INTRODUCTION
Imperata cylindrica (L.) Raeuschel, also known as speargrass in West Africa, alang-alang in Asia, and cogongrass in America, is a pernicious perennial grassy weed of significant importance in tropical and subtropical zones, as well as in some warm parts of the temperate regions of the world (Holm et al. 1977; Garritty et al. 1997). In these ecologies, I. cylindrica occurs in a wide range of habitats, which include degraded forests, grasslands, arable land, and young plantations. Normally, the grass does not occur in closed forests but frequently appears within a few years once the forests are opened up for agriculture or lumbering (Ivens, 1980). I. cylindrica is considered as the worst weed of southeastern Asia and the moist savanna of West Africa (Garriety et al. 1997; Terry et al. 1997). It is classified as the tenth most infamous weed in the world, which affects farmers who practice slash-and-burn agriculture (Holm et al. 1977) and among the nine grassy weed species that require additional effort beyond that needed to control other weeds. It is noxious because of its wide distribution and adaptation to a wide range of climatic conditions and soils, its high competitive ability with many crops, and its resistance to control. Technologies for combating I. cylindrica have been developed but very few have been widely adopted by small-scale farmers (Brook, 1989, Townson, 1991, Terry et al. 1997). This short review discusses the biological characteristics of I. cylindrica that have implications for its control and reviews the research progress and management options in small-scale farms in developing countries.
ECONOMIC IMPORTANCE
The harmful effects of I. cylindrica on crops are well documented (Holm et al. 1977; Townson, 1991). It negatively affects the growth of teak, cocoa, kola, coffee, cashew, oil palm, coconut, rubber, and Gmelina arborea (Komolafe, 1976; Holm et al. 1977; Townson, 1991). Yields of annual crops are severely reduced by competition from I. cylindrica. It caused yield reductions of 51-62 percent in maize when the crop was weeded 2-4 times (Akobundu and Ekeleme, 2000). Higher maize grain yield losses (80-100 percent) have also been reported (Koch et al. 1990; Udensi et al. 1999). Complete crop failure usually occurs when crops are grown in slashed plots without additional weeding. In cassava yield losses of 50-90 percent have been reported (Koch et al. 1990; Chikoye et al. 2001). In soybean, Avav (2000), reported yield losses of 29-53 percent in the middle belt of Nigeria.
In addition to crop yield losses, I. cylindrica increases the cost of crop production, reduces the market value of damaged tuber and root crops, and increases the risk of fire in perennial crops, plantations, and forest reserves. It readily burns, even when still green, destroying other vegetation while it regenerates very rapidly from its underground rhizome system, thereby displacing other plant species. Recurrent bush fires during the dry season cause considerable loss of organic matter, which results in soil degradation. It reduces the size of farms to that which can be adequately weeded by available labour. Mechanical injuries to the skin caused by rhizome ramets reduce the efficiency of planting, fertilizer application, staking, weeding, and harvesting in highly infested areas, resulting in increased labour demand and abandonment of land (Holm et al. 1977; Terry et al. 1997).
CHARACTERISTICS OF I. CYLINDRICA
Townson (1991) and Terry et al. (1997) reviewed the biological characteristics that make I cylindrica very successful. The weed shows wide genetic variability that allows it to adapt to a wide range of ecological and management conditions. It possesses five taxonomic varieties with var. major in Asia and var. africana in West African being the most serious. I cylindrica reproduces sexually from seed and vegetatively by rhizomes. Flowering is common after exposure to stress such as burning, overgrazing, drought and repeated slashing. It can produce as many as 3 000 seeds, which have little, or no dormancy period and which can remain viable for over a year (Santiago, 1965). The aggressive and invasive nature of I. cylindrica is attributed to its rhizomes. These are normally concentrated in the upper 15-20 cm of soil where they can remain dormant but viable for a long time (Ivens, 1980). Rhizomes have a high regenerative ability because of the numerous buds that readily sprout into new shoots after fragmentation by tillage or any other form of disturbance. Rhizomes are resistant to fire because of deep soil burial. Deep burial also makes I. cylindrica very resistant to most control strategies (Holm et al. 1977; Ivens, 1980). The ability of rhizome fragments to regenerate decreases with a reduction in length of rhizome segment. Longer rhizomes have better chances of sprouting because they have more carbohydrate reserves than short fragments (Ivens, 1975).
I. cylindrica can grow on soils with a wide range of nutrients, moisture and pH (Santoso et al 1997). Although sometimes reported to be a weed of poor soils, I. cylindrica probably dominates these areas because of lack of competition from other plant species that cannot survive on marginal land (Santoso et al. 1997). It is a poor competitor and is easily suppressed by other species on fertile soils (Eussen and Wirjahardja, 1973). It does not tolerate shaded environments because it assimilates carbon via the C4 photosynthetic pathway (Paul and Elmore, 1984). It is a strong competitor for growth factors such as water, nutrients, and light because it sprouts and grows more rapidly than most crops (C3 plants).
