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Conservation agriculture in dry areas
Field Crops Research 132 (2012) 1–6
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Preface
Conservation agriculture in dry areas
1. Introduction In general, there are three critical components which define Conservation Agriculture (CA): (1) retaining full or as close as possible full ground cover, (2) no soil compacting and disturbance, and (3) maintaining diverse crop rotations. Major advantages reported for CA are reduced wind and water erosion of topsoil, increased water use efficiency through improved water infiltration and retention, increased nutrient use efficiency through enhanced nutrient cycling and fertilizer placements adjacent to seed, reduced oscillation of surface soil temperatures, increased soil organic matter and diverse soil biology, reduced fuel, labor and overall crop establishment costs, and more timely operations (Hobbs et al., 2008). These incentives could make CA a viable alternative in dry areas, where it would help to address the challenges of scarce and degraded natural resources. However, the supposed benefits of CA are not universal and Giller et al. (2009) pointed out circumstances where it did not appear to be advantageous, particularly for smallholder, resource-poor farmers. Such farmers find it generally difficult to maintain a soil cover of crop residue or a cover crop due to the competing requirements for such biomass for fodder, fuel or building material. Appropriate herbicides are often not available to, or affordable by, resource-poor farmers, resulting in increased weeding costs and weed constraints if tillage is reduced under CA. Although CA is considered as a primary driver to sustainable farming, a practical, location – specific and not rigid approach is needed. The global area currently under CA is reaching 120 M ha, corresponding to about 8.5% of arable cropped land, spread across all continents and agro-ecologies, including the dry Mediterraneantype environments (Kassam et al., 2012). Growing scientific and empirical evidence shows that significant productivity, economic, social and environmental benefits can be harnessed through the adoption of CA principles for sustainable production intensification in dry environments. Crop yield potential under CA management practices in rainfed systems is often greater than with conventional tillage (CT) systems; particularly in dry environments where sub-optimal rainfall limits yield (Farooq et al., 2011). A recent review of long-term and multilocation experiments confirmed these trends by showing slight increases over time in crop yields under CA relative to CT, indicating that CA can compete with CT on a crop production basis, in addition to the well-established environmental benefits associated with CA (Farooq et al., 2011). The improved performance of CA relative to CT in drier climates fits with the model proposed by Lal (1985), although high variability in the 0378-4290/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2012.03.002
data emphasized the importance of location specificity of tillage studies and that the model proposed was a ‘tentative’ generalization for discussion, followed with further experimentation and demonstration. Despite tangible benefits that CA can potentially contribute to agro-ecosystems, there remains a debate whether and under what conditions to promote CAfor smallholder farming systems, especially in sub-Saharan Africa (Giller et al., 2011). The low adoption of CA in parts of semi-arid Africa could be due to the fact that CA is often promoted as a package, without proper fine-tuning of technologies and adaptation to local circumstances (Tittonell et al., 2012). Local production constraints often limit implementation of CA principles by resource-poor farmers, resulting in lower than expected benefits. Factors that reduce the adoption from the benefits of CA include weeds, feed demand by animals, root and foliar diseases and inhibitory organism in the soil. System trade-offs in the allocation of resources at farm and village levels can be important factors determining the success of CA introduction for a given farming system (Giller et al., 2011). Additional socioeconomic constraints such as conditions of market access, institutional and policy issues, and other parameters related to partnerships and interactions among stakeholders are important factors for the introduction of CA at national and regional levels. Recent suggestions that ex-ante analysis of situations for where and how CA could fit into smallholding farming systems would require an innovative and interdisciplinary research in order to identify and address major knowledge gaps and system tradeoffs. Farmers who use CA management practices are seeking to improve their profitability, productivity and preserve the value of their natural resources. The objective of this special issue is to enrich the ongoing debate on the role of CA especially in smallholder farming systems by synthesizing the recent research findings of CA in dry areas, by analyzing agronomic, socio-economic, and agro-ecological determinants leading to its success or failure, and by identifying potential entry points for priorities for future research on CA in dry areas.
2. Does CA improve soil water balance? The productivity benefits from CA are mainly associated with its positive environmental and soil effects compared to conventional systems, including reduced erosion, runoff and surface crusting, increased aggregate distribution and stability, and increased infiltration and soil water content and water use efficiency (Hobbs et al.,
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2008; Thierfelder and Wall, 2010; Farooq et al., 2011). Water conservation is considered a key element of CA, especially in dryland areas exposed to erratic and unreliable rainfall. To demonstrate the effects of mulching on infiltration of rainfall, water balance studies are needed to analyze rainfall capture, soil storage and crop water use, including simple measurements of rainfall productivity of CA compared to conventional farming methods (Rockstrom et al., 2009). A general tendency of improved rainfall productivity was reported under CA in dry locations, which could be explained by a water harvesting effect, leading to a strategy for in situ moisture conservation. This strategy was confirmed in various smallholder farming systems in the savannah agro-ecosystems of East and Southern Africa (Rockstrom et al., 2009). The importance of crop residue retention in the CA system for improving soil stability and water balance was also demonstrated in the highlands of Mexico (Govaerts et al., 2009). The description of CA as a water-harvesting technology is challenged by the findings of Baudron et al. (2012) in a semi-arid area of Zimbabwe. CA did not affect cotton productivity during the first two years of the experiment, which received average or above average rainfall, and had a negative effect on crop yield in both on-farm trials and farmers’ fields during the drier season. There was no difference in water runoff between CA and CT on a relatively fine-textured soil, but significantly more runoff with CA on a coarser-textured soil due to soil surface crusting. Although most soils in the study area fall into the same category, the authors concluded that soil crusting might be avoided by increasing mulch production (ground cover) beyond what was achieved in their study. Araya et al. (2012) reported the results of a medium-term tillage experiment carried out on a vertisol to quantify changes in runoff, soil loss and crop yield due to CA in a sub-humid catchment in the northern Ethiopian highlands. A treatment combining permanent raised beds with a 30% standing crop residue and no-tillage seeding resulted in significantly less soil losses due to erosion and water runoff, leading to better crop performance, but it took at least five years of cropping before the difference was significant. Similarly, long-term adoption of CA in dryland cropping in the semi-arid US Great Plains resulted in greater precipitation storage and use efficiency, which led to greater cropping intensity, higher productivity, more diverse crop rotations, and improvements in soil properties (Hansen et al., 2012). The effect of cover cropping on soil water balance components has been documented in many locations around the world, but has not yet been explored in dryland grain production systems in regions with a Mediterranean climate. Ward et al. (2012) investigated the use of cover crops, grown solely to increase ground cover and not harvested for grain or biomass, under a Mediterranean-type climate in southwestern Australia. They examined the impact of cover crops and residue retention on evapotranspiration—over the summer fallow period and during the winter and spring crop growth period—and on deep drainage from subsequent crops, on two contrasting soil types. In contrast to previously published research, cover crops and residue retention had little impact on total evaporation during summer and autumn, although there were occasional short-term impacts on the rate of evaporation shortly after rainfall. There was also limited evidence of changes in evaporation during early crop growth. Drainage under crops grown after cover crops was not consistently different to drainage from crops grown after conventional crops. The authors concluded that the inclusion of cover crops in Mediterranean farming systems is unlikely to have major impacts on water balance, but may increase overall sustainability of the farming system. Simulation modeling can be a useful tool to analyze complex interactions influencing the effects of CA soil water balance and
crop water use (Scopel et al., 2004; Tittonell et al., 2012). Sommer et al. (2012) applied the crop–soil simulation model CropSyst to study soil water dynamics and water balance of wheat grown after barley in northern Syria under zero tillage (ZT) compared to CT. Simulation results showed that in 25 of 30 years, ZT yields were higher than those of CT, but ZT and residue retention had only a minor effect on reducing the amount of seasonal precipitation lost by unproductive soil evaporation. The authors argued that the impact of ZT on water balance might be limited to soils with selfmulching characteristics that are common in the Mediterranean regions.
