Conservation tillage for sustainable agriculture

Soil & Tillage Research 61 (2001) 93±107

Conservation tillage for sustainable agriculture An agrarian revolution gathers momentum in Africa Richard Fowlera,*, Johan Rockstromb a

Agricultural Research Council of South Africa, Grain Crops Institute, ARC-GCI, P/bag X 9059, Pietermaritzburg 3200, South Africa b Regional Land Management Unit (RELMA), PO Box 63403, Nairobi, Kenya

Abstract Before the arrival of European agricultural technology, technologies addressing the need for more ef®cient capture and utilisation of resources were being developed in Africa. Land invasions and displacements, the introduction of technologies more suited to European conditions, and increased use of the mouldboard plough and hand hoe slowed and almost halted this revolution. In recent decades farmers have in some areas regained security of tenure, recognised the potential value of indigenous knowledge, and begun to understand the degradation caused by soil inversion and other `modern' technologies. Conservation tillage is de®ned as any cropping system which results in conservation of natural or other resources, and sustainable agriculture as the use of agricultural practices which conserve water and soil and are environmentally nondegrading, technically appropriate, economically viable and socially acceptable. Synergising of the will, ®ndings and knowledge of African farmers and agriculturists with the experience and understanding of other conservation tillage researchers, advisers and practitioners throughout the world has resulted in this agrarian revolution gathering momentum. The potential contributions of conservation tillage to sustainable agriculture, and the role of the African Conservation Tillage (ACT) network in assisting and facilitating the process, and identifying and prioritising research needs, are discussed. # 2001 Published by Elsevier Science B.V. Keywords: Conservation tillage; Conservation farming; Conservation agriculture; Sustainable agriculture; Technology transfer

1. Introduction Population pressures and geographical limitations slowed African tribal migrations, and forced crop producers to rely more on existing than possible future resources. As a result, traditional agrarian systems such as burn/planting stick agriculture tended to become slash/burn/planting stick. Shifting cultivation was reduced, fallows introduced, and more envirofriendly systems of water, nutrient and soil manage*

Corresponding author. Tel.: ‡27-33-355-9410; fax: ‡27-33-343-4281. E-mail address: [email protected] (R. Fowler).

ment, such as pitting initiated (Whiteside, 1998). The arrival of European settlers further reduced land availability, but the advent of the manual digging hoe and animal (later tractor) drawn mouldboard plough temporarily enabled food supply to satisfy demand. Agricultural advisers, however, promoted the idea of a ®ne even seedbed as the basis for successful crop production and commercial farmers proceeded to pulverise their soils with increasingly powerful and destructive equipment. In many instances crop residues were removed or burned so as not to interfere in the process. Soil structure, organic matter and biota were damaged, water holding and cation exchange capacity reduced, soil loss and deserti®cation escalated, and yield and

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biomass production decreased. Water and wind erosion increased downstream pollution of rivers and the consequent silting of reservoirs. Contour banks, storm water drains, windbreaks and other `conservation works' were subsequently deemed to be the answer, and were often established with the assistance of subsidies and legislation. To make way for the commercial farmers (and in some instances to reduce competition at the beginning of the 20th century) colonial governments began to concentrate indigenous populations into `reserves' or `homelands'. These were often in marginal areas whose soils, rainfall or topography rendered them less suited to cropping. The increased population pressure often resulted in over-exploitation. Policies to combat this degradation involved the consolidation of arable lands, for example in Zimbabwe, as early as 1926 (Norton, 1995), and conservation works to reduce erosion. Where animal traction or tractor services were available, planting on the contour was advocated, but some 90% of African farmers continued to rely on hand hoes for primary cultivation. Subsidised tractor schemes were introduced in many countries to overcome the lack of machinery, but these proved unsustainable, almost without exception. When they attained independence, therefore, the better arable land in most African countries was largely farmed by an expatriate minority of commercial farmers using mouldboard or disc ploughs to prepare ®ne even seedbeds, with contour drainage ways to limit soil loss. The indigenous majority of resource poor semi-subsistence farmers cropped the remaining land using hand hoes or animal-drawn ploughs in an attempt to produce seedbeds similar to their commercial counterparts, with disproportionate areas of their small allotments occupied by government-imposed contour works. After independence, indigenous farmers in many countries rebelled against the use and maintenance of conservation structures, regarding them as colonial instruments imposed on them against their will (Nyagumbo, 1998) occupying a large proportion of scarce land. Successive governments found the maintenance of a state-®nanced conservation works programme increasingly unaffordable, and the collapse of subsidised tractor schemes often resulted in a return to the use of hand hoes for land preparation, taking up to 160 days ha 1 ®eld labour (Findlay and