I. cylindrica is very successful in areas that are frequently burnt, overgrazed or intensively cultivated. Fire is an inexpensive and effective tool for clearing forests for agricultural activities. Burning increases soil fertility for a short period of time. Fire is used to remove excess vegetation in slash-and-burn agricultural systems. Pastoral farmers burn grasslands to stimulate the growth of young grass for their livestock while hunters may use fire to expose animals hiding in forests or grasslands. Fires usually become uncontrollable and spread to other areas resulting in large social costs, e.g. soil degradation. Burning destroys only the foliage of I. cylindrica; after burning, the weed sprouts again and produces fresh shoots and flowers. Burning also destroys other plant species that would otherwise compete with I. cylindrica. The rhizomes are resistant to burning, and are the primary means of perennation (Wibowo et al. 1997).
MANAGEMENT STRATEGIES
Methods of I. cylindrica control have been reviewed extensively by Brook, (1989), Townson (1991), Terry et al. (1997) and others. The key objective of any management strategy should be the destruction of rhizomes, which are the main organs by which the weed perennates and spreads. Control strategies should also be based on an integrated approach, as no single method can control I. cylindrica in a sustainable manner. Technology to control I. cylindrica has been developed and has been used successfully in large estates or commercial farms where there is an ample supply of labour, capital and herbicides. However adoption levels by small-scale farmers are still low. Selected management strategies are summarized below.
Fire prevention/control
Fire prevention or control is an important factor for I. cylindrica-dominated grasslands because it hastens the rate of natural succession to secondary forest that would eventually shade and suppress the weed. Preventing the start of wildfires and the suppression of those that start can reduce burning. Fire prevention schemes should normally be targeted at people since they cause most fires. These schemes can be based on education and enforcement (Anon. 1996). Education should increase awareness of why people should not start fires and train people in fire prevention practices. Enforcement should ensure that people who live in fire-prone areas comply with the fire codes and regulations. Wibowo et al. (1997) suggested that fire prevention strategies of enforcement and education should be developed and applied at field/farm, community and government levels. At farm/field level, the use of firebreaks created by removing foliage by slashing, tillage and controlled burning can prevent fires from spreading to large areas. In agroforestry settings, this may be achieved by growing food crops between trees. The community and government institutions should be more concerned with enforcement of fire codes, regulation, and educational campaigns. In low-input agriculture based on slash-and-burn, there is a need to develop cheaper ways of clearing vegetation other than burning.
Slashing/cutting
Slashing followed by burning is a common practice for clearing the foliage of I. cylindrica on arable farms before tillage or sowing. To be effective and exhaust carbohydrate reserves in the rhizome, slashing must be repeated at frequent intervals. For example, Soerjani (1970) suggested an interval of two weeks over a period of three years. Slashing is labour intensive requiring as many as 75-man days/ha and cannot be applied to large areas (Brook, 1998). Repeated slashing also induces flowering and hence can facilitate the spread of the weed. Slashing should be integrated with other options to reduce the amount of labour required.
Pressing/Flattening
Pressing is accomplished by bending the culms (stems) of I. cylindrica at ground level. If the stems are tall (1 m or more), their own weight helps keep the grass flattened (Terry et al. 1997). In the middle belt of Nigeria, farmers bend the foliage of I. cylindrica at the beginning of the rainy season immediately followed by shallow tillage where soil is placed over the stems to keep them flattened. Tillage is carried out four weeks later, to completely cover the foliage and rhizomes. (T. Avav, personal communication). Regrowth of I. cylindrica after flattening is 20-60 percent lower than after slashing and it is cheaper and faster than slashing. It reduces the fire risk and facilitates the establishment of cover crops. Pressing can be done using planks, logs or drums (Friday et al. 1999).
