method costs only US$4. If farmers can afford it, this may be a solution. Another problem could be Striga resistance to the proposed herbicide (imazethapyr) in the medium term. One may argue that rates are very low and that resistance will not take place, but experience has shown that although rates are important, the nature of the herbicide is important too. ALS and ACCase inhibitors have a big selection pressure and may evolve herbicide resistance in four to five years’ time. It is true that no perfect method exists, but we should be aware of problems may arise in an ‘x’ period of time in order to prevent them. Problems of resistance can quickly be overcome in agriculture in the advanced economies, but not in developing countries, because of a lack of resources.

It is for this reason that Striga parasitic weeds should be managed through the implementation of an integrated approach, where containment should play a basic role in the control and reduction of its seed banks (Ransom, 1996) while maintaining or increasing farm productivity. Containment can be achieved through the use of clean seeds and farm implements, while Striga seed banks can be reduced by preventing reproduction through hand-weeding and/or the application of 2,4-D in cereal crops sown alone, inducing suicidal germination with effective trap crops, and by enhancing the natural demise of seed by increasing the biological activity of the soil. Maintaining on-farm productivity is achieved by using resistant/tolerant cultivars, through strategic use of non-host crops, by delaying attachment with seed-dressed herbicides, and by improving the fertility status of the soil. Because of the varied agro-ecologies and farming systems where Striga is found, localized adaptation and integration of these components is required. Drs. Jurgen Kroeschel and Abuelgasim Elzein have provided an excellent overview of the problem of parasitic weeds in this volume, and have given interesting information of various control methods including a detailed one on biological control through the use of Phytomyza fly and/or Fusarium oxysporium isolates.

Taking into consideration the problems posed by Orobanche spp. in countries in North Africa and the Near East, we are also pleased to provide the following summarized information from the Regional Workshop organized by ICARDA-FAO, held in Rabat, Morocco, (April 2003) on the status of the control of these parasitic weeds. The main highlights are:

Algiers: O. crenata is a major problem in faba bean, lentil, chickpea and peas. This parasitic species has a high interspecific and low intraspecific polymorphism. Main control measures consist in the delayed planting of legumes and the use of post-emergence glyphosate in two applications. Some trials are going on for testing the efficacy of imidazolinone herbicides, such as imazapyr and imazethapyr. Some research has been carried out on the possible use of Smicronyx insects for biocontrol of Orobanche.

Egypt: Faba bean is the main crop affected by Orobanche crenata. The Egyptians have developed four resistant cultivars. They use delayed sowing date in irrigated areas only, and apply glyphosate at reduced rates (64 g a.i./ha) in two applications in some areas. The fly Phytomiza orobanchia is present in the country, but no work has been carried out towards increasing its population.

Ethiopia: Three Orobanche species have major incidence in the country: O. minor in ornamentals and sunflower, and O. ramosa and O. cernua in tomato where they may cause losses up to 60 percent of crop yields. These species also affect pulses (919 000 ha of faba bean, chickpea, lentil and others). At present, the Ethiopian specialists have concentrated their work in identifying resistant cultivars and the use of cultural control methods, mainly crop rotation.

Iran: Specialists from this country reported 36 species of Orobanche, the most important of which is Orobanche aegyptiaca affecting lentil, lucerne and chickpea. Other species present are O. nana in almonds (Prunus amygdalus), O. ramosa in cucurbit crops and O. cernua in sunflower and tobacco. Research is being carried out on the use of trap crops in crop rotation, use of Fusarium and/or Phytomiza for biocontrol, as well as crop rotation with rice and the use of flooding.

Morocco: Four species are predominant, namely Orobanche ramosa/O. aegyptiaca in solanaceous crops, O. crenata in pulses and O. foetida in clover. The last species can also be found in waste areas as a parasite of several wild plant species. The country has participated in a research programme with GTZ, which appears to have produced many publications and useful results regarding the use of resistant faba beans cultivars, delayed planting in irrigated areas, use of glyphosate and some field demonstrations, and other measures in the use of the Phytomiza fly.

Sudan: The main species are O. crenata and O. ramosa. The former affects various pulses (faba bean, chickpea and lentil) and the latter is mainly found in areas of solanaceous crops, such as pepper, tomato, potato, eggplant and tobacco. Main research is concentrated on resistant cultivars, delayed planting of faba beans, and the use of imidazolinones herbicides, such as imazethapyr (50-76-100 g. a.i./ha) pre-planting soil incorporated or pre-emergence. Trials with post-emergence glyphosate have also been conducted.