Hutchinson, 1999) or animal plough systems, involving a 35 km walk behind the plough. Under the colonial powers the responsibility for soil conservation and agricultural production was frequently separated. As a result conservation was initially focussed on removing water with a minimal silt load from arable areas (Whiteside, 1998). Only comparatively recently has it come to be appreciated that soil water is the major constraint limiting crop production, especially in semi-arid areas and regions prone to drought or erratic rainfall, and that, by slowing or eliminating water ¯ow, the vector responsible for most soil loss is reduced or removed. To enhance food security and reduce dependence on external donors, sustainable agriculture has become imperative. Consideration has been given to conservation tillage in many countries, but emphasis has often been placed on systems developed under markedly different agro-ecological and socio-economic conditions. There is, however, an encouraging and growing awareness throughout the continent of the potential value of components of various conservation tillage systems. Farmers have unique understandings and experience of their speci®c circumstances, so the on-farm testing of a range of techniques, both indigenous and exotic, to meet farmer identi®ed and prioritised needs can assist farmers to develop sustainable and acceptable systems. An agrarian revolution is therefore in progress, albeit in its infancy. The African Conservation Tillage (ACT) network was established to encourage and facilitate this process. This paper outlines some of the contributions conservation tillage can make to sustainable agriculture, lists some of the initiatives which led to revival of the revolution, and discusses how ACT can assist in promoting and facilitating the adaptation and adoption of conservation tillage in Africa. 2. Conservation tillage for sustainable agriculture Conservation tillage is the generic term given to soil management systems which aims to conserve natural resources. The maintenance of at least 30% soil surface cover by plant residues is often regarded as essential for conservation of soil and water, but may not affect the usage of other resources such as time, fuel, and money. Residue levels alone, therefore,

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do not adequately de®ne all conservation tillage practices (Baker et al., 1996). For this reason Erenstein (1999) claims the term `conservation tillage' is a misnomer and should be replaced by `crop residue mulching'. Others claim the terms `conservation farming' or `conservation agriculture' describe a more holistic system. In Africa, especially in semi-arid areas where all forms of biomass may be harvestable components of human or animal survival, crop residues are often viewed as communal assets. While the production and preservation of soil cover is desirable, the critical component of conservation tillage in such areas is the minimisation of soil disturbance, and the term `conservation tillage' reminds agriculturists that the primary causes of resource loss and environmental degradation on the arable lands of Africa are the hand hoe and mouldboard plough. The Food and Agricultural Organisation of the United Nations de®nes sustainable agriculture as the use of agricultural practices which conserve water and soil and are environmentally non-degrading, technically appropriate, economically viable and socially acceptable. 2.1. Water conservation The output from any crop production system is dependent on the most limiting input. In the arid and semi-arid regions in Africa that input is frequently soil water, so the key to dryland crop production is the capture and storage of precipitation (Willcocks and Twomlow, 1991; Oldreive, 1995). Water ¯ow partitioning in a typical African conventionally tilled crop production system is illustrated in Fig. 1 (Rockstrom, 1999). Surface runoff frequently accounts for up to one quarter of rainfall (and more for individual events) as a result of crust and pan formation, high intensity rainfall events and poor soil cover. Evaporation losses from the soil surface can account for up to half the rainfall, a proportion that increases with intensity of cultivation and soil degradation. After accounting for drainage, only 15±30% is available for biomass production. As shown by Rockstrom (1998), this can fall to 5±10% of rainfall in degraded maize production systems where yields may be only 0.3±0.5 t ha 1. Distribution and reliability are often more important than total rainfall (Twomlow et al., 1999; Lawrance et al., 1999). Conservation tillage can slow surface water ¯ow by increasing soil surface rough-

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Fig. 1. Overview of rainfall partitioning in the semi-arid tropics in Sub-Saharan Africa. R: seasonal rainfall, Es: soil evaporation and evaporation from interception, Ec: crop transpiration, Roff: surface runoff, and D: deep percolation beyond the root zone of the crop (after Rockstrom, 1999).

ness and water in®ltration rate; reduce evaporation losses by reducing soil disturbance and mulching; and increase soil water holding capacity by increasing the volume of soil macropores and reducing the oxidation of organic matter. 2.2. Soil conservation Of the 5.2 billion hectare in world-wide dryland crop production, 70% are degraded, and 24 billion tons of topsoil are lost annually (Convention to Combat Deserti®cation, 1995). Just over 1 billion hectare in Africa (73% of the continent's drylands) are moderately or severely affected by degradation. Steep slopes in Ethiopia lose soil at up to 290 t ha 1 yr 1, and gentle slopes in West Africa lose 10±20 t ha 1 yr 1 (IBRD, 1989). Even the Lilongwe plains in Malawi are estimated in places to be losing 35 t ha 1 yr 1 (Siacinji-Musiwa, 1999). The key problem in tropical agriculture is the steady decline in soil fertility resulting from soil erosion associated with conventional tillage (Derpsch and Moriya, 1998). Many small-scale farms in Africa are severely affected by sheet and rill erosion, and use of the mouldboard plough is a major contributor to both water erosion (e.g. Elwell and Stocking, 1988; Brunner et al., 1998) and wind erosion (Keib, 1998; Fowler, 1999). Studies in Zimbabwe and elsewhere have shown that conservation tillage techniques can reduce soil