Tillage
Tillage, if not preceded by slashing or burning, has the role of knocking-down the foliage as well as damaging the rhizomes and preventing their regrowth into new shoots by fragmentation, desiccation, and deep burial. Tillage should be to a depth of about 30-40 cm, since most rhizomes are found above this depth. Rhizomes should be broken into the shortest fragments possible and buried as deeply as possible. Ivens (1975), reported that rhizome sections of 2-3 nodes could not sprout and 77-84 percent of these rotted within two months when planted at a depth of 7.5 cm. Longer rhizome segments need to buried to a depth of 20 cm. Numerous tillage operations may be necessary for complete control, depending on prevailing conditions and other control options available. For example, Terry et al. (1997) recommended disk ploughing twice to a depth of 30-40 cm at an interval of two weeks and at right angles to the first cut, followed by harrowing twice at intervals of two weeks. Tillage is best done at the beginning of the dry season when most of the plants’ biomass is in the rhizomes and desiccation is most effective (Terry et al. 1997). In root and tuber crops in West Africa, farmers normally ridge or mound their fields at the end of the rainy season (October and November) while the soil is soft. This operation fragments the rhizomes into small sections, and brings them out to the soil surface where they are desiccated by sunlight over a period of 4-5 months and generally reduces re-infestation during the subsequent rainy season (T. Avav, personal communication). In small-scale farms, most tillage operations are carried out using hand-held hoes or ox-drawn implements while on commercial farms, tractor-drawn implements may be used. The disadvantages of using tillage to control I. cylindrica are: (i) manual tillage by hoes is laborious and does not affect the rhizomes; (ii) it takes a long time to get acceptable control; (iii) it has be repeated several times; and (iv) it is expensive and may promote soil erosion (Townson, 1991; Terry et al. 1997).
Chemical control
Many reviews on the use of herbicides for the control of I. cylindrica are available (Brook, 1989; Townson, 1991; Terry et al. 1997). Herbicides are quicker, cost-effective, and disturb the soil less where erosion may be of concern (Townson, 1991). Several herbicides have been tested alone (for example, paraquat, fluazifop-butyl, glufosinate-ammonia, dalapon, imazapyr, glyphosate, sulfometuron, nicosulfuron, and rimsulfuron) or in mixtures for the control of I. cylindrica. A few of these have shown poor-to-good control, depending on the rate of application, climate, and soil type. Repeated or sequential applications are usually necessary to have good control of I. cylindrica. Imazapyr and glyphosate appear to be the most promising herbicides for I. cylindrica control because of their ability to translocate to the underground rhizomes. Imazapyr at 0.5 -1.0 kg/ha and glyphosate at 1.0-1.8 kg/ha provide good control lasting up to 12 months, depending on soil type, application rate, and environmental conditions (Udensi et al. 1999; Terry et al. 1997). The long-lasting soil activity of imazapyr may be good in plantations but not in arable farming where it inhibits the establishment of arable crops. Glyphosate is the mostly widely used chemical for I. cylindrica control worldwide. It may be attractive to smallholder farmers because it has little or no soil activity and thus no carry-over effects to crops sown after its use. In addition, the efficacy of the herbicide is not dependent upon the volume of the carrier; hence it can be applied using weed wipers or in low volume as well as high volume sprays. After glyphosate application, supplementary weeding is still required in the crop to control shoots that escape the initial pre-planting application. Various innovations in application technology have also been evaluated to increase the efficacy of most herbicides. These include the use of adjuvants, low and ultra-low volume sprayers, and rope wipers. The results have been variable and sometimes conflicting. Despite the several advantages of glyphosate, it suffers from some disadvantages in that it has a high cost relative to other herbicides, and also it requires a rain-free period of six hours after application. Fluazifop-butyl (fusilade) is an option for post-emergence control in soybean. For example, at 0.375 kg/ha it provided 51 percent to 83 percent control of I. cylindrica in soybean, which was comparable to pre-planting application of glyphosate at 2.16 kg/ha in Nigeria (Avav, 2000). Shilling and Gaffney, (1995), reported that fulisade suppressed I. cylindrica for only three months. In maize, post-emergence application of nicosulfuron gave good control at rates of 70-400 kg/ha in West Africa (A. F. Lum, personal communication). The use of herbicides requires capital for purchasing sprayers and herbicides, new skills, and technical support, not all of which are available to most small scale farmers. The farmer’s only option is to use labour-based control methods, which are tedious, and only practical on a small scale.
Shade-based management practices
I. cylindrica is sensitive to shading and therefore it usually dies when subjected to shading for a long time. It may take up to 8-10 years for the weed to die out and be replaced by natural forest (Dalziel and Hutchinson, 1937). This sensitivity to shading can be exploited in its control by the use of fast growing cover crops, shrubs, or trees. The use of planted fallows for I. cylindrica suppression has been reported extensively by Koch et al. 1990; Anon. 1996; Macdicken et al. 1997; Akobundu et al. 2000; Chikoye et al. 2001 and others. Promising species include Mucuna spp., Calapogonium mucunoides, Centrosema pubescens, Pueraria sp., Lablab purpureus, Psophocarphus palustris, Stylosanthes guyanensis, Cajanus cajan, Crotalaria spp. and Moghania macrophylla. Mucuna spp. are prominent among the cover crops that have been promoted to smother weeds particularly in West Africa, because of their ease of establishment, faster growth rate, and higher biomass production. Important lessons learned from the use of cover crops in West African experiences are as follows:
Some cover crops need to be weeded until they form a canopy that has ability to suppress the weed.