3. Residue management and crop rotation Crop and soil management in dry areas generally involves conventional and sometimes excessive soil tillage and a lack of appropriate crop residue management, which frequently results in deterioration of soil physical structure and loss of soil fertility (Lal, 2007). Low crop yields due to continuous monocropping and deteriorating soil health in smallholder farmers fields have led to a search for more sustainable production practices with greater resource use efficiency. The shifting of crop production systems to CA and retention of crop residues has been reported to induce progressive qualitative and quantitative variations in soil organic cover, which impacts soil water balance, biological activity, soil organic matter build-up and fertility replenishment (Kassam et al., 2012). However, it is obvious that organic resources are often the most limiting factor in dry areas, where crop residues are generally used to feed livestock. Lahmar et al. (2012) argue that CA may not fully succeed in the Sahel region unless alternative sources of biomass are identified. They suggest a double phase of aggradation/conservation, by gradually rehabilitating the biomass production function of the soil through increased nutrient input and traditional water harvesting measures, encouraging regeneration of native evergreen multipurpose woody shrubs often associated with crops. A shift to the classical, less labor intensive CA practices would occur once appropriate levels of soil fertility and water capture are sufficient to allow an increase in the primary productivity of the agroecosystem. Another alternative approach was recently tested by the Agroecology-Based Aggradation–Conservation Agriculture (ABACO) initiative, which engages farmers in the design and implementation of locallysuited CA practices aimed at establishing a long-term strategy for soil rehabilitation (Tittonell et al., 2012). This approach is carried out through local innovation platforms coordinated through the African Conservation Tillage Network (www.act-africa.org), and relies on agro-ecologically intensive measures for increased water productivity and soil rehabilitation in semi-arid regions of Africa. Under semi-arid conditions in southern Zimbabwe, Mupangwa et al. (2012) reported that maize grain yield increased with increased mulch cover in those seasons that had below average rainfall, although the tillage system itself had no significant effect on maize yield. Mulching at a rate of 2–4 tha−1 led to optimum yields in seasons with below average rainfall. Under CA, a maize–cowpea–sorghum–maize rotation had significantly higher grain yield than first year maize, a maize–maize–monocrop and a maize–cowpea–maize rotation. In the dry semi-humid regions of northern China, Wang et al. (2012) observed that the benefits of no-till or reduced tillage practices on soil physical conditions were more pronounced when in combination with residue application. However, they also reported that surface application of crop residue for maize may delay
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seedling emergence because of low soil temperatures, and concluded that incorporation of residue in combination with reduced tillage might be a better solution in that particular environment. Baudron et al. (2012) reported that retention of sorghum residues and inclusion of N2 -fixing legumes in cotton-based cropping systems resulted in reduced N export by cropping from CA fields compared with conventional farmer practices in semi-arid Zimbabwe. The authors concluded that this may result in long-term beneficial effects of CA on crop yields, and discussed the possibility of increasing the short-term benefits of CA by improving the sorghum–intercrop association. Ngwira et al. (2012) carried out on-farm evaluation of short term maize–legume intercropping systems under conservation agriculture in Malawi. They found that CA had a positive effect on maize grain yield at two sites during the drier seasons. While the authors could not estimate the effects on water balances in farmer-managed experiments, they speculated that yield differences between CA and CT could be attributed to tillage and crop residue cover since other farm operations were generally similar. Hansen et al. (2012) reported that in Colorado, a no-till rotation of winter wheat–maize–fallow increased total annualized grain yield by 75% compared to winter wheat-summer fallow. In addition, soil erosion was reduced to just 25% of that from a conventional tillage wheat–summer fallow system. Cover crops have been successfully integrated into CA systems in many parts of the world, as a means of providing surface cover, improving soil fertility and suppressing weeds. Flower et al. (2012) examined the effect of different crop sequences, which included oat cover crops and grass pasture, on soil N dynamics and showed that N mineralization following oat cover crops was similar to that following wheat and barley. The authors concluded that a managed pasture appears to be a better option than oat cover crops because of the relatively low cost and increased soil water storage. Ward et al. (2012) concluded that the inclusion of cover crops may increase overall sustainability of Mediterranean farming systems, although it was unlikely to have significant impacts on soil water balance. CA is widely adopted for soybean-based cropping systems in Brazil. Zotarelli et al. (2012) analyzed the effect of CA on N2 -fixation in soybean and other legumes, in order to test the hypothesis that a system richer in N would bring about a positive effect on soil C stocks. Grain yields under CA and CT were different in some years, being higher under CA for soybean and under CT for maize. Legume reliance on N2 -fixation was consistently higher under CA than under CT. The results highlight the importance of CA for enhancing N2 -fixation inputs to the system and the need to better consider N balance as the key driver of C stock changes in the soil. In addition, it suggests that in this study CA had the consequence of avoiding soil C loss rather than increasing soil C stocks. The combined interaction of tillage and vegetative barriers on soil and water conservation, and their effects on crop yield were investigated in the sub-humid zone of the Central Kenya Highlands (Guto et al., 2012). The authors concluded that a strategy for the successful introduction of vegetative barriers into water-deficient farming environments should ensure minimum competition for water between crops and barriers. A judicious choice should be made between adapted tree species such as Napier, which are more efficient at capturing rainwater but compete with crops for available water, and species such as Leucaena which seem to compete less for water with the crops, but are less efficient at capturing rainwater.