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loss to sustainable levels. Vogel (1994b) reported that no-till tied ridging resulted in sheet erosion loss of 0.5 t ha 1 yr 1, compared with up to 9.5 t ha 1 yr 1 following a mouldboard plough/hoe system. Mulch ripping also reduced sheet erosion losses. In comparisons over six seasons in Zimbabwe, mean soil loss (t ha 1 yr 1 ) and water loss (% of cumulative rainfall) were 6.4 and 3.4 (mulch ripping), 6.0 and 2.7 (no-till tied ridging), 26.2 and 12.7 (hand hoeing), 22.8 and 8.6 (clean ripping) and 60.4 and 11.5 (conventional mouldboard ploughing), respectively (Chuma and Hagmann, 1995). Nyagumbo (1999) found no-till tied ridging and mulch ripping could maintain soil loss levels at below 5 t ha 1 yr 1, reducing soil loss by 84±90 and 72±89%, respectively, compared to that on conventionally tilled areas. 2.3. Greenhouse effect Tillage releases CO2 into the atmosphere, contributing to global warming (Derpsch and Moriya, 1998). Soil organic matter is the major terrestrial sink for C, N, P and S, and soil biota the major factor responsible for the mineralisation, transportation and immobilisation of these elements (Van der Merwe and de Villiers, 1998). No-till may increase the ®xation of carbon. It may also increase emissions of nitrous oxide due to anaerobic conditions as a result of soil compaction and the lack of soil disturbance and residue incorporation (Ball et al., 1999). If conservation tillage resulted in at least 30% plant residue cover of the soil surface of 75, 50 and 25% of the cropland of the USA, other developed countries and developing countries, respectively, it could result in an annual global sequestration of 1500 MMT of C by the year 2020 according to Lal (1997). The sequestration of carbon in soils under conservation tillage, especially when combined with the judicious use of cover crops, crop rotations and crop residues, could have a signi®cant effect on the greenhouse effect and the sustainability of agriculture in the tropics and sub-tropics. 2.4. Appropriateness 2.4.1. Field preparation Of the total energy used in crop production in North Africa in 1987, 69% was derived from people, 17%

from animals, and 14% from tractors (S.Twomlow personal communication., 1999). In sub-Saharan Africa this ratio was 89:10:1. Findlay and Hutchinson (1999) estimated that 80±100 person days per hectare might be needed to prepare a land for planting with hand hoes. Animal-drawn mouldboard ploughing may take 2±3 days, whereas tractor ploughing may require only 2±3 h. Although it is often recommended that farmers should plough immediately after harvest, most farmers wait until the ®rst rains before commencing seedbed preparation. Because the majority of African farmers have no direct access to animal or motorised traction, seedbeds are often prepared too late, the cropping season shortened, and crop yields reduced (Ellis-Jones and Mudhara, 1997). Under conservation tillage the area cultivated is reduced and cultivation is usually shallower than conventional tillage. Herbicides may be used in some systems (Findlay and Hutchinson, 1999), hand hoes in others, and those that have animal-drawn ploughs can ®t simple and inexpensive tynes or sub-soilers (Bwalya, 1999). 2.4.2. Planting In conventional tillage systems hand planting may be done with planting sticks or hand hoes. Alternatively, seed (and organic or inorganic fertiliser) may be placed in the appropriate furrow during ploughing. Where soil is suf®ciently friable, planting sticks may be used in conservation tillage systems. Alternatively, planting furrows can be opened using conventional or specially adapted V-shaped hand hoes. Inexpensive kits are available in some southern African countries to convert animal-drawn ploughs into ripper planters, and work is underway in South Africa to develop simple conversion kits which will enable owners of traditional animal-drawn single furrow planters to change these into inexpensive ripper planters. Timely planting is essential for ef®cient utilisation of resources such as moisture, temperature, and day length. This is facilitated by early seedbed preparation, or systems which require no prior tillage. 2.4.3. Weed control Delays in the control of weeds, especially soon after planting, can markedly reduce biomass production and crop yield. Weeding accounted for more than 60% of the labour used for maize production in