Variations occur within and between cover crops’ ability to suppress I. cylindrica (Chikoye and Ekeleme, 2001).
Benefits (reduced weed pressure) are realized after at least 1-3 seasons or years of growing cover crops, depending on location, type of cover crop, and cover crop accession (Chikoye et al. 2002).
Labour investments for weeding subsequent crops may be reduced by up to 50 percent (Akobundu et al. 2000).
Cover crops are effective only when the mulch is not burnt by bush fires during the dry season.
The smothering effect of cover crops on I. cylindrica is sometimes as effective as glyphosate at 1.8 kg/ha (Udensi et al. 1999).
Chemical control is cheaper than the use of cover crops in areas where labour is scarce (Chikoye et al. 2002).
Yields of crops grown after leguminous cover crops are usually higher than plots without cover crops owing to reduced weed pressure and probably increased soil nitrogen and moisture retention (Chikoye et al. 2002).
Despite the demonstrated benefits of cover crops on I. cylindrica, the technology has not been widely adopted by farmers, except for certain parts of Benin, West Africa. Some factors that prevent the adoption of Mucuna spp. are:
it does not completely eradicate I. cylindrica;
volunteers can smother companion crops and lead to serious yield losses;
scarcity of land and land-tenure system;
toxicity of grain for human and animal consumption;
destruction of accumulated mulch by bush fire in the dry season, and
inability to inter-crop Mucuna spp. with short-stature crops such as cowpea or yam.
Furthermore, repeated use of Mucuna spp. on the same land may lead to outbreaks of pests and diseases (Vissoh et al. 1998).
Alley cropping is an agroforestry system in which food crops are grown in alleys formed by hedgerows of trees or shrubs (Kang et al. 1981). Farmers may easily adopt this technology because it retains the basic features of bush fallow such as nutrient recycling and weed suppression. Examples of trees and shrubs that have been tested for alley cropping are Gliricidia sepium, Gliricidia maculata, Leucaena leucocephala, Flemingia congesta, Senna siamea, Alchornea cordifolia, Acioa barteri, Gmelina arborea, and Peltophorum pterocarpum. A number of workers have reported the effectiveness of alley cropping against I. cylindrica and other weeds (Aken’Ova and Atta-Krah, 1986; Anoka et al. 1991). For example, Anoka et al. (1991) reported that after shading for two years by hedgerows of Gliricidia sepium and Leucaena leucocephala, the shoot biomass of I. cylindrica decreased by 78-81 percent while reduction in rhizome biomass was 90-96 percent. Despite the numerous advantages of alley cropping, the technology has not yet gained wide acceptance by the smallholder farmers of West Africa. Some of the important factors that do not favour the adoption of the technology include the additional labour needed for the establishment, pruning and general management of hedges, the reduction of the area of land available for producing food crops, and the reduced crop yields resulting from competition and the allelopathic effects of hedgerows (Kang et al. 1981).
Shading of I. cylindrica using food crops may overcome some of the shortcomings of green manure type of cover crops. Cajanus cajan was reported to be effective in reducing the rhizomes of I. cylindrica in Asia and Africa (Koch et al. 1990; Macdicken et al. 1997). Intercropping Citrullus lanatus and Vigna unguiculata effectively suppressed the weed for eight weeks in West Africa (Chikoye et al. 2001). Spacing maize in narrow rows (50 cm) was found to lower the dry matter of I. cylindrica by 42 percent compared with the recommended row spacing of 76 cm (Chikoye, unpublished data). In young plantations of 2-3 years, farmers normally plant food crops (maize, rice, groundnuts, and soybean) to reduce open spaces and hence suppress I. cylindrica (Anon., 1996). Intercropping may not be possible in older plantations because of severe competition. The use of intercropping and narrow row spacing is recommended after populations of I. cylindrica have been reduced by tillage or chemical control.
Integrated management
No single method described above can control I. cylindrica in a sustainable manner. The best way to control I. cylindrica is by implementing an integrated approach that employs a variety of options, which should be attuned to the individual farmer’s agronomic and socio-economic conditions (soil type, climate, costs, local practices, and preferences). For example, the growth of the weed can be suppressed by flattening, tillage or chemical control followed by planting competitive cover crops as well as food crops that prevent re-invasion. Successful use of cover crops requires that burning be strictly prevented. Improvements in soil nitrogen from the use of leguminous cover crops will increase crop vigour and enable crops to be more competitive against I. cylindrica. As Terry (1994) stated, the technology for tackling the problem of I. cylindrica is available. More efforts should be directed toward the promotion of these control practices in areas invaded by this weed.
ACKNOWLEDGEMENT
This manuscript is published with approval from the International Institute of Tropical Agriculture.
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