4. Weed management and mechanization One of the major challenges reducing the adoption of CA, especially in smallholdings, is related to weeds and weed management
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under reduced or no tillage (Giller et al., 2011). The dynamics of the weed population under CA is entirely different from conventional systems; the eco-physiological responses of weeds and their interaction with crops under CA management practices tend to be more complex and not always well understood (Chauhan et al., 2006). After sowing a crop using CA, a large proportion of the weed seed bank generally remains on or close to the soil surface. Perennial weeds can also become a major problem in the absence of tillage and proper timely management. On the other hand, crop residues, when uniformly covering the soil surface under CA could potentially suppress weed seedling emergence, delay time to emergence, and allow the crop to benefit from an initial advantage in terms of early vigor over weeds (Chauhan et al., 2006). Weed seeds present on the soil surface under CA are also generally more prone to desiccation and greater predation activity of insects (Baraibar et al., 2009). The application of pre-emergence herbicides can potentially limit weed development under CA, but it can also result in low herbicide efficacy due to interception of a considerable proportion of the applied herbicide by crop residues. Evolved weed resistance to herbicides is also one of the greatest challenges to long-term sustainability of CA in intensive systems. However, several examples of successful weed management under CA have been reported (Farooq et al., 2011). In most dryland production systems in developing countries, adoption of CA can be hindered by the availability of effective herbicides. Alternative approaches such as stale-seed-bed technique, uniform and dense crop establishment, use of cover crops and crop residues, crop rotations and practices for enhanced crop competiveness with a combination of pre- and post-emergence herbicides and adapted small-scale planters could be integrated to develop sustainable weed management strategies under CA systems (Johansen et al., 2012). Flower et al. (2012) reported that oat cover crops and grass pasture were effective at managing weeds, even in continuous cereal rotations. Two consecutive years of cover crop were required for good annual control of ryegrass (Lolium rigidum Gaud.) in a predominantly cereal rotation. The authors also found that the timing of when cover crops were killed by herbicide was crucial for good weed control, as failure to prevent weed seed set resulted in significantly lower weed control. Also, late killing of the cover crop reduced soil water storage. The inclusion of an oat cover crop in the rotation reduced the three-year average gross margin; but managed pasture, with herbicide control of weed seed set, appears to be a better option than oat cover crops because of the relatively low cost and increased soil water storage. However, the authors concluded that the profitability and sustainability of these cropping systems needs to be evaluated over a longer period (Flower et al., 2012). The development of specific CA planting equipment combined with herbicide technology has enabled weed control that is traditionally achieved through tillage in large-scale commercial agriculture. However, large-scale mechanized options are generally not available to smallholder farmers, impeding the adoption of CA practices (Giller et al., 2011). Johansen et al. (2012) reported on the adoption of two-wheel tractors in South Asia, replacing animal-drawn plowing in smallholder farms. Although mechanization speeds up the tillage operation and hence the turnaround time between crops and may increase opportunities for crop intensification, problems associated with full tillage remain. Recently, planter attachments for two-wheel tractors have been developed which permit seed and fertilizer placement with minimum to ZT in a single pass. Evaluations have demonstrated that use of these implements can produce acceptable crop yields equal to or higher than those obtained with CT and hand broadcasting of seed and fertilizer. In addition, fuel and labor costs, seed and fertilizer inputs and turnaround time between crops could also be reduced, and thus
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constitute an incentive for the adoption of CA in smallholdings (Johansen et al., 2012). Johansen et al. (2012) also reported on the successful introduction of animal-drawn rippers and direct seeders in sub-Saharan Africa, originally developed for small-scale farmers in Brazil. This small-scale farmer mechanization can significantly reduce labor for planting if fields are not prepared with the moldboard plow, and benefits may be even greater if herbicides can be effectively used for weed control. It is clear that CA adoption with twowheel tractor-mounted planters and animal-drawn direct-seeding equipment is also constrained by weed management issues. Problems of availability and the safe and effective use of herbicides by resource-poor farmers exist and there is a need to develop further integrated weed management strategies that can be combined with small-scale planters. In addition, there is a need to optimize the performance of small-scale planters to suit farmers’ needs in different agro-ecological environments. It is also evident that the availability and use of small-scale mechanization such as jab-planters, ox- or donkey-drawn rippers depends on the development of an adequate infrastructure for agricultural support through local agro-dealer shops and implement hiring services (Giller et al., 2011). 5. Breeding for CA Most crop varieties used for CA in developing countries were developed under conventional systems, and an important question is whether these varieties are suitable under reduced or no tillage. The genetic improvement of crops for specialized agronomic practices is dependent upon a genotype × tillage practice interaction. Without such an interaction crops could be developed under full tillage and expected to adapt to ZT systems where crop residues are retained. If, on the other hand, interactions exist, breeding specifically for crop varieties adapted to CA conditions may be necessary. Trethowan et al. (2012) observed highly significant genotype × tillage practice and genotype × planting method interactions for grain yield and several grain quality attributes in wheat. These results clearly indicate that wheat cultivars with improved adaptation to CA could be developed with more relevant market quality. A QTL analysis identified several loci associated with specific adaptation to tillage regimes, with a major QTL explaining up to 25% of the variation for yield within an individual cross. The authors concluded that it is possible to breed cultivars with specific adaption to CA although genetic control of adaptation appears to be highly complex and greatly influenced by environment, soil type, planting method and crop rotation (Trethowan et al., 2012). In this context, breeding of crop cultivars suitable for CA, with enhanced early vigor and improved canopy architecture could also strengthen weed competitiveness of these cultivars under CA. Research is needed to find genotypes which are not only suitable for CA but can also form an early dense canopy leaving less space and light for weeds. Reduced light availability at the soil surface can restrict germination and subsequent growth of weed seedlings (Chauhan et al., 2006). Developing crop varieties resistant to residue-borne diseases, with a high litter degradation rate or that could grow vigorously in unplowed soil could also be valuable (Farooq et al., 2011). It is well known that CA-based crop management strategies can change the disease spectrum in some environments. Plant breeders have responded by developing cultivars with a broader range of disease resistance (Trethowan et al., 2012). 6. Crop–livestock integration One of the key principles of CA is the maintenance of soil cover by retaining a proportion of crop residue on the field as mulch. Yet