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semi-arid Zimbabwe (Riches et al., 1997). Oldreive (1993) found four workers could keep an area of 1.62 ha 1 weed free without undue effort, but a regular routine was needed to prevent weeds becoming established, more dif®cult to remove and competitive with the crop. Early winter ploughing followed by mouldboard ploughing in spring, resulted in weed free ®elds at planting, but this led to a rapid proliferation of annual weeds (Vogel, 1994a). The wide spread occurrence of Cynodon dactylon on communal lands in southern and eastern Africa is associated with shallow primary cultivations with animal-drawn ploughs. Mabasa et al. (1995) reported that excellent Cynodon control can be achieved by a single application of glyphosate post-harvest or prior to spring tillage and appropriate timing of tillage. Findlay (1998) found that use of pre- and post-plant herbicides in a no-till situation required only 15% of the time otherwise required for seedbed preparation and weed control with a hand hoe. The reduction in labour days required per hectare of rice in Senegal was 53±60%. The lack of soil inversion in most conservation tillage systems results in weed seeds remaining on the soil surface where they are more exposed to weather and animals. Where crop or other plant residues are retained the soil surface is usually cooler and wetter, resulting in slower weed growth and greater opportunities for control. In conservation tillage, annual weeds become less prevalent and the population of perennial weeds increases more slowly if adequate precautions are taken to limit weed seed production. Systems incorporating the judicious use of mulches and crop rotations may almost eliminate the need for weed control (Calegari et al., 1998). In situ mulching helped control weeds, and crops such as Mucuna pruriens and Pueraria phaseoloides can be used to suppress weeds during fallow periods (Couper, 1995). Using Mucuna to replace weeds as the dominant species following harvest could ®x nitrogen and reduce weed pressure and production costs (Chalulo et al., 1998). 2.4.4. Insect and disease The presence of plant residues on the soil surface has often been given as the reason for pest and disease outbreaks in conservation tillage. However, predators will also proliferate, and instances have been recorded

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where problems have decreased under conservation tillage. Thus Lamarca (1996) found the incidence of take-all disease caused by Gaeumannomyces graminis was reduced under no-till compared to traditional tillage, which he attributed to reduced pathogen incidence as a result of increased soil organic matter content. In America, a reduction in Colorado potato beetle damage on tomatoes was noted, as was a reduction in early blight of potatoes, attributed to reduced soil splash in mulch-based systems (Magdoff and van Es, 2000). In South Africa, reports have been received of reduced termite damage due to the pest's preference for dead plant material. 2.4.5. Surface residues Erenstein (1999) advocated crop residue mulching for improved resource conservation and productivity. Plant residues on the soil surface affect soil temperature and moisture, and consequently crop and weed germination, speed of emergence and root growth. They affect soil water and gas ¯ow, soil structure, residue decomposition, nutrient cycling and availability, weed spectrum and competition, and plant disease dynamics (Lafond and Derksen, 1996). In semi-arid tropical regions mulch may be crucial in reducing the deleterious effects of intensive summer rainfall, and may reduce high temperature injury to emerging seedlings by slowing evaporation (McCown, 1996). In cooler areas early in the season, however, Berry et al. (1987) found increased soil temperatures at 50 mm depth, and more rapid seedling emergence and development with less residue cover. The bene®ts derived from mulch depend on the agro-ecological zone. Where marginal or erratic rainfall or drought is a problem, the major bene®ts are increased moisture capture and retention, with weed suppression in these and more humid areas. Leguminous residue mulches are most likely to increase nutrient supply. The minimum quantity of mulch needed for short-term moisture conservation bene®t is 5 t ha 1, which is often dif®cult to achieve due to alternative uses for the residue, especially in semi-arid regions. Smaller quantities (2±3 t ha 1 ) may improve soil physical properties in the long term if applied each year. Cost of mulching is critical to adoption, and large-scale mulching of ®eld crops is only likely to be achieved where in situ production is suf®cient for both mulch and household needs (Carsky et al., 1998).

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Surface mulch is extremely dif®cult to maintain in some semi-arid areas due to uncontrolled livestock, termites, and the rapid mineralisation of organic matter (Couper, 1995). Crop residues are often essential fodder for livestock (Molapong et al., 1998), and other problems include grazing by wild life, low yields and use as fuel (Findlay and Hutchinson, 1999). Tillage practices which preserve higher levels of surface residues retain more water (Berry et al., 1985). Ideally, soils should at all times have a minimum of 30% plant residue cover, but Oldreive (1993) maintains that even 10% is better than using a mouldboard plough because of the damage this does to soil structure. 2.5. Economic viability Reduced cost of production is probably the main reason for adopting no-till (Derpsch and Moriya, 1998; Steiner, 1998). Minimum tillage research in Ethiopia has shown that even if crop yields are similar to those of traditional systems, the gross margins of minimum tillage are almost doubled due to smaller inputs (A. Astatke, personal communication, 2000). Increased retention and availability of soil water, combined with the improvement in soil structure and biotic activity, tend to increase long-term mean yields in conservation tillage systems. Initially, however, as the system adjusts, yields may be lower, and the immediate bene®t is the lower costs incurred, especially in seedbed preparation. The introduction of a high input no-tillage system to farmers dependent on hand hoeing for seedbed preparation and weed control can result in a labour saving of 120 days per hectare (Findlay and Hutchinson, 1999). To many resource poor farmers this may not be obvious, as these are labour costs, which are seldom quanti®ed in monetary terms. Comparing commercial tillage systems in South Africa, however, Lawrance et al. (1999) found production costs to be least with no-till systems and increasing with tillage. On-farm trials in Tanzania by the Sasakawa Africa Association and Monsanto International demonstrated that a pre-plant glyphosate spray increased the yields of no-till maize and rice by only 12.7 and 6%, respectively, compared with manual weeding, but cost reduction increased net income by 24.5 and 11.0% (Findlay and Hutchinson, 1999).