Syria: Tomato is seriously affected by O. ramosa/O. aegyptiaca and the main control method is hand-pulling in tomato, with or without black plastic mulch cover.

Turkey: There are four parasitic Orobanche species: O. ramosa/O. aegyptiaca in solanaceous, pulses and cucurbit crops, O. cernua in sunflower and O. crenata in faba beans and chickpea. Solarization is used widely as well as the Phytomiza fly. In some areas of tobacco and sunflower, imazapyr has been used at a reduced rate (0.1 kg a.i./ha).

Tunisia: This country shares the same problems with Orobanche as does Morocco. O. foetida y O. crenata are the main species affecting mainly Vicia crops and faba bean. The research programme conducted in the country is also similar to the one already described for Morocco. Various herbicides, such as glyphosate or sulphosate, and imidazolinones have been tested. Intercropping of faba bean with fenugreek seems to be good. The main practical control method is hand-pulling.

AQUATIC WEED PROBLEMS AND CONTROL

Floating weeds

Problems with aquatic weeds, particularly of the floating species, have increased enormously in many developing countries of Asia, Africa and even Latin America during the last ten years.

The major weed problem, and probably the major one worldwide, is the introduction and invasion of water hyacinth (Eichhornia crassipes) in many regions of the world. At present important water bodies in Africa are heavily infested by this weed, including Lake Victoria in East Africa, the river Congo in Central Africa, the river Niger and several other rivers in West Africa. The river Nile continues to be affected by the infestation of this weed. Huge amounts of the plant are mechanically removed annually from the Nile in Egypt.

In Latin America, the plant has disseminated rapidly in several countries of Mexico, Central America and the Caribbean. Although some natural enemies of the plant are found in these areas, heavy water pollution in dams and rivers enable the hyacinth to proliferate and to infest water bodies totally. Asia does not escape the problem either. Several regions of mainland China are seriously infested by the weed while nearly all the water bodies in Sri Lanka, India, the Philippines, Thailand, Malaysia and Vietnam present medium- to high-levels of infestation.

Other important floating weeds are water fern (Salvinia molesta) and water lettuce (Pistia stratiotes). These plants quickly reproduce in hot- climatic conditions, and may cover whole areas of water bodies over short periods of time. Water fern has been a serious problem in various country-sites, among them Papua New Guinea and more recently in the river Senegal. Water lettuce is found everywhere in tropical areas, and it reproduces quickly in heavily polluted water bodies.

Because of these existing problems, there are various national and regional programmes implemented for the control of floating weeds. The biggest has been the one initiated in Uganda with the support of FAO to control water hyacinth in Lake Victoria. The initial FAO project in 1991 led to another two FAO projects and it is now a component of big management programme for wetlands in Lake Victoria, involving three countries, Uganda, Kenya and Tanzania. Similarly other countries, such as Ghana, Côte d’Ivoire, Mali, Nigeria, Niger, Bénin, Togo and Burkina Faso have conducted either national or regional projects, again supported by FAO and in collaboration with other relevant institutions, such as CSIRO, IIBC and IITA. Here the recommended approach has always been integrated management, where the major component is classical biological control through the introduction of specific insects, such as weevils, Neochetina eichhorniae and N. bruchi. To this end, technical personnel have been trained in methods of insect rearing and release, and relevant rearing units have been set up.

Although biological control helps to reduce the infestation of water hyacinth, it is not enough in many areas because of the existing high stand of the weed and highly polluted water bodies. At this stage, other short-term control measures are required to integrate with biological control. These could consist in manual or mechanical removal, and a rational use of low-toxic herbicides. In Ghana, the use of glyphosate to control water hyacinth in areas with heavy infestations is accepted and approved, while mechanical harvesting is currently practised in Uganda. Manual removal is very frequently used in areas close to power supply stations, e.g. Sotuba in Mali and Owen Fall station in Uganda, where completely clean water is a necessity.

Experience with water hyacinth clearly shows that long-term programmes are required to reduce substantially the infestation of this weed. Single releases of the insects on the hyacinth will not alone solve the problem. A programme will need to be established for rearing/releasing the insects, with the involvement of local communities affected by the weed. Monitoring is another important activity that will need to be conducted on a regular basis, with the aim of arriving at suitable decisions.