crop residues are often treated as a commodity and are in great demand in smallholding farming systems, especially in dry areas where biomass is generally a limiting factor. Using harvested crop residues as livestock feeds, combined with free grazing during the lean period, keeps the soil surface bare and prone to erosion and degradation. Valbuena et al. (2012) addresses the knowledge gap of potential trade-offs of crop residues used as mulch in mixed crop–livestock systems, based on village surveys at 12 sites in 9 different countries across Sub-Sahara Africa and South Asia. They showed that biomass production tends to be more substantial in high-density sites, covering demands for livestock feed and allowing part of the residues to be used as mulch. In medium-density sites, although population and livestock densities are relatively less, biomass is scarce and pressure on land and feed are high, increasing the pressure on crop residues and their opportunity cost as mulch. In low-density areas, population and livestock densities are relatively low and communal feed and fuel resources exist, resulting in lower potential pressure on residues on an area basis. Yet, biomass production is low and farmers largely rely on crop residues to feed livestock during the long dry season, implying substantial opportunity costs to their use as mulch (Valbuena et al., 2012). These authors concluded that despite the CA potential benefit for smallholder farmers across the density gradient, the introduction of mulching practices appeared potentially easier at sites where biomass production is high enough to fulfill existing demands for feed and fuel. In sites with relatively high feed and fuel pressure, the introduction of CA would probably require increasing biomass production and/or developing alternative sources to alleviate the opportunity costs of leaving some crop residues as mulch (Lahmar et al., 2012). Further research needs to be devoted to better integration of crop–fodder–tree–livestock production systems in dry areas. Possible soil damage by livestock on cropped land can also be a concern for mixed crop-livestock producers (Farooq et al., 2011) but is not a topic considered in this issue. A recent literature review suggested that treading by livestock increases soil strength and bulk density and reduces macro-porosity and infiltration rate (Bell et al., 2011). However, the effects are mostly confined to the soil surface (<0.1 m) and are short-lived due to amelioration through natural processes or tillage. Few studies report yield penalties in subsequent crops, possibly because effects are too small in magnitude or depth to influence plant growth significantly, or because of the highly season-dependent nature of the crop response. Overall, most experimental evidence shows that grazing livestock has little effect on subsequent crop yields, which would only be possible on structurally degraded soils where surface cover falls below critical levels (Bell et al., 2011). However, the integration of livestock into CA systems requires further research to quantify the potential effects on soil properties and water infiltration.
7. Future thrusts for CA adoption in dry areas Global efforts for the development and dissemination of CA technologies have taken several decades and adoption is still increasing worldwide, including in many of the arid and semiarid areas (Kassam et al., 2012; Kienzler et al., 2012; Mrabet et al., 2012; Wang et al., 2012; Hansen et al., 2012; Llewellyn et al., 2012). These reports are also confirmed by recent evidence for higher crop yields with CA relative to conventional systems in drier climates, as opposed to wet climates (Farooq et al., 2011). However, the lack of precise knowledge on the effects and interactions of minimal soil disturbance, permanent residue cover, planned crop rotations, integrated weed management, and socioeconomic factors, which are all key CA components, can seriously hinder its adoption (Giller
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et al., 2011; Tittonell et al., 2012; Lahmar et al., 2012; Valbuena et al., 2012). Llewellyn et al. (2012) analyzed enabling factors and drivers that have influenced the adoption of CA-related practices, including demand-induced innovation by farmers and agricultural engineers, enabling agronomic technologies such as herbicides and crop disease resistance, extension, and economic forces. Drawing upon existing studies from various regions of the world where CA-related practices have been widely adopted and detailed quantitative analyses of CA adoption in Australia, the authors concluded that the same extension and diffusion processes that led to the current high proportion of farmers in some regions are not likely to lead to highly extensive use of CA in all regions (Llewellyn et al., 2012). As pointed out by Kienzler et al. (2012), in the case of Central Asia, the existence of a general blueprint for a CA approach is highly unlikely because of diverse political, socio-economic and agroecological contexts. Therefore, the adaptation of CA practices to various farming systems requires a shift in research paradigms and current mainstream thinking in favor of cost-effective technologies and agricultural implements specifically suited for small-scale farm enterprises and resource conserving practices. Moreover, successful implementation of site-specific CA practices demands strong support for farmers and awareness raising activities among policymakers.
8. Conclusions The principles that underlie CA are well known and have been tested and validated in various regions around the world, but CA components for crop and agro-ecological resource management are complex and location-specific, including crop residue management, cultivar selection and crop choice for rotation, strategies for nutrient management, tactics for weed management, disease and pest management, and soil water management practices. These components must be managed and fine-tuned for relevant cropping systems, so that the practices are compatible with the principles of CA and most important meet the requirements and are feasible for the farmers. Research and development should focus on better understanding the effects and interaction among all these systems components and to develop site-specific CA options. More importantly, there is a critical need for an active farmer participatory approach in the comprehensive assessment of ecological and socio-economic conditions in which CA could be adapted especially under smallholder farming systems. This should be carried out in the target region in order to identify the most relevant and compatible crop management component technologies for that area. The positive impacts of CA can be harnessed through innovative systems research and development, to enhance agricultural systems productivity, profitability and sustainability for the future.
References Araya, T., Cornelis, W.M., Nyssen, J., Govaerts, B., Getnet, F., Bauer, H., Raes, D., Aregay, N., Haile, M., Deckers, J., 2012. Medium-term effects of Conservation Agriculture based cropping systems for sustainable soil and water management and crop productivity in the Ethiopian highlands. Field Crop Res. 132, 53– 62. Baraibar, B., Westerman, P.R., Carrión, E., Recasens, J., 2009. Effects of tillage and irrigation in cereal fields on weed seed removal by seed predators. J. Appl. Ecol. 46, 380–387. Baudron, F., Tittonell, P., Corbeels, M., Letourmy, P., Giller, K.E., 2012. Comparative performance of conservation agriculture and current smallholder farming practices in semi-arid Zimbabwe. Field Crop Res. 132, 117– 128. Bell, L.W., Kirkegaard, J.A., Swan, A., Hunt, J.R., Huth, N.I., Fettell, N.A., 2011. Impacts of soil damage by grazing livestock on crop productivity. Soil Till. Res. 113, 19–29.
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Chauhan, B.S., Gill, G., Preston, C., 2006. Tillage system effects on weed ecology, herbicide activity and persistence: a review. Aust. J. Exp. Agr. 46, 1557– 1570. Farooq, M., Flower, K.C., Jabran, K., Wahid, A., Siddique, K.H.M., 2011. Crop yield and weed management in rainfed conservation agriculture. Soil Till. Res. 117, 172–183. Flower, K.C., Cordingley, N., Ward, P.R., Weeks, C., 2012. Nitrogen, weed management and economics 1 with cover crops in conservation agriculture in a Mediterranean climate. Field Crop Res. 132, 63–75. Giller, K.E., Witter, E., Corbeels, M., Tittonell, P., 2009. Conservation agriculture and smallholder farming in Africa: the heretics’ view. Field Crop Res. 114, 23– 34. Giller, K.E., Corbeels, M., Nyamangara, J., Triomphe, B., Affholder, F., Scopel, E., Tittonell, P., 2011. A research agenda to explore the role of conservation agriculture in African smallholder farming systems. Field Crop Res. 124, 468– 472. Govaerts, B., Sayre, K.D., Goudeseune, B., Corte, P.D., Lichter, K., Dendooven, L., Deckers, J., 2009. Conservation agriculture as a sustainable option for the central Mexican highlands. Soil Till. Res. 103, 222–230. Guto, S.N., De Ridder, N., Giller, K.E., Pypers, P., Vanlauwe, B., 2012. Minimum tillage and vegetative barrier effects on crop yields in relation to soil water content in the Central Kenya Highlands. Field Crop Res. 132, 129–138. Hansen, N.C., Allen, B.L., Baumhardt, R.L., Lyon, D.J., 2012. Research achievements and adoption of no-till, dryland cropping in the Semi-Arid US Great Plains. Field Crop Res. 132, 196–203. Hobbs, P.R., Sayre, K., Gupta, R., 2008. The role of conservation agriculture in sustainable agriculture. Philos. Trans. R. Soc. London Biol. 363, 543– 555. Johansen, C., Haque, M.E., Bell, R.W., Thierfelder, C., Esdaile, R.J., 2012. Conservation agriculture for small holder rainfed farming: opportunities and constraints of new mechanized seeding systems. Field Crop Res. 132, 18–32. Kassam, A., Friedrich, T., Derpsch, R., Lahmar, R., Mrabet, R., Serraj, R., Basch, G., González-Sánchez, E.J., 2012. Conservation agriculture in the dry Mediterranean climate. Field Crop Res. 132, 7–17. Kienzler, K.M., Lamers, J.P.A., McDonald, A., Mirzabaev, A., Ibragimov, N., Egamberdiev, O., Ruzibaev, E., Akramkhanov, A., 2012. Conservation agriculture in Central Asia – What do we know and where do we go? Field Crop Res. 132, 95–105. Lahmar, R., Bationo, A.B., Lamso, N.D., Guéro, Y., Tittonell, P., 2012. Tailoring conservation agriculture technologies to West Africa semi-arid zones: building on traditional local practices for soil restoration. Field Crop Res. 132, 158– 167. Lal, R., 1985. Soil surface management. In: Muchow, R.C. (Ed.), Agro-Research for the Semi-Arid Tropics: North-West Australia. University of Queensland Press, pp. 273–300. Lal, R., 2007. Constraints to adopting no-till farming in developing countries. Soil Till. Res. 94, 1–3. Llewellyn, R.S., D’Emden, F.H., Kuehne, G., 2012. Extensive use of no-tillage systems. Field Crop Res. 132, 204–212. Mrabet, R., Moussadek, R., Fadlaoui, A., Van Ranst, E., 2012. Conservation agriculture 1 in dry areas of Morocco. Field Crop Res. 132, 84–94. Mupangwa, W., Twomlow, S., Walker, S., 2012. Reduced tillage, mulching and rotational effects on maize (Zea mays L.), cowpea (Vignaunguiculata (Walp) L.) and sorghum (Sorghum bicolor L. (Moench)) yields under semi-arid conditions. Field Crop Res. 132, 139–148. Ngwira, A., Aune, J.B., Mkwinda, S., 2012. On-farm evaluation of yield and economic benefit of short term maize legume intercropping systems under conservation agriculture in Malawi. Field Crop Res. 132, 149–157. Rockstrom, J., Kaumbutho, P., Mwalley, J., Nzabi, A.W., Temesgen, M., Mawenya, L., Barron, J., Mutua, J., Damgaard-Larsen, S., 2009. Conservation farming strategies in East and Southern Africa: yields and rain water productivity from on-farm action research. Soil Till. Res. 103, 23–32. Scopel, E., Da Silva, F., Corbeels, M., Affholder, F., Maraux, F., 2004. Modelling crop residue mulching effects on water use and production of maize under semi-arid and humid tropical conditions. Agronomie 24, 383–395. Sommer, R., Piggin, C., Haddad, A., Hajdibo, A., Hayek, P., Khalil, Y., 2012. Simulating the effects of minimum tillage and crop residue retention 1 on water relations and yield of wheat under rainfed semiarid Mediterranean conditions. Field Crop Res. 132, 40–52. Thierfelder, C., Wall, P.C., 2010. Rotation in conservation agriculture systems of Zambia: effects on soil quality and water relations. Exp. Agric. 46, 309–325. Tittonell, P., Scopel, E., Andrieu, N., Posthumus, H., Mapfumo, P., Corbeels, M., Van Halsema, G.E., Lahmar, R., Lugandu, S., Rakotoarisoa, J., Mtambanengwe, F., Pound, B., Chikowo, R., Naudin, K., Triomphe, B., Mkomwa, S., 2012. Agroecologybased aggradation-1 conservation agriculture (ABACO): targeting innovations to combat soil degradation and food insecurity in semi-arid Africa. Field Crop Res. 132, 168–174. Trethowan, R.M., Mahmood, T., Ali, Z., Oldach, K., Garcia, A.G., 2012. Breeding wheat cultivars better adapted to conservation agriculture. Field Crop Res. 132, 76–83. Valbuena, D., Erenstein, O., Homann-KeeTui, S., Abdoulaye, T., Claessens, L., Duncan, A.J., Gérard, B., Rufino, M., Teufel, N., Van Rooyen, A., Van Wijk, M.T., 2012. Conservation Agriculture in mixed crop-livestock systems: scoping 1 crop residue trade-offs in Sub-Saharan Africa and South Asia. Field Crop Res. 132, 175–184. Wang, X., Wu, H., Dai, K., Dingchen, Z., Feng, Z., Zhao, Q., Wu, X., Jin, K., Cai, D., Oenema, O., Hoogmoed, W.B., 2012. Tillage and crop residue effects on rainfed wheat and maize production in northern China. Field Crop Res. 132, 106–116.