The long-term bene®ts of conservation tillage may exceed the apparent short-term gains from practices which ignore soil conservation. Although yield under no-tillage or conservation tillage may initially be less than conventional tillage, soil productivity is likely to be sustained for a longer period (F.K. Salako, personal communication, 2000). 2.6. Social acceptability Especially in sub-Saharan Africa, male migration to employment in the towns has resulted in many rural areas with more female than male-headed households. This has resulted in messages from a male-dominated extension service frequently being directed at absentee farmers whose needs are different from the actual farmers. Conservation tillage technologies are gender neutral, but they can be biased during project formulation and implementation (Lubwama, 1999). Men are responsible for ®eld preparation (and handling of oxen) and women for plant and weed. Small-scale farmers in Africa, especially female household heads with domestic chores, seek agricultural technologies like conservation tillage that reduce the effort and time required but result in improved food security. An extension system developed in Zimbabwe, Training for Transformation, empowers local people to take control of their own lives and stresses the importance of participation by both men and women in development (Hagmann et al., 1997). Other sociocultural factors affecting the adoption of conservation tillage include the fear of change, risk aversion, and weaknesses in the promotional and technical abilities of extensionists (Kaumbutho et al., 1999). Traditional systems of land use and ownership are frequently cited as a reason for low productivity (Wegerif, 1998). An analysis of 15 case studies of land use in east, west and southern Africa by Scoones and Toulmin (1999), however, found land tenure was rarely mentioned as a factor in poor soil fertility management, suggesting that current systems provided suf®cient guarantees of permanence of tenure. Socio-economic and cultural constraints play a major role in the adoption or rejection of innovations, but farmer participation in the identi®cation of the system components best suited to their speci®c needs can overcome this problem (Nyagumbo, 1999). When

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the bene®ts are identi®ed and approved by potential users, the effort and time saving implications of conservation tillage are soon recognised. Off-farm bene®ts such as cleaner streams and reservoirs, and the resultant lower costs of puri®cation and storage, are also most acceptable to the wider community. 3. Revival of the agrarian revolution Conservation tillage in Africa originated in many areas centuries ago, as farmers began to observe the degradation process and understand the need to ®nd sustainable alternatives (Kayombo et al., 1999; Okoba et al., 1998). The Ubuntu philosophy that `the person that I am is a result of where I am' led to an appreciation of the symbiosis between humankind and the environment. Philosophies such as these combined with the practical and immediate bene®ts led, for example, to the basin and pitting technologies found in many parts of Africa, and the trenching techniques developed in the Cameroons. To a large extent it was the advent of colonialism, European-trained agriculturists and the mouldboard plough which slowed the development and adoption of these practices. Many communities were moved from traditional farming areas to settlements where they did not belong and for which they felt no responsibility. The Western agricultural revolution based on fossil fuel energy and the economies of scale started to permeate the continent and, combined with the mouldboard plough and paternalistic advice, resulted in pulverisation of the soil and disempowerment of the farmers. In common with similar agrarian revolutions, notably the Brazilian no-till experience, it was the farmers whose survival depended on the success of the production systems who initiated change. Some continued with time-honoured systems such as Low External Input Sustainable Agriculture, inter-cropping, multicropping or water capture systems, while others experimented with innovations and alternatives. Evidence from overseas encouraged agriculturists in various regions to investigate the possibilities of conservation tillage in commercial, and subsequently smallholder farming systems during the 1970s. Nodes of research and development emerged, notably in west, then south and, more recently, east Africa.

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The International Institute for Tropical Agriculture in Nigeria probably established the ®rst formal conservation tillage trials in Africa. This institute focussed attention on no-till arable crop production in humid zones, and numerous trials demonstrated the bene®ts of no-till on water and soil conservation, crop establishment and yield (Lal, 1992). In the sub-humid and semi-arid zones however, zerotillage was not an unquali®ed success (Dunham, 1979; Ogborn, 1982; Ike, 1986). Surface mulch, an important component of zero-tillage systems, is dif®cult to retain in these regions because crop residues are frequently required as fodder. Plant residues attract free ranging animals, and these remove mulch and compact the seedbed. In addition, the ®ne sandy and silty soils of the sub-humid and semi-arid regions also have topsoils prone to crusting, and compact sub-soils (Adeoye, 1986) which require periodical tillage (Adeoye, 1982; Adeoye and Mohammed-Saleem, 1990). The Institute for Agricultural Research (IAR), Samaru, Northern Nigeria, compared soil properties, crop yield and weed growth under different tillage systems (Dunham, 1979). This data generally showed more rapid soil deterioration under conventional tractor tillage than under reduced tillage and zero-tillage systems. Weed control was superior with zero-tillage in the ®rst year. Mean crop yields under tractor cultivation were similar to yields under hoe and bullock cultivation, but yield from zero-tillage was distinctly inferior to the other systems. Initial work in South Africa focussed on control of wind erosion on the sandy soils of the central plateau (Joubert, 1979). During the mid-1970s the South African Grain Crops Research Institute (now the Agricultural Research Council's Grain Crops Institute) established tillage trials to compare the effect of different tillage systems on commercial grain yields in various parts of the South African `Maize Triangle'. This was followed by work by the Agricultural Research Trust in Zimbabwe and the Golden Valley Agricultural Research Trust in Zambia. The British Silsoe Institute and GTZ also initiated small-scale farmer trials in Zimbabwe, and other institutes and organisations, such as the Swedish SIDA and French CIRAD, started conservation tillage research in other parts of Africa. Adoption, however, remained slow, perhaps because researchers advocated complete zero-tillage

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packages, rather than components, and also preferred demonstrations to on-farm trials. Once researchers started to respect the wisdom and experience of farmers, however, participatory planning and execution of on-farm trials was introduced and the site- and circumstance-speci®c nature of small-scale farming systems began to be better understood. Adaptation and adoption rates subsequently increased. 4. Facilitating the revolution A characteristic of this revolution has been its fragmentation. While many African professionals and their western-based advisers are aware of the development and widespread adoption of conservation tillage in South America, most know little of the developments on their own continent. This is especially true of indigenous technologies, only recently recognised as a potential source of innovative ideas. In 1998 a group of concerned organisations convened a workshop in Zimbabwe to discuss conservation tillage for sustainable agriculture. In addition to the approximately 70 participants from 15 African countries, 12 from other continents brought a global perspective. The participants concluded that much of the necessary technology already existed and the major constraint was its transfer. Policies and possibly legislation needed to be developed, and research needed to be identi®ed. The best way to further these aims was considered to be the initiation and development of a network. 4.1. Establishment of the African Conservation Tillage network Participants at the 1998 Zimbabwe workshop af®rmed that the development of an effective network could inter alia provide dynamic comparative advantage, avoid duplication by international agricultural research centres, and facilitate adaptive research and technology transfer by national research initiatives and regional centres (Lal, 1992). Claveran (1998) brie¯y summarised the activities of the Latin American Conservation Agriculture Network (RELACO) and Simalenga (1998) stressed the need for networks to service the needs of members, and encourage farmer participation.

A group of primarily African promoters and practitioners initiated the ACT network in April 2000. ACTs main objectives are to identify, disseminate and promote the adaptation and adoption of resource conserving tillage practices in Africa. The network sees as its primary task the opening up of channels of communication, but also plans activities to stimulate the establishment of national conservation tillage networks and the identi®cation, adaptation and adoption of conservation tillage techniques. It plans to concentrate its resources in eastern and southern Africa initially (2000±2002), then western and central Africa (2003±2005), and ®nally northern Africa (2006±2008). Membership is open to anyone interested in the practice or promotion of conservation tillage in Africa, and care is being taken to ensure that all those interested in promoting its objectives play a signi®cant role in, and are adequately served by, the network. 4.2. Research needs Simple adaptation and introduction of existing technologies cannot solve all problems faced by communal farmers. New technologies are also required to improve productivity while conserving the resource base. Priority must be given to project identi®cation, design and appraisal with active involvement and consultation with intended bene®ciaries and stake holders (EllisJones and Mudhara, 1995). Some long-term experiments are also essential in the investigation of sustainable production systems (Poulton, 1995). Published conservation tillage research has been carried out primarily in temperate regions for largescale producers with adequate ®nancial and other resources. In Africa more research is required into the effects of soil type and climate (especially rainfall impact and distribution), crop residue management (especially in crop/livestock systems) and equipment development (Benites, 1998). Many resource poor farmers, especially in Namibia and Zimbabwe, have sandy infertile soils, and methods of building up organic matter and nutrients need urgent investigation (Rigourd and Sappe, 1999). Chuma and Hagmann (1998) identi®ed green manuring, legume-based inter-crops for weed and fertility management, strip cropping, hard pan management options, weed management systems, and single-ox harnesses as some of the other areas requiring investigation.

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Commercial farmers in South Africa believed more information is required on the introduction, proliferation and protection of earthworms, as well as the characterisation of macrofauna according to their potential effects on soils and crops; allelopathy of crops and weeds; crop rotations; characterisation of cover crops; prevention and redemption of surface acidity; together with the identi®cation and development of appropriate hand-, animal- and fossil-fuel powered implements (Fowler, 1999). Bobobee (1999) noted the lack of appropriate animal-drawn implements in Ghana, as did Fall and Faye (1999) in West Africa. Zambia has a programme developing such implements (Bwalya, 1999). In Zambia, Benites et al. (2000a) recommended that integrated ripping and pot-holing/cover crop systems should be developed; cover crops screened for different purposes; equipment adapted for no-till planting; improved fallows tested for soil fertility restoration; integrated insect, disease and weed control systems developed; and appropriate soil amelioration strategies developed to obviate increased soil acidity. There is a need to identify potential cover crops (indigenous and exotic) for different agro-ecological zones and to test and validate promising system components used in other regions (Benites et al., 2000b). Zero-tillage effects on crop production in the sub-humid and semi-arid zones are not well understood, and there is a need to establish more precise ecological limits for conservation tillage systems. The de®nition, production and maintenance of minimum effective levels of soil cover also needs investigation (Adeoye, personal communication, 2000). In recent times, there has been a more concerted interest in determining the quality and impact of residues (Kumar and Goh, 2000), but these efforts tend to focus on nutrient cycling, rather than conservation tillage. Residue quality also needs consideration in terms of its potential to act as a host for crop pathogens (F.K. Salako, personal communication, 2000), and tillage system effects on weed spectra and competitiveness need attention (Rigourd et al., 1999). Research should be devised in full consultation with farmers and conducted by farmers on their ®elds, where this is feasible. This will usually entail larger plots, simpli®ed trial designs and control treatments that re¯ect farm practice and respect farmer knowledge, practices and conditions (Mashavira et al., 1995).

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4.3. Technology transfer needs The success of an agricultural technology in one area generates optimism and feeds the desire for quick ®xes elsewhere. This often leads to extrapolation of practices to areas where they are not suitable (Erenstein, 1999). Transfer of appropriate information should be prioritised to facilitate the agrarian revolution in Africa (Findlay and Hutchinson, 1999). Wenner (1988) mentions that there are ®ve important pre-conditions in determining farmer motivation to participate in soil conservation, namely soil erosion should be visible and considerable; short-term economic gain should be possible; loss of land (e.g. the use of terraces) should not be part of the solution; investment (in money, time, energy, etc.) should be pro®table and traditions must be respected. For farmers, the future is based on the present, and promotional messages must build on tradition (L. Mosenene, personal communication, 1999). The signi®cance of soil and water loss is often not obvious where yields are normally poor. Farmers in these situations want to ensure that innovation costs are small and achieve the bene®ts of yield stabilisation (Misika and Mwenya, 1999). Participatory approaches need to make use of existing local knowledge and creativity in the form of guided and supported innovation (Critchley, 1999). Identi®cation and recognition of local traditions or indigenous knowledge is important, but it is the possibility of building on these that has real potential. A logical ®rst step could be to use `farmer measurable indicators' to assess the effectiveness of these measures, before working with innovative farmers towards improvements and, once proven, disseminating them, using the land users themselves to spread the message (Critchley and Netshikovhela, 1998). Muliokela (1998) in Zambia recommended increasing farmer responsibility for the development, improvement and dissemination of conservation tillage technologies. The traditional top-down extension approach has been identi®ed as one of the key factors hampering adoption (Nyagumbo, 1998; Shetto, 1998), and the best approach to technology transfer involved participatory strategies, including farmer experimentation. One such approach is the Kuturaya (let us try) system, where farmers' indigenous technical knowledge is utilised as the basis for innovation (Hagmann

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et al., 1996). For this approach to succeed, however, the social environment should be conducive, the intervention must involve communities not individuals, the activities must involve all potential players, extension and research must be involved as equal partners with the farmers, a trust-building entry point is required, and a holistic approach to land husbandry engendered (Chuma et al., 1998). A farming systems approach with active farmer participation will ensure technology is developed to the satisfaction of the primary client, the farmer (Nduli, 1998). The promotion of a high external input no-till system in Ghana, for instance, consisted of preseason training, establishment of on-farm demonstrations, ®eld days, farm tours, workshops and seminars, in all of which researchers, extensionists and farmers are encouraged to play major roles. Increasing farmer participation resulted in a high rate of adoption (Boa-Amponsem et al., 1998). Extensionists in sub-Saharan Africa are entering the 21st century with a serious credibility problem, often exacerbated by macroeconomic reforms which might include the removal of subsidies, downsizing and decentralisation of public services. Participatory development methods also challenge traditional approaches which are based on a one way ¯ow of information from `experts' to farmers. The belief that extensionists are `advisors', and that only science-based technologies are worth communicating to farmers, ignores and denigrates the indigenous technical knowledge on which most resource poor farmers depend for survival. There is an urgent need to improve the skills of extension workers and help them engage in meaningful and effective dialogues with farmers (Opio-Odongo, 1999). Useful extension messages should communicate options; be tailored for different management levels; be holistic in their approach to problems, and address ecological, socio-economic, and sustainability issues in addition to technological matters (Goericke, 1995). They must take into account the practical, social and economic constraints faced by resource poor farmers (Mashavira et al., 1995). Methods used to disseminate technologies in Zambia include videos, farmer ®eld schools, village extension groups, demonstration plots, on-farm trials, ®eld days, radio and school clubs, posters, shows, newsletters, etc. (Mulenga, 1998). Norton (1995) also

emphasised the need for tillage recommendations to be site-speci®c, and developed a systematic method of selecting and recommending tillage systems based on soil type, farmer resources, climate and crop. He also noted the ``social risk'' associated with technology adoption. Instead of allowing community members to succeed and improve, communities tend to pull innovators back into the ``status quo'', a possible spiritual dimension to development often noted but seldom addressed. Kaoma-Sprenkels et al. (1999) found that a critical mass of adopters was required for a technology to spread in Zambia. Rate of adoption will be accelerated if the technology can lead to an increase in area cropped and a reduction in drudgery, energy and time. It will be further in¯uenced by cost, compatibility with existing socio-cultural practices and the accessibility of information support services (Bwalya, 1999). Rapid adoption will result if farmer priorities are addressed. In northern Tanzania, for example, the stability of production was substantially improved by breaking up the compacted layers found after ploughing, and this resulted in widespread adoption of conservation tillage technologies (Jonsson, 1998). 4.4. Policy and legislative needs Resource-use decisions made by households and communities in pursuit of survival and security may be in¯uenced by outside policies, technologies and institutions. Poverty reduction requires the revival of the communal stake in the natural resource base, restoring local control over local resources, and the use of local perspectives and traditional knowledge systems in development activities (Ellis-Jones, 1999). Policy instruments, not necessarily legislative, may be used to bring about positive change. The Sasakawa-Global 2000 strategy of enlisting ministerial support for its programmes is a good demonstration of this approach (W. Haag, personal communication, 1999). Resource conservation systems that have relied on economic incentives and legal sanctions have proved unsustainable especially in developing countries (Kamar, 1998). Such approaches tend to ignore farm household and institutional requirements, and neglect the complexity of African farming systems (IFAD, 1992). A multi-disciplinary, participatory, and ¯exible approach is essential if farm households'

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preferences and constraints are to be addressed and resource utilisation maximised (Erenstein, 1999, pp. 252±253). Most governments recognise their responsibility to protect the environment from exploitation. Prescriptive and punitive legislation, however, has tended to cause more harm than good, with Kenya's compulsory terracing programmes fuelling the ®ght for independence (Critchley, 1999). South Africa's enforced contour banks were also seen as part of the previous government's subjugative ploy, and deliberately abandoned in some areas. In Zambia on the other hand, Chiti (1998) suggested that conservation tillage has suffered from inadequate policy and legislation support, including the fragmentation of responsibilities, uncoordinated programmes, and lack of explicit legislation. The adoption of conservation tillage (or `farming') as a national strategy may be more successful (Jonsson, personal communication, 2000). The lack of enforcement of legislation is often a fundamental problem (Erenstein, 1999). Farmers have frequently seen traditional land tenure systems and the inability to use land as a security for loans as constraints to the adoption of medium or longterm development strategies. Systems of land ownership are currently under review in many countries (e.g. Uganda) (Okurut, 1998), and Shetto (1998) has advocated the review of land tenure and inheritance rights to increase personal responsibility for sustainability in Tanzania. Governments must educate everyone, including farmers, about environmental issues such as water saving, watershed management and maintenance of bio-diversity (Findlay and Hutchinson, 1999). Governments must develop policies that encourage rather than penalise farmers, avoiding subsidies that may sti¯e private enterprise but still prove unaffordable. Above all governments must seek to eradicate the receiver mentality brought about by previous subsidisation policies (Nyagumbo, 1997). 5. Discussion and conclusions Increasing abuse of Africa's natural resources is propelling the countries of the continent into a spiral of decreasing food security and increasing aid dependence. To slow and reverse this spiral, the degradation

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and loss of Africa's agricultural resources must be urgently addressed. The adaptation and adoption of conservation tillage techniques can reduce and reverse current trends, but these options need to be identi®ed and communicated to resource poor farmers. To facilitate this process, participative research is needed into many aspects of conservation systems, particularly the socio-economic constraints hampering the production and maintenance of surface mulches. Effective and acceptable conservation tillage systems must be identi®ed and characterised in terms of soil, climate and socio-economic conditions, so these can be communicated to aspiring conservation tillage practitioners. There is great potential for the development of geographic information system overlays relevant to the agro-ecological parameters of conservation tillage to identify areas where similar practices might be effective, on a world-wide basis. Agricultural advisers in the public and private sectors, together with potential conservation tillage practitioners need exposure to successful conservation tillage practitioners, training in participatory research-extension techniques and information on practices which have potential under their speci®c circumstances. All of these measures can accelerate this revolution, which is still fragmented and embryonic. Numerous constraints can slow or disrupt it, including poor communication facilities, regional con¯icts, the AIDS pandemic, and the growing gap between rural rich (livestock owners) and poor (crop producers). Other factors such as fuel costs, devaluation of local currencies, and urban migration can have both positive and negative effects on adoption. Promoters and practitioners need to be aware, for example, that greater fossil fuel costs increase the advantage of conservation tillage in terms of reduced energy inputs, but also increase the cost of herbicides. The ACT network has the potential to be a major factor in the adoption of environmentally friendly technologies and hence a major resource to assist the agrarian revolution. Once the network has secured adequate funding it needs to encourage the use of better research and development processes, to ®nd effective ways of communicating with all stakeholders, and ef®cient mechanisms of marshalling, researching and making available all pertinent information, especially to potential practitioners.

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