Much better results have been obtained for the control of water fern and water lettuce using biological control agents, such as weevils, Neohydronomous affinis for the control of water lettuce, and Cyrtobagous salviniae for water fern. Good results have been achieved with both insects for the control of these two species in the river Senegal (Labrada and Fornasari, 2003). Over a period of 12-14 months, high control has been obtained in both cases following a massive release of insects.

Rooting weeds

In some areas where water depth is reduced various Typha species may become a serious problem.

The river Senegal has experienced heavy stands of water lettuce (Pistia stratiotes) and water fern (Salvinia molesta), both solved through the use of biocontrol agents, but it now has an increased infestation of Typha. This Typha infestation prevents easy and safe withdrawal of water from the river. Such stands also provide a congenial habitat for snakes, and the local population is, of course, fearful of this danger.

Unfortunately, there is no effective biological agent for the control of Typha, so control at this stage should be based on manual or mechanical removal. In Mexico and elsewhere, repeated mechanical cutting at a level of water surface during a one-year period is a good method to substantially reduce the stand of Typha spp. This procedure exhausts the existing nutritional reserves in the subterranean parts of the plant, eradicating the greater part of it.

The feasibility of the method here will depend on whether local communities will be able to afford the purchase of this equipment, as well as the fuel required for its operation.

Submerged weeds

There are several important submerged weeds, which cause detrimental impacts on water use. These species affect aquatic ecosystem and navigation by forming dense canopies. Dense mats seriously affect water quality, decreasing oxygen and increasing temperatures under the mats. Created canopies provide a good habitat for breeding mosquitoes. Some recreational activities, such as boating, or commercial, such as fishing, are also heavily affected.

The major submerged species are Elodea spp., Egeria spp., Hydrilla verticillata, various species of Potamogeton, Myriophyllum spicatum, Ceratophyllum demersum, and macrophytic and unicellular algae. High infestations of these plants occur in polluted and oxygen-poor waters.

There are several herbicides recommended for the control of submerged weeds, among them the granular dichlobenil and Z-propenal, usually known as acrolein, terbutryne and diquat. These compounds are applied directly to water and are effective in static waters. Diquat is not so efficient in the control of submerged Potamogeton. The best results with dichlobenil are obtained when the plants are first mechanically removed, and the granular herbicide is applied immediately after removal. Another good option for the control of Potamogeton is the use of herbivorous Grass Carp (Ctenopharyngodon idella).

In the United States, the exotic Hydrilla verticillata (native to parts of Asia, Africa, and Australia) has become a serious problem in several water bodies in California and Florida (Anon. 2003). Three herbicides here are approved for the control of Hydrilla. They are the systemic and slow-action fluridone, which is a very costly compound; the fast-acting Endothall, and Copper compounds, which are used alone or mixed with endothal. Copper is also effective in the control of algae. None of these herbicides affect the seeds, tubers, and turions of the weed, and repeated applications are needed to control hydrilla regrowth. Mechanical control is not the best method because the plant spreads through fragmentation. The most effective method against Hydrilla is the use of sterile, triploid grass carp (Anon., 2003). Other biocontrol agents have been explored since 1981 in the United States, one of them being the Hydrilla tuber weevil Bagous affinis. The adult lays its eggs on rotting wood and other organic matter and after hatching the larvae burrows through the sediment until it encounters a hydrilla tuber (Bennett and Buckingham, 1991). The insect feeds on the tuber, where it completes its whole cycle, finally destroying the tuber. The Hydrilla leafmining fly, Hydrellia pakistanae originally from Pakistan and India, was released in southern Florida in 1987 (Balciunas and Center, 1988). It has been found to attack the plant, but its impact is still unknown. Results from these insect releases are still being evaluated.

Increased use of biological control of weeds

A number of biological control programmes directed at various aquatic weeds have been successfully conducted worldwide. Some other terrestrial species, such as Chromolaena odorata are under control, using the insect Parechetes pseudoinsulata (Muniappan, 1994) in Guam. P. pseudoinsulata, the biological control agent of C. odorata released at Fumesua in the Ashanti region of Ghana in 1991 has effectively controlled the weed in a pilot project. The successful establishment of P. pseudoinsulata was the result of continued releases of large numbers of the insect on Chromolaena. The feeding activities of the insect have reduced the populations of Chromolaena from an average of 85 percent in infested fields to 32.9 percent in places where control of the weed has been achieved. Populations of other herb species in danger of extinction have increased from 13.0 percent to 38.0 percent, as well as grasses from 2.0 percent to 29.1 percent. (Timbilla, 1996). The introduction of P. pseudoinsulata to Indonesia has so far been successful only in North Sumatera while in Java it has not been reported as being established successfully (Tjitrosemito, 1996).

Another bioagent, the stem-galling tephritid fly Procecidochares connexa was released in Indonesia for the control of C. odorata for the first time near Marihat in northern Sumatra in January, 1995. It established quickly and easily, and now appears to be having an impact on the weed, especially where it occurs in conjunction with P. pseudoinsulata. Field releases were made using adult flies (Wilson and Widayanto, 1996).

Some attempts are being made for development in the control of Striga (Watson et al. 1998). A team of scientists funded by the International Development Research Centre has found that the fungus (Fusarium oxysporum) in the soil in Mali can suppress the weed’s growth. In a pilot study, the fungus was grown on sorghum straw and then spread on farmers’ fields at sowing time. Results obtained have indicated a suppression of Striga growth, resulting in dramatically increased sorghum yields.

ORGANIC AGRICULTURE

During recent years, organic agriculture has hugely increased in many regions of the world, and this trend has not yet ceased. The total area of organically grown crops is estimated to be around 8 189 717 ha (Table 1). Organic agriculture should be seen as a process depending highly on naturally-occurring biological processes. Therefore, the objective is to stimulate these processes to obtain maximum suppression of pest problems. Obviously, the use of chemical pesticides is contrary to the concept and the practice of organic agriculture, and human intervention in the process should therefore be carried out only very carefully.

Weed management in organic agriculture is not an easy task, particularly in areas where labour for hand-weeding is short or not affordable. However, the principle should be the same as in any conventional cropping system, i.e. weed competition needs to be prevented in order to obtain maximum crop yields. This necessarily implies weeding with non-chemical materials but, this has to be carried out exactly at the right time to eliminate weeds during the so-called ‘critical period of weed competition.’ Organic systems also require the use of preventive methods before growing the crop and to establish a reasonable crop rotation. Stale seed bed preparation in order to kill the weeds mechanically or manually is a very good option to delay the start of weed competition. The use of cover crops and green manure, in addition to increasing soil fertility, may help to control some weed species. The most common methods used to prevent weed competition in organically grown crops are high seeding rates, narrow seed spacing/cross seeding, and companion cropping with small-seeded legumes. In temperate areas sweet clover, medics and alfalfa are the most common companion crops. Other rotational sequences for crops grown in tropical and sub-tropical areas should be developed to guarantee vigorous growth of the crops as well as fewer pest problems, including weeds.

Table 1

Areas of organic agriculture worldwide
(Data processed from Yussefi Minou and Willer Helga, 2002)

USE OF HERBICIDES AND HERBICIDE RESISTANCE

For several economical reasons the agrochemical industry is not so active in promoting new herbicide molecules as it was in the past. The development of a new compound and the cost of its registration (detailed toxicological information with more requirements than in the past) make the process more costly economically. Thus the improvement of weed management, particularly in some developed economies in recent years, has been based mainly on the use of new integrated procedures, plus the introduction of transgenic herbicide-resistant crops.

In developing countries the use of herbicides has increased in crops such as direct-seeded rice, maize, cotton and soybean, mainly in Latin American and Asian countries, while in Africa the use of herbicides is still very poor.

The problems of herbicide resistance are increasing in several crop areas and regions of the world. There is a very good Website (http://www.weedscience.org/in.asp), which provides updated information on cases of resistance all over the world. In addition, the problem of resistance and its management is also discussed here in this present publication by Dr. Bernal Valverde.

Information available on cases of resistance also enables scientists to develop materials for preventing herbicide resistance in some crops. Research has also been initiated by FAO in order to provide comprehensive advice to the agricultural extension services of the developing countries.

THE USE OF TRANSGENIC HERBICIDE-RESISTANT CROPS (HRCs)

During the last eight years, genetically enhanced crops with resistance to broad-spectrum post-emergence herbicides such as glyphosate, have been developed through genetic engineering. This development enables farmers to use a non-selective herbicide applied selectively over the already emerged crop, and to easily implement zero tillage with subsequent soil protection. The use of some herbicides as glyphosate makes it possible to reduce the use of soil-acting herbicides and associated problems of carryover.

Herbicide-resistant crops offer the potential for simpler weed control, more effective management of problematic and resistant weeds, more timely weed control with potential to employ critical period, increased usage of minimum or zero tillage and avoidance of yield loss caused by current ‘selective’ herbicides (FAO, 1998).

Although the benefits are certain, herbicide-resistant crops (HRCs) may also pose direct risks to human health, and may cause indirect, ecological, and evolutionary risks, which have not received enough attention from regulatory agencies (Ford Denison, 1999).

There are several concerns with regard to the consequences of development and deployment of transgenic herbicide-resistant crops. Objections to the use of these transgenic crops rest on several issues related to the associated risks (FAO, 2001), such as the potential transfer of genes from herbicide resistant crops (HRC) to wild relatives, thus creating super weeds; and the possibility of HRC volunteers becoming weeds in subsequent crops. So at this point the risks may be various, and among these:

• troubles in managing herbicide-resistant volunteers;

• selection of weed flora in favour of less susceptible species or those with the ability to avoid the herbicide used in the resistant crop;

• evolution of herbicide resistant biotypes of existing previously-sensitive species;

• increased dependency of growers on herbicides;

• potential loss of landraces;

• potential development of multiple resistance (gene stacking).

In order to prevent such problems, a technical team from the Royal Veterinary and Agricultural University (KVL), Denmark, sponsored by FAO, prepared the initial draft of procedures for assessment of ecological risk of herbicide-resistant crops, which were then revised by several specialists from all over the world for their comments and suggestions, and thoroughly discussed and improved at a workshop organised by FAO and the Royal Veterinary and Agricultural University (KVL), Department of Agricultural Sciences (Weed Science), held in Copenhagen, Denmark, in September 2000. The final document is still in the final stage of its preparation, and it will establish the methods for assessing HRCs before their introduction into the agricultural practices followed in all countries.

These procedures identify two scenarios: Where the HRC is to be released into an agricultural system where there are compatible wild relatives or weed species, and where HRC is to be released in another agricultural system where there is minimal risk of gene flow to other species. Five keys were prepared for this assessment, each of them compatible with a specific likelihood.

A better overview of this issue is provided in this volume by Drs. K. Madsen-Haugen and J. Streibig, from Denmark.

CONSERVATION AGRICULTURE

The so-called conservation agriculture is gaining a lot of recognition among farmers all over the world. Grain crops as wheat, barley, maize, rice and soybean are grown in some parts of the world under this system, which consists of the judicious use of crop rotation with minimum or zero tillage, including the use of green manures and cover crops. This approach is beneficial to effectively protect and increase soil fertility. It is wrong to identify conservation agriculture as the practice of zero or minimum tillage. In fact these procedures are part of the system, but if they are implemented in areas of monocropping then this cannot be considered conservation agriculture. The use of herbicide-resistant crops (HRCs) combined with the application of broad-spectrum herbicides make the process of conservation agriculture easier, but it also runs the risk of bringing about new weed problems, either by a shift in the weed populations or the presence of species able to evolve resistance to the herbicides in use. Again it is not always necessary to use HRCs and herbicides, particularly when the system has been practised for ten years or more.

Although this development is real, several weeds still remain which seriously affect the production of a number of crops such as rice, with its incidence of red/weedy rice, and Echinochloa spp., parasitic Orobanche weeds found in sunflower, faba beans and solanaceous crops; parasitic Striga spp. in cereals in Africa south of the Sahara, and Imperata cylindrica in many areas in Africa.

The switch to zero tillage or direct seeding practices concerns many producers when it comes to controlling weeds. The loss of tillage as a method of weed control means that producers must adjust crop rotations, herbicide use, and other cultural practices to compensate. Perennial weeds may become a serious problem to overcome, and there is a need to implement additional cultural methods, such as the use of covers. Some annual weeds, such as wild oat and volunteer canola, also grow well under zero-till conditions.

Valuable information on cultural and preventive weed control methods is provided by Dr. Paolo Bàrberi in this volume.

BIBLIOGRAPHY

Anon. 2003. Hydrilla verticillata (Hydrilla). The Western Aquatic Plant Management Society. (available at http://www.wapms.org/plants/hydrilla.html)

Balciunas, J. K., & Center, T.D. 1988. Australian insects to control Hydrilla, an update.

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