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Ward, P.R., Flower, K.C., Cordingley, N., Weeks, C., Micin, S.F., 2012. Soil water balance with cover crops and conservation agriculture in a Mediterranean climate. Field Crop Res. 132, 33–39. Zotarelli, L., Zatorre, N.P., Boddey, R.M., Urquiaga, S., Jantalia, C.P., Franchini, J.C., Alves, B.J.R., 2012. Influence of no-tillage and frequency of a green manure legume in crop rotations for balancing N outputs and preserving soil organic C stocks. Field Crop Res. 132, 185–195.
Rachid Serraj a,∗ Kadambot H.M. Siddique b a International Centre for Agricultural Research in the Dry Areas (ICARDA), PO Box 5466, Aleppo, Syria
b
The UWA Institute of Agriculture, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia ∗ Corresponding author. E-mail address: [email protected] (R. Serraj)
Available online 27 March 2012
Contents lists available at SciVerse ScienceDirect
Field Crops Research journal homepage: www.elsevier.com/locate/fcr
Preface
Conservation agriculture in dry areas
1. Introduction In general, there are three critical components which define Conservation Agriculture (CA): (1) retaining full or as close as possible full ground cover, (2) no soil compacting and disturbance, and (3) maintaining diverse crop rotations. Major advantages reported for CA are reduced wind and water erosion of topsoil, increased water use efficiency through improved water infiltration and retention, increased nutrient use efficiency through enhanced nutrient cycling and fertilizer placements adjacent to seed, reduced oscillation of surface soil temperatures, increased soil organic matter and diverse soil biology, reduced fuel, labor and overall crop establishment costs, and more timely operations (Hobbs et al., 2008). These incentives could make CA a viable alternative in dry areas, where it would help to address the challenges of scarce and degraded natural resources. However, the supposed benefits of CA are not universal and Giller et al. (2009) pointed out circumstances where it did not appear to be advantageous, particularly for smallholder, resource-poor farmers. Such farmers find it generally difficult to maintain a soil cover of crop residue or a cover crop due to the competing requirements for such biomass for fodder, fuel or building material. Appropriate herbicides are often not available to, or affordable by, resource-poor farmers, resulting in increased weeding costs and weed constraints if tillage is reduced under CA. Although CA is considered as a primary driver to sustainable farming, a practical, location – specific and not rigid approach is needed. The global area currently under CA is reaching 120 M ha, corresponding to about 8.5% of arable cropped land, spread across all continents and agro-ecologies, including the dry Mediterraneantype environments (Kassam et al., 2012). Growing scientific and empirical evidence shows that significant productivity, economic, social and environmental benefits can be harnessed through the adoption of CA principles for sustainable production intensification in dry environments. Crop yield potential under CA management practices in rainfed systems is often greater than with conventional tillage (CT) systems; particularly in dry environments where sub-optimal rainfall limits yield (Farooq et al., 2011). A recent review of long-term and multilocation experiments confirmed these trends by showing slight increases over time in crop yields under CA relative to CT, indicating that CA can compete with CT on a crop production basis, in addition to the well-established environmental benefits associated with CA (Farooq et al., 2011). The improved performance of CA relative to CT in drier climates fits with the model proposed by Lal (1985), although high variability in the 0378-4290/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2012.03.002
data emphasized the importance of location specificity of tillage studies and that the model proposed was a ‘tentative’ generalization for discussion, followed with further experimentation and demonstration. Despite tangible benefits that CA can potentially contribute to agro-ecosystems, there remains a debate whether and under what conditions to promote CAfor smallholder farming systems, especially in sub-Saharan Africa (Giller et al., 2011). The low adoption of CA in parts of semi-arid Africa could be due to the fact that CA is often promoted as a package, without proper fine-tuning of technologies and adaptation to local circumstances (Tittonell et al., 2012). Local production constraints often limit implementation of CA principles by resource-poor farmers, resulting in lower than expected benefits. Factors that reduce the adoption from the benefits of CA include weeds, feed demand by animals, root and foliar diseases and inhibitory organism in the soil. System trade-offs in the allocation of resources at farm and village levels can be important factors determining the success of CA introduction for a given farming system (Giller et al., 2011). Additional socioeconomic constraints such as conditions of market access, institutional and policy issues, and other parameters related to partnerships and interactions among stakeholders are important factors for the introduction of CA at national and regional levels. Recent suggestions that ex-ante analysis of situations for where and how CA could fit into smallholding farming systems would require an innovative and interdisciplinary research in order to identify and address major knowledge gaps and system tradeoffs. Farmers who use CA management practices are seeking to improve their profitability, productivity and preserve the value of their natural resources. The objective of this special issue is to enrich the ongoing debate on the role of CA especially in smallholder farming systems by synthesizing the recent research findings of CA in dry areas, by analyzing agronomic, socio-economic, and agro-ecological determinants leading to its success or failure, and by identifying potential entry points for priorities for future research on CA in dry areas.
2. Does CA improve soil water balance? The productivity benefits from CA are mainly associated with its positive environmental and soil effects compared to conventional systems, including reduced erosion, runoff and surface crusting, increased aggregate distribution and stability, and increased infiltration and soil water content and water use efficiency (Hobbs et al.,
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2008; Thierfelder and Wall, 2010; Farooq et al., 2011). Water conservation is considered a key element of CA, especially in dryland areas exposed to erratic and unreliable rainfall. To demonstrate the effects of mulching on infiltration of rainfall, water balance studies are needed to analyze rainfall capture, soil storage and crop water use, including simple measurements of rainfall productivity of CA compared to conventional farming methods (Rockstrom et al., 2009). A general tendency of improved rainfall productivity was reported under CA in dry locations, which could be explained by a water harvesting effect, leading to a strategy for in situ moisture conservation. This strategy was confirmed in various smallholder farming systems in the savannah agro-ecosystems of East and Southern Africa (Rockstrom et al., 2009). The importance of crop residue retention in the CA system for improving soil stability and water balance was also demonstrated in the highlands of Mexico (Govaerts et al., 2009). The description of CA as a water-harvesting technology is challenged by the findings of Baudron et al. (2012) in a semi-arid area of Zimbabwe. CA did not affect cotton productivity during the first two years of the experiment, which received average or above average rainfall, and had a negative effect on crop yield in both on-farm trials and farmers’ fields during the drier season. There was no difference in water runoff between CA and CT on a relatively fine-textured soil, but significantly more runoff with CA on a coarser-textured soil due to soil surface crusting. Although most soils in the study area fall into the same category, the authors concluded that soil crusting might be avoided by increasing mulch production (ground cover) beyond what was achieved in their study. Araya et al. (2012) reported the results of a medium-term tillage experiment carried out on a vertisol to quantify changes in runoff, soil loss and crop yield due to CA in a sub-humid catchment in the northern Ethiopian highlands. A treatment combining permanent raised beds with a 30% standing crop residue and no-tillage seeding resulted in significantly less soil losses due to erosion and water runoff, leading to better crop performance, but it took at least five years of cropping before the difference was significant. Similarly, long-term adoption of CA in dryland cropping in the semi-arid US Great Plains resulted in greater precipitation storage and use efficiency, which led to greater cropping intensity, higher productivity, more diverse crop rotations, and improvements in soil properties (Hansen et al., 2012). The effect of cover cropping on soil water balance components has been documented in many locations around the world, but has not yet been explored in dryland grain production systems in regions with a Mediterranean climate. Ward et al. (2012) investigated the use of cover crops, grown solely to increase ground cover and not harvested for grain or biomass, under a Mediterranean-type climate in southwestern Australia. They examined the impact of cover crops and residue retention on evapotranspiration—over the summer fallow period and during the winter and spring crop growth period—and on deep drainage from subsequent crops, on two contrasting soil types. In contrast to previously published research, cover crops and residue retention had little impact on total evaporation during summer and autumn, although there were occasional short-term impacts on the rate of evaporation shortly after rainfall. There was also limited evidence of changes in evaporation during early crop growth. Drainage under crops grown after cover crops was not consistently different to drainage from crops grown after conventional crops. The authors concluded that the inclusion of cover crops in Mediterranean farming systems is unlikely to have major impacts on water balance, but may increase overall sustainability of the farming system. Simulation modeling can be a useful tool to analyze complex interactions influencing the effects of CA soil water balance and
crop water use (Scopel et al., 2004; Tittonell et al., 2012). Sommer et al. (2012) applied the crop–soil simulation model CropSyst to study soil water dynamics and water balance of wheat grown after barley in northern Syria under zero tillage (ZT) compared to CT. Simulation results showed that in 25 of 30 years, ZT yields were higher than those of CT, but ZT and residue retention had only a minor effect on reducing the amount of seasonal precipitation lost by unproductive soil evaporation. The authors argued that the impact of ZT on water balance might be limited to soils with selfmulching characteristics that are common in the Mediterranean regions.
3. Residue management and crop rotation Crop and soil management in dry areas generally involves conventional and sometimes excessive soil tillage and a lack of appropriate crop residue management, which frequently results in deterioration of soil physical structure and loss of soil fertility (Lal, 2007). Low crop yields due to continuous monocropping and deteriorating soil health in smallholder farmers fields have led to a search for more sustainable production practices with greater resource use efficiency. The shifting of crop production systems to CA and retention of crop residues has been reported to induce progressive qualitative and quantitative variations in soil organic cover, which impacts soil water balance, biological activity, soil organic matter build-up and fertility replenishment (Kassam et al., 2012). However, it is obvious that organic resources are often the most limiting factor in dry areas, where crop residues are generally used to feed livestock. Lahmar et al. (2012) argue that CA may not fully succeed in the Sahel region unless alternative sources of biomass are identified. They suggest a double phase of aggradation/conservation, by gradually rehabilitating the biomass production function of the soil through increased nutrient input and traditional water harvesting measures, encouraging regeneration of native evergreen multipurpose woody shrubs often associated with crops. A shift to the classical, less labor intensive CA practices would occur once appropriate levels of soil fertility and water capture are sufficient to allow an increase in the primary productivity of the agroecosystem. Another alternative approach was recently tested by the Agroecology-Based Aggradation–Conservation Agriculture (ABACO) initiative, which engages farmers in the design and implementation of locallysuited CA practices aimed at establishing a long-term strategy for soil rehabilitation (Tittonell et al., 2012). This approach is carried out through local innovation platforms coordinated through the African Conservation Tillage Network (www.act-africa.org), and relies on agro-ecologically intensive measures for increased water productivity and soil rehabilitation in semi-arid regions of Africa. Under semi-arid conditions in southern Zimbabwe, Mupangwa et al. (2012) reported that maize grain yield increased with increased mulch cover in those seasons that had below average rainfall, although the tillage system itself had no significant effect on maize yield. Mulching at a rate of 2–4 tha−1 led to optimum yields in seasons with below average rainfall. Under CA, a maize–cowpea–sorghum–maize rotation had significantly higher grain yield than first year maize, a maize–maize–monocrop and a maize–cowpea–maize rotation. In the dry semi-humid regions of northern China, Wang et al. (2012) observed that the benefits of no-till or reduced tillage practices on soil physical conditions were more pronounced when in combination with residue application. However, they also reported that surface application of crop residue for maize may delay
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seedling emergence because of low soil temperatures, and concluded that incorporation of residue in combination with reduced tillage might be a better solution in that particular environment. Baudron et al. (2012) reported that retention of sorghum residues and inclusion of N2 -fixing legumes in cotton-based cropping systems resulted in reduced N export by cropping from CA fields compared with conventional farmer practices in semi-arid Zimbabwe. The authors concluded that this may result in long-term beneficial effects of CA on crop yields, and discussed the possibility of increasing the short-term benefits of CA by improving the sorghum–intercrop association. Ngwira et al. (2012) carried out on-farm evaluation of short term maize–legume intercropping systems under conservation agriculture in Malawi. They found that CA had a positive effect on maize grain yield at two sites during the drier seasons. While the authors could not estimate the effects on water balances in farmer-managed experiments, they speculated that yield differences between CA and CT could be attributed to tillage and crop residue cover since other farm operations were generally similar. Hansen et al. (2012) reported that in Colorado, a no-till rotation of winter wheat–maize–fallow increased total annualized grain yield by 75% compared to winter wheat-summer fallow. In addition, soil erosion was reduced to just 25% of that from a conventional tillage wheat–summer fallow system. Cover crops have been successfully integrated into CA systems in many parts of the world, as a means of providing surface cover, improving soil fertility and suppressing weeds. Flower et al. (2012) examined the effect of different crop sequences, which included oat cover crops and grass pasture, on soil N dynamics and showed that N mineralization following oat cover crops was similar to that following wheat and barley. The authors concluded that a managed pasture appears to be a better option than oat cover crops because of the relatively low cost and increased soil water storage. Ward et al. (2012) concluded that the inclusion of cover crops may increase overall sustainability of Mediterranean farming systems, although it was unlikely to have significant impacts on soil water balance. CA is widely adopted for soybean-based cropping systems in Brazil. Zotarelli et al. (2012) analyzed the effect of CA on N2 -fixation in soybean and other legumes, in order to test the hypothesis that a system richer in N would bring about a positive effect on soil C stocks. Grain yields under CA and CT were different in some years, being higher under CA for soybean and under CT for maize. Legume reliance on N2 -fixation was consistently higher under CA than under CT. The results highlight the importance of CA for enhancing N2 -fixation inputs to the system and the need to better consider N balance as the key driver of C stock changes in the soil. In addition, it suggests that in this study CA had the consequence of avoiding soil C loss rather than increasing soil C stocks. The combined interaction of tillage and vegetative barriers on soil and water conservation, and their effects on crop yield were investigated in the sub-humid zone of the Central Kenya Highlands (Guto et al., 2012). The authors concluded that a strategy for the successful introduction of vegetative barriers into water-deficient farming environments should ensure minimum competition for water between crops and barriers. A judicious choice should be made between adapted tree species such as Napier, which are more efficient at capturing rainwater but compete with crops for available water, and species such as Leucaena which seem to compete less for water with the crops, but are less efficient at capturing rainwater.
4. Weed management and mechanization One of the major challenges reducing the adoption of CA, especially in smallholdings, is related to weeds and weed management
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under reduced or no tillage (Giller et al., 2011). The dynamics of the weed population under CA is entirely different from conventional systems; the eco-physiological responses of weeds and their interaction with crops under CA management practices tend to be more complex and not always well understood (Chauhan et al., 2006). After sowing a crop using CA, a large proportion of the weed seed bank generally remains on or close to the soil surface. Perennial weeds can also become a major problem in the absence of tillage and proper timely management. On the other hand, crop residues, when uniformly covering the soil surface under CA could potentially suppress weed seedling emergence, delay time to emergence, and allow the crop to benefit from an initial advantage in terms of early vigor over weeds (Chauhan et al., 2006). Weed seeds present on the soil surface under CA are also generally more prone to desiccation and greater predation activity of insects (Baraibar et al., 2009). The application of pre-emergence herbicides can potentially limit weed development under CA, but it can also result in low herbicide efficacy due to interception of a considerable proportion of the applied herbicide by crop residues. Evolved weed resistance to herbicides is also one of the greatest challenges to long-term sustainability of CA in intensive systems. However, several examples of successful weed management under CA have been reported (Farooq et al., 2011). In most dryland production systems in developing countries, adoption of CA can be hindered by the availability of effective herbicides. Alternative approaches such as stale-seed-bed technique, uniform and dense crop establishment, use of cover crops and crop residues, crop rotations and practices for enhanced crop competiveness with a combination of pre- and post-emergence herbicides and adapted small-scale planters could be integrated to develop sustainable weed management strategies under CA systems (Johansen et al., 2012). Flower et al. (2012) reported that oat cover crops and grass pasture were effective at managing weeds, even in continuous cereal rotations. Two consecutive years of cover crop were required for good annual control of ryegrass (Lolium rigidum Gaud.) in a predominantly cereal rotation. The authors also found that the timing of when cover crops were killed by herbicide was crucial for good weed control, as failure to prevent weed seed set resulted in significantly lower weed control. Also, late killing of the cover crop reduced soil water storage. The inclusion of an oat cover crop in the rotation reduced the three-year average gross margin; but managed pasture, with herbicide control of weed seed set, appears to be a better option than oat cover crops because of the relatively low cost and increased soil water storage. However, the authors concluded that the profitability and sustainability of these cropping systems needs to be evaluated over a longer period (Flower et al., 2012). The development of specific CA planting equipment combined with herbicide technology has enabled weed control that is traditionally achieved through tillage in large-scale commercial agriculture. However, large-scale mechanized options are generally not available to smallholder farmers, impeding the adoption of CA practices (Giller et al., 2011). Johansen et al. (2012) reported on the adoption of two-wheel tractors in South Asia, replacing animal-drawn plowing in smallholder farms. Although mechanization speeds up the tillage operation and hence the turnaround time between crops and may increase opportunities for crop intensification, problems associated with full tillage remain. Recently, planter attachments for two-wheel tractors have been developed which permit seed and fertilizer placement with minimum to ZT in a single pass. Evaluations have demonstrated that use of these implements can produce acceptable crop yields equal to or higher than those obtained with CT and hand broadcasting of seed and fertilizer. In addition, fuel and labor costs, seed and fertilizer inputs and turnaround time between crops could also be reduced, and thus
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constitute an incentive for the adoption of CA in smallholdings (Johansen et al., 2012). Johansen et al. (2012) also reported on the successful introduction of animal-drawn rippers and direct seeders in sub-Saharan Africa, originally developed for small-scale farmers in Brazil. This small-scale farmer mechanization can significantly reduce labor for planting if fields are not prepared with the moldboard plow, and benefits may be even greater if herbicides can be effectively used for weed control. It is clear that CA adoption with twowheel tractor-mounted planters and animal-drawn direct-seeding equipment is also constrained by weed management issues. Problems of availability and the safe and effective use of herbicides by resource-poor farmers exist and there is a need to develop further integrated weed management strategies that can be combined with small-scale planters. In addition, there is a need to optimize the performance of small-scale planters to suit farmers’ needs in different agro-ecological environments. It is also evident that the availability and use of small-scale mechanization such as jab-planters, ox- or donkey-drawn rippers depends on the development of an adequate infrastructure for agricultural support through local agro-dealer shops and implement hiring services (Giller et al., 2011). 5. Breeding for CA Most crop varieties used for CA in developing countries were developed under conventional systems, and an important question is whether these varieties are suitable under reduced or no tillage. The genetic improvement of crops for specialized agronomic practices is dependent upon a genotype × tillage practice interaction. Without such an interaction crops could be developed under full tillage and expected to adapt to ZT systems where crop residues are retained. If, on the other hand, interactions exist, breeding specifically for crop varieties adapted to CA conditions may be necessary. Trethowