Control strategies. Advantages of CA on sustainable weed management ... In
addition, at harvest, weed seeds often contaminate the crop produce. Thus,.
Conservation of natural resources for sustainable Agriculture what you should know about…
Integrated Weed Management Importance of weeds Basic knowledge on Integrated Weed Management Weed Inventory Weed Bio-Ecology Weed Interference Control strategies Advantages of CA on sustainable weed management References Further reading
Importance of weeds and their management Those plants that interfere with human activity in crop and non-crop areas are considered weeds. Weeds compete with crops for soil nutrients, water and light; they host insects and plant pathogens harmful to crop plants, and their root exudates and/or leaf leachates may be toxic to crop plants. Weeds also hinder crop harvest and increase the costs of such operations. In addition, at harvest, weed seeds often contaminate the crop produce. Thus, the presence of weeds in crop areas reduces the efficiency of inputs such as fertilizer and irrigation water, enhances the density of other pest organisms, and finally severely reduces crop yield and quality (Labrada and Parker 1999). In any cropping system various operations are devoted to the control of weeds. Land preparation and inter-row cultivation procedures are mostly aimed at controlling weeds. However, in conservation agriculture the number of tillage operations is reduced and thus the weeds should be controlled by other means. It is thought that reduction of tillage operations may bring about higher weed infestation. The fact is that conservation agriculture demands new approaches for preserving soil fertility as well as for weed management. Indeed, reduced or minimum tillage, not well implemented, can increase weed infestation, particularly of perennial weeds, when combined with natural fallow. Conservation agriculture does not only base its efficacy on reduced tillage, but in combining it with the use of cover crops and crop rotation. It has been seen that minimum tillage may reduce weed stand better that the application of conventional tillage practics (figure 1). FIGURE 1 Number of weeds under different tillage regimes and different cover, 39 days after sowing (Skora Neto, 1993). 10 00
-2
We ed d ensity (pla nts m )
12 33 80 0
F a l lo w V e tch
60 0
L u p in O a ts
40 0
M u cu n a O a ts+ v e tch
20 0
0 D ir e c t s e e d i n g
C o n ve n tio n a l tilla g e
M i n im u m till a g e
Figure 1 clearly shows that Mucuna pruriens as a cover crop, used in direct seeding (Tools, machinery and equipment module), can effectively suppress weeds. However, Mucuna may increase weed infestation when combined with conventional or reduced tillage practices. For effective weed management it is extremely important to understand weed behaviour and its competition with crops.
Basic knowledge for integrated weed management Understanding weed behaviour helps to design adequate control. The main components to be taken into consideration are: 1. 2. 3. 4.
Identification of weeds and their level of infestation. Biology and ecology of the prevalent weed species. The competitive effects of prevalent weed species. Technically effective, economically viable and environmentally safe control strategies.
Weeds usually compete with the commercial crop for water, light, space and nutrients. These resources are obtained in a certain 'biological space'. Competition between weeds and the crop is variable and depends on the capacity of plants to occupy this space. Various characteristics are related to the success of weed species (Patterson, 1985): long dormancy period high seed scattering ability high genetic diversity so adapted to wide range of conditions high rate of reproduction reproduction through both seeds and vegetative material vigorous and rapid growth abilty to survive and reproduce under environmentally unfriendly conditions But the real success of weeds depends on their ability to invade and colonize - or dominate and persist - an area (Cousens and Mortimer, 1995). The mechanism of seed dormancy is the principal characteristic that ensures the survival of the weed species in agricultural fields. Without dormancy certain conditions could lead to the extinction of the species. Thus, dormancy ensures the maintenance of a certain seed bank in the soil, that is able to form a population in different periods of time and under different conditions. The soil is a reservoir for weed seeds as every year weeds set seed and scatter them over the area. These seeds: x stay on the surface, or x are incorporated with superficial tillage activities, or x are incorporated with deep tillage activities With direct seeding the reservoir of seeds differs from conventional tillage, because: x the weed seeds stay on the soil surface, where they are susceptible to attacks from insects, birds and soil organisms and to atmospheric influences x the soil stays covered with residues, which prevents light to reach the seeds and thus reduces germination x weed seeds already at certain depths are not brought to the surface again, where they could germinate x perennial weeds are no longer redistributed through equipment Weeds adapt themselves constantly to changes in their environment and a change from conventional tillage to conservation agriculture will generate a change in variety of species. With a mulch layer on the soil surface, as in Conservation Agriculture, the change in soil humidity and temperature and the interception of sunbeams are the principal physical factors that affect the germination of weed seeds. One of the chemical changes in the soil that affect the germination of weed seeds is the release of allelopatic substances.
Weed inventory A fundamental basis for sound weed management is to know the species present and the level of infestation. Weed identification can be important in terms of distinguishing perennial or parasitic weeds which will not respond to the standard traditional weeding practices, while more precise determination, even of annual weed species can be vital for the optimum selection of any herbicide. The exact levels of infestation are not generally so critical, but may need to be determined where economic thresholds have been established. Weeds can be counted or evaluated visually through an appropriate scoring system.
Weed Bio-Ecology It is vital to know the patterns of the different phases of the main weed species. These phases include: x dormancy x germination x seedling development x emergence x vegetative growth x flowering x seed setting x maturity, and x seed dispersal. Favourable or unfavourable influences of biotic and abiotic factors on each phase need to be understood. Terrestrial weed species persist in the soil by virtue of dormant structures, typically seeds or vegetative perennating organs such as rhizomes, tubers and taproots. In dense infestations, the banks of seeds or underground meristems from which new plants may be recruited into adult populations can be exceptionally large. Rao (1968) estimated that in Cyperus rotundus L. tuber populations of 10 million per hectare were possible, whilst Soerjani (1970) calculated that Imperata cylindrica may annually produce six tones of rhizomes per hectare. Typically seed banks of annual weeds in arable soils contain from 1000 - 10,000 seeds per m2 whilst in grassland the upper limit to this range may extend at least to 1 million per m2 (Mortimer 1994). Losses from the seed bank may result from germination, loss of viability in situ, and predation or fungal attack. Whilst the longevity of seeds of some weed species in the soil is known to be considerable (e.g. at least 20 years in Striga), many studies have shown that there is a constant death risk to seeds buried in the soil and the survivorship of viable buried seed populations may be conveniently described by the half life (the time taken for the population to decline by half, akin to radioactive decay). Meticulous experimentation (Roberts and Dawkins 1967; Roberts and Feast 1973) involving burial and retrieval of seeds have shown that half lives are species-specific and vary with depth of burial, tending to increase with depth in many species; and decrease with increasing frequency of soil cultivation. From an examination of fifteen common weed species of Nigerian farmland, Marks and Nwachuku (1986) concluded that tropical weed species may have considerably less seed longevity than temperate ones, although large data sets remain scarce. Eleven out of fifteen species exhibited half lives of less than 8 months and most seed banks were severely depleted after two years. Examination of the fates of buried seeds suggested that losses were due, in the main, to death in situ of dormant seeds. Such high decay rates clearly indicate the merits of fallow periods as a weed control technique in tropical agriculture and the implementation of conservation agriculture systems. In contrast to buried seed populations, the longevity of banks of meristem on underground perennating organs of weeds (e.g. tubers, rhizomes, creeping roots) may be considerable, particularly where apical dominance may suppress the development buds. The persistence of dormant buds is very much dependent upon the fate of the above ground shoots to which organs are attached. Regular cultivation that fragments perennial plants may release buds from internal dormancy and serve to exhaust bud banks.
The ability to display discontinuous germination is a well known feature of many (but not all) weed species. Episodic seedling emergence from a persistent bank of propagules is a life history characteristic that may confer reproductive advantage in unpredictable habitats so as to maximise the chance of seeding adult plants (Figure 2). FIGURE 2 Scheme of weed cycle, its reproduction and ways of keeping up the weed seed bank. Seed rain
Migration
Viability Adults
Seed losses
Seedling survival Seedling Seedling emergence Seed bank
Migration
Seed survival
The possession of seed dormancy mechanisms confers two important ecological opportunities to weed species. The first is the ability to resist periods of adverse conditions and the second is the synchronisation of resistant and non-resistant stages with appropriate environmental conditions to maximise the chance of seedling establishment. Strategically, dormancy may be predictive or consequential. Predictive seed dormancy in weeds is generally referred to as innate dormancy (Harper 1959) and reflects adaptation to predictable seasonal environments, seeds entering dormancy in advance of adverse conditions. Contrastingly consequential seed dormancy (enforced or induced) reflects a response to adverse conditions and inevitably leads to persistent seed banks as opposed to transient ones (Grime 1989) of lateral or adventitious Germination patterns may result in discrete flushes of seedling emergence or, as is often the case of emergence of seedlings, in cohorts over an extended period. The chance of survivorship to flowering may relate to time of emergence in response to climate as well as weed control practices. In Avena fatua there is a higher risk of natural mortality of autumn emerged seedlings due to overwintering conditions than in spring emerging plants. Critical comparative analyses of the relative growth rates of weeds and crops are relatively few and often difficult to interpret because of variation in experimental conditions. Whilst rapid growth in the vegetative stage is to be expected in weed species, this is not to presume that weeds may uniformly accumulate biomass or leaf area at a faster rate than accompanying crops. For instance, Cousens et al. (1991) have shown that growth rates of above ground plant biomass in wheat and barley outstrips that in A. fatua but this dominance is reversed in later stages of development. The time required to reach reproductive maturity in weeds varies considerably and may be similar to the life of the companion crop or be considerably shorter. In the tropics, weed life cycles may be extremely short. Echinochloa colona (L.) Link, Setaria verticillata (L.) P. Beauv. and Dactyloctenium aegyptium (L.) Willd. may approach flowering in 30-45 days whilst Rottboellia cochinchinensis may produce mature seed within 50 days of establishment (Fisher et al. 1985). Similar short duration life cycles may be observed in
weeds of temperate latitudes (e.g. Capsella bursa-pastoris L.) but weeds of major importance tend to have an extended growing season approaching at least 6 months. Since weeds that rapidly establish with the crop may be strong competitors and reduce crop yield, agriculturalists have long favoured prophylactic weed control procedures as seen in the preparation of clean crop seed beds. Tillage practices serve to destroy existing weeds and to distribute weed seeds at depth within the soil profile from which they may fail to emerge. The fraction of seeds that germinate from the buried seed bank and successfully establish seedlings is very often a small fraction (1 -10%) of the total available in species exhibiting a buried seed bank. In a few species such as Bromus spp. and Agrostemma githago L. the reverse is true and populations annually renew from a transient seed bank. In these species soil inversion by deep ploughing may place seeds at depths from which seedlings fail to emerge and populations rapidly decline. In consequence tillage practices may exert considerable mortality on weed seed populations and a weed flora may rapidly respond to changes in tillage practice (Forcella and Lindstrom 1988). According to the adopted tillage weed seeds will be placed differently in the soil. Under minimum or zero tillage, weed seeds will be found in shallow soil layers, something that enables easier seed germination. Conventional operations tend to incorporate seeds at the soil depth of 15-25 cm. Most of these seeds do not germinate at such depth, and wait until they are brought back close to the soil surface to germinate. Seed and bud production constitute the multiplicative phase in the life cycle, and regulation of reproductive output arises naturally from intra and interspecific competition in the weed crop community and from losses during and after harvest. Studies of weed-crop competition suggest that seed production per plant of many arable weeds may be densitydependent over the range of 1 - 100 adult plants per square metre. Losses of seeds at harvest may arise through removal of weeds by crop combining and may account for significant fractions of seed (up to 40 % of seed production, Howard et al 1991) in species which do not disseminate prior to harvest.
Weed Interference Weeds cause “hidden” losses in contrast to the damage done by insects, rodents, plant diseases and most other pests, the farmer does not see the productivity decline due to weeds. The importance of timely weeding to remove most if not all of the negative effects of weeds on crops has also been "hidden" in that it is often unrecognized by farmers. An understanding of the principles and some of the complexities of the interactions between plants will increase the awareness of the importance of weed interference in agricultural systems. In recent years weed scientists have done numerous studies on the inhibitory effects certain chemicals in weeds may have on crops or other weeds. This phenomenon is known as allelopathy and may have significant effects on some species in certain situations. The combined effects of competition and allelopathy are called weed interference. The awareness that weeds compete with crops is probably as old as the domestication of crops and the development of non-nomadic agriculture. Weed management was born to increase productivity by removing competition. It is well-established that weeds cause most injury to crops during certain crop growth stages and control during this period is especially important. One of the most studied aspects of interference is that of the length of weedy and weed-free periods (figure 3). For several annual crops, the critical period of competition is approximately equal to the first one-third to one-half of the life cycle of the crop. For example, in rice and maize which often take 100 to 120 days to mature, keeping the crop free of weeds for 30 to 40 days usually assures near maximum productivity (Doll 1994). First ascending and descending sections in figure 3 show the Critical Period of Weed Competition. The last ascending arrow shows weed-free period, which indicates that for obtaining high crop yields there is no need to weed the whole cycle. FIGURE 3 Critical period of weed competition.
100
% yield
80 weedy
60
weeded
40 20 0 0
20
40
60
80
100
120
Days after sowing
This general rule of the critical period of competition being one-third to half the life cycle of the crop varies considerably among crops. For example, cassava is planted in relatively wide rows and grows slowly compared to most staple food crops. In Cameroon, three weedings in cassava at 4, 12 and 20 weeks after planting gave optimum yields (Ambe et al. 1992). If only two weedings were done, timings of 4 and 12 weeks the crop yielded twice as
much as two weedings at 2 and 12 weeks. Thus weeding in this crop can start later than in other crops with less impact on yield. The critical period also varies between species. Aggressive perennial weeds such as Cyperus rotundus L. or Convolvulus arvensis L. will need more frequent weeding than would annuals because perennials regrow several times from food reserves in underground storage organs. On the other hand, fields with very low weed pressure do not require as intensive nor lengthy control practices as do those with abundant weed pressure. If soil nutrients and moisture are abundant, weed competition is less important. However, in many tropical and subtropical areas, soils are nutrient-poor and water may be scarce and thus competition is critical. On the other hand, applying fertilizers or irrigation water to increase crop yield will fail to reach maximum benefits unless weeds are adequately managed. PLATE 1 Soil tillage results in the incorporation of seeds and vegetative material at a depth that is related to the type of tillage. However, the total extinction of a weed is a very difficult objective to achieve. S. Vaneph
Control strategies and Integrated Weed Management Integrated Weed Manaement (IWM) is defined as a system of sustainable weed management that combines judiciously various control strategies in order to reduce the impact of the weeds to an economically acceptable level. The concept of integrated weed management (IWM) has been around for a long time but has not been taken very seriously. The major reason is that herbicides have generally been effective and relatively non-laborious means for weed control in crops. Traditionally, tillage and other control operations have been integrated with herbicide use as a means of weed control. Producers adopting reduced or zero tillage systems can no longer depend on these practices as components of an IWM system. The goals of an IWM system should be to reduce the movement of weed seeds into the soil and to reduce the impact of weeds on crops to an economically acceptable level. The emphasis should be on management rather than eradication. There are two major approaches for weed control: x preventive weed control x control techniques pre and post crop planting Preventive methods Prevention and sanitation are very important components of an IWM system. The use of clean crop seeds free of weed seeds and preventing the entry of machinery from heavily infested fields into low infested ones are among the major preventive activities to be undertaken. The problem is the weed seed bank in the soil, and anything done to reduce it, will undoubtedly result in less weed interference and better crop growth. Other control strategies The main weed control methods applied before and during the crop cycle are: 1. Cultural methods (crop rotation, good crop stand and row-spacing, intercropping, cover crops, mulches and others). 2. Physical control (mechanical and manual weeding). 3. Chemical control through the use of herbicides. Crop rotation.. This is a key method for controlling weeds. With the introduction of herbicides it was thought that crop rotation could be avoided, but life has demonstrated the opposite. Good preceding crops are important in the reduction of weed infestation and helps crops to compete better with weeds. Normally certain weeds are better adapted to the environment created by a particular crop. Monocropping tends to increase population of those weeds well adapted to the crop. Crop rotations that include crops morphologically and physiologically different, serve to break the cycle and adaptation of several weed species. Crop rotation has a clear effect on the suppression of weeds as is shown in figure 4.
Number of broad leaf weeds per 2m
FIGURE 4 Effect of rotation and tillage practices on the number of broad leaf weeds (Ruedell, 1995) CT= conventional tillage; DS= direct seeding 90
CT without rotation CT with rotation DS without rotation DS with rotation
60
30
0 0
3
5
9
Time (years)
FIGURE 5 Effect of rotation and tillage practices on the number of grass weeds (Ruedell, 1995) CT= conventional tillage; DS= direct seeding Number of grass weed plants per m2
900
CT without rotation CT with rotation DS without rotation DS with rotation
600
300
0 0
3
Time (years)
5
9
Crop stand An extremely important measure is to have a good crop stand and, subsequently, good row spacing. Any space left in the field will be normally occupied by weeds, and their reproduction may become a reservoir of seeds and a factor for their further spread. Good crop stand is also a guarantee of crop ability to compete with weeds even emerging early in the crop cycle. Narrow spacing in case of cereals may also be a possible means for preventing weed development. The tendency in the past was to keep 70-90 cm row to enable farmers passing with their machinery for inter-row cultivation, something that changes when using conservation agriculture. Covers or natural mulches are produced mainly by leaving crop residues on the land, chopping or slashing them if needed, for example with a simple cutting roller. These materials are not incorporated into the soil as in conventional agriculture, but they are gradually consumed by the soil mesofauna. Besides protecting the soil and the crop against erosion and water loss by runoff or evaporation, the soil cover also inhibits the germination of many weed seeds, minimising weed competition with the crop. During the first couple of years of conservation agriculture on a field, usually the stock of viable weed seeds near the soil surface gradually declines. Some cover crop residues contain compounds known as allelochemicals, which suppress the growth of other plants (Almeida, 1988). For example, rye straw suppresses the growth of many broadleaf weeds, but care should be taken, because allelopathic effect may also be exerted over some
susceptible crops, such as vegetables and legumes. Other cereals, such as oats and wheat also have demonstrated allelopathic properties under some conditions. PLATE 2 The amount of soil cover in conservation agriculture saves time and labour during weeding operations. A. Calegari
The best way to take advantage of allelopathy is to mow or spray the cover and manage it as mulch, instead of incorporating it. Most research shows the allelopathic effect may last about a month. The selection of cover crops is of vital importance in areas where other control methods, e.g. use of herbicides, are not affordable by farmers. Therefore a good crop rotation using cover crops during fallow periods is a sustainable way to also reduce weed pressure. In Paraguay herbicide use was reduced by taking up sunflower (Helianthus annuus) and Crotalaria juncea as cover crops in production systems of soyabean/wheat/soyabean and maize/wheat/soyabean (Kliewer, et al. 1998). PLATE 3 Not well-managed residues of black oats on the left, allows the germination and growth of Cyperus rotundus. On the right the weed gets less chance to germinate because of the allelopathic effect and the physical barrier the residues provide. A.J. Bot
Grass covers like black oats (Avena strigosa) and rye (Secale cereale) are highly efficient in suppressing weed seed germination. Their effectiveness is usually higher than that of legume covers. However, some other crops, such as oil radish and lupin, have shown a high depressive effect over many weeds.
Percentage of the area covered by wee
FIGURE 7 Weed Infestation under different covers (Almeida, 1988). 100 80 60 40 20 0 Fallow
Wheat
Lupin
Field pea Serradella Lathyrus Oil radish
Rye
Oats
The type of cover may also affect the composition of weed species as shown in figure 8. Mulches of broad leaf species affect less population of grass weeds, while broad leaf weeds prevail under cover of grass mulches.
Percentage of area covered by weeds
FIGURE 8 Percentage of soil covered by different weed species under different covers, 100 days after cover crop management (Almeida, 1991). 100
Graminae Broad leaf species
80 60 40 20 0 Lupin
Oil radish Field pea Lathyrus
Rye
Oats
Wheat
Physical control. Although conservation agriculture implies less use of machinery and implements for tillage, some manual weed control is still to be carried out, particularly in areas of small farmers. Farmers use several tools, among them knife roller or machete. More recently some other more productive tools have been developed like the motorized hand mower (plate 4), which can be used to control weeds in between crop rows. Farmers also use a roller with cutting blades to flatten and/or eliminate a cover crop. In some areas where conservation agriculture has been established more than five years ago, the use of chemical products is not anymore required and manual weeding using these tools is an economically feasible option. Herbicide use. Herbicides play an important role in controlling weeds during the first years after the adoption of conservation agriculture, at least, in large cropping areas where handweeding would be inefficient. Three to four years after starting CA, herbicide may still need to be applied in some environments, based on a location-specific knowledge of weeds.
In Brazil, where the extent of conservation agriculture has grown to more than 10 million ha over the last two decades, even after one or two years, the amount of herbicides used are generally reduced to about ten percent of the usual recommendations (through application in trouble spots only), and to zero after a few more years. Some farmers continue using herbicides, however, for example instead of using a roller with cutting blades to flatten and kill a cover crop. Several herbicides are used pre-sowing, pre-emergence or post-emergence and depending on the selectivity of the herbicide to be used. Soil-acting herbicides are mainly used in pre-sowing or pre-emergence treatments while post-emergence ones lack long residual effect in soils. More information about the characteristics of herbicide groups, their translocation, behaviour in soil and methods of application are found in chapter 10 “Herbicides” written by J.C. Caseley (see Weed Management for Developing Countries, FAO, 1994). With the introduction and use of herbicide-tolerant crops, in some countries as USA, Canada and Argentina, some broad-spectrum herbicides, such as gluphosinateammonium and glyphosate, are sprayed over the crops shortly after weed emergence. These herbicides lack soil activity against germinating weeds and may be necessary to be used twice according to weed flushes in the fields. FIGURE 9 Herbicide use in conventional systems and conservation agriculture (Quezungual) in Lempira Sur, Honduras (CDRULA, 2000). Herbicide use for land preparation Total herbicide use during production cy cle
-1
Cost of herbicides (U$ ha )
60
40
20
0
Burned fallow
Slashed fallow
Quezungual
Early herbicide application to eliminate weed competition in any system is a guarantee for vigorous early crop growth. Rational use of herbicides increases the productivity of the whole cropping process. Herbicides used correctly and at the right rates normally do not pose any problem to the environment. Soil-acting herbicides regularly decompose in soil in a period of 4-6 weeks after their application while most post-emergence ones are quickly dissipated in soil. The major problem with the repeated use of a single herbicide is the possibility of some weed species to evolve resistance. Several herbicide groups, such as sulphonyl ureas, imidazolinones, graminicides as ‘fops’ and ‘dims’, posses a great selection pressure and are able to create problems of resistance in periods of four-six years of their repeated
application. The problems of resistance are prevented mainly by crop rotation and avoiding the use of the same herbicide repeatedly. In conservation agriculture, herbicides can also be used to manage cover crops and to control their regrowth.
Advantages of conservation agriculture for sustainable weed management In conservation agriculture various agronomic procedures are included, which at the end favour reduction of weed growth. On one hand, crop rotation is useful to break the life cycle of weeds adapted to a particular crop and soil covers create a particular environment that inhibit weed seed germination either by preventing sunlight to the seeds or through the exudation of allelopathic substances. In addition, the use of herbicides is reduced and in long-term may be eliminated completely, and in small farm areas, weed pressure and manual weeding are greatly reduced. At early adoption of conservation agriculture, some perennial weeds may become a problem, and this will require use of particular systemic herbicides in order to exhaust their underground propagules. Once soil is not bare, such a population will tend to be reduced. Conservation agriculture has the following advantages: x weed seeds are no longer spread and incorporated in the soil, nor dug up to the soil surface or redistributed through roots parts x it allows the integration of different practices, which makes the system more sustainable PLATE 4 A motorized hand mower can be used to control weeds between crop rows. It is less efficient than a hoe but has a bigger operational return. A.J. Bot
Additional advice on weed management: 1. 2. 3. 4.
Survey regularly your areas to record major weed species in the field. Keep in mind that crop rotation is the key for good weed management. Select cover crops considering the prevailing weed infestation. Do no leave non-cropped spaces in the field and use correct seed densities as well as row-spacing. 5. Most post-emergence herbicides should be used at early weed emergence, although some systemic compounds as glyphosate are preferred to use a couple of weeks after weed emergence. 6. Pre-emergence and or pre-sowing soil-acting herbicides are best used with appropriate soil moisture.
7. To prevent problems of resistance it is important to avoid the use of the same herbicide repeatedly and year after year. 8. It is important to keep in mind that weed management should have as one of the main objectives sustainable reduction of weed seed bank and not only the control of weeds interfering during the critical periods of competition. 9. Perennial weed species may require the integration of various control methods to get the required reduction of their stand. 10. Preventive methods at field level should not either be neglected. FIGURE 10 Evolution of the number of weeds after the adoption of conservation agriculture (Skora Neto and Darolt, 1995).
Number of weed plants per m 2
600
400
200
0 1
2
3
4
5
6
T ime (years)
Figure 10 shows the reduction of weeds over time after the adoption of CA in four years. A population consisting of Brachiaria sp., Euphorbia sp., Digitaria sp., Richardia sp. and Sida sp. was reduced by 95 percent.
References Almeida, F.S. 1991. Controle de plantas daninhas em plantio direto. IAPAR Circular 67, Londrina. Almeida, F.S. 1988. A alelopatia e as plantas. IAPAR Circular 53, Londrina. Ambe J., A. Agboola and S. Hahn 1992. Studies of weeding frequency in cassava in Cameroon. Tropical Pest Management 38:302-304. 38 Balota, E.L., M. Kanashiro and A. Calegari. 1996. Adubos verdes de inverno na cultura do milho e a microbiologia do solo. In: I Congresso Brasileiro de Plantio Direto para uma Agricultura Sustentável. Ponta Grossa. Resumos expandidos p12-14. Caseley J.C. 1994. Weed Management for Developing Countries. Edited R. Labrada, J. C. Caseley & C. Parker, Plant Production and Protection Paper No. 120, FAO, Rome, pp. 183-223. CDRCDR-ULA. ULA 2000. Servicios financieros rurales y economía campesina sostenible. Un estudio de caso en el departamento de Lempira, Honduras. I. Informe principal. Centro de Estudios para el Desarrollo Rural, Universidad Libre de Amsterdam. 80pp. Cousens R.D., S.E. Weaver, T.D. Martin, Martin, A.M. Blair and B.J. Wilson 1991. Dynamics of competition between wild oats (Avena fatua L.) and winter cereals. Weed Research 31:203-210. 31 Doll J. D D. 1994 Dynamics and Complexity of Weed Competition. Weed Management for Developing Countries. Edited R. Labrada, J. C. Caseley & C. Parker, Plant Production and Protection Paper No. 120, FAO, Rome, pp. 29-34. Forcella F. and M.J. Lindstrom 1988. Weed seed populations in ridge and conventional tillage. Weed Science 36:500-503. 36 Grime J.P. 1989. Seed banks in ecological perspective In: M.A. Leck, V.T. Parker and R.L. Simpson (Eds.) Ecology of Soil Seed Banks pp xv-xxii. Academic Press. Harper J.L. 1959. The ecological significance of dormancy and its importance in weed control. Proceedings, 4th International Conference Crop Protection pp 415-520. Kliewer,I., J. Casaccia, F. Vallejos. 1998. Viabilidade da redução do uso de herbicidas e custos no controle de plantas daninhas nas culturas de trigo e soja no sistema de plantio direto, através do emprego de adubos verdes de curto período. In: I Seminário Nacional sobre Manejo e Controle de Plantas Daninhas. Resumos. Aldeia Norte (Ed), Passo Fundo. p.120-123. Labrada R. & Parker C C. 1994. Weed Control in the context of Integrated Pest Management. Weed Management for Developing Countries. Edited R. Labrada, J. C. Caseley & C. Parker, Plant Production and Protection Paper No. 120, FAO, Rome, pp. 3-8. Marks M.K. and A.C. Nwachuku 1986. Seed bank characteristics in a group of tropical weeds. Weed Research 26:151-157. 26 Mortimer A. M M. 1994. The Classification and Ecology of Weeds. Weed Management for Developing Countries. Edited R. Labrada, J. C. Caseley & C. Parker, Plant Production and Protection Paper No. 120, FAO, Rome, pp. 7-26. Rao J. 1968. Studies on the development of tubers in nutgrass and their starch content at different soil depths. Madras Agricultural Journal 55:19-23. 55 Roberts H.A. and P.A. Dawkins 1967. Effect of cultivation on the numbers of viable weed seeds in the soil. Weed Research 7: 290- 301. Roberts H.A. and P.M. Feast 1973. Changes in the numbers of viable weed seeds in soil under different regimes. Weed Research 13:298-303. 13 Ruedell, J. 1995.Plantio direto na região de Cruz Alta. FUNDACEP. 134pp. Skora Neto F. 1993 Coberturas vegetais em differentes sistemas de preparo do solo no controle de plantas daninhas. In: I Encontro Latino Americano de Plantio Direto na Pequena Propriedade. Ponta Grossa. Anais, p 173-188.
Skora Neto, F. and M.R. Darolt. 1995. Estratégias de controle de plantas daninhas em pequenas propriedades. In: I Seminário internacional do sistema plantio direto. Passo Fundo. Resumos p. 155-156. Soerjani M. 1970. Alang-alang, Imperata cyclindrica (L.) Beauv., pattern of growth as related to its problem of control. BIOTROP Bulletin 1, Regional Centre for Tropical Biology, P.O. Box 17, Bogor, Indonesia.
Further reading Report of the Expert Consultation on Weed Ecology and Management FAO, Rome, 2224 September 1997, FAO Plant Production and Protection Division Available in PDF from http://www.fao.org/ag/AGp/agpp/IPM/Weeds Report of the Technical Meeting on Benefits and Risks of Transgenic Herbicide Resistant Crops FAO, Rome, Italy, 16-18 November 1998. The report of the technical meeting on HRCs presents summaries and papers on the benefits and risks of HRCs use; and on regulations necessary for the introduction of HRCs. Available in PDF from http://www.fao.org/ag/AGp/agpp/IPM/Weeds http://www.fao.org/ag/AGp/agpp/IPM/Weeds Draft Guidelines for Hazard Assessment of Herbicide and Insect Resistant Crops. Available in PDF (English, French, Spanish) from http://www.fao.org/ag/AGp/agpp/IPM/Weeds Weed management management for developing countries. countries 1994. Edited by R. Labrada, J.C. Caseley and C. Parker, FAO Plant Production and Protection Paper 120. The book contains 18 chapters written by several outstanding weed scientists from all over the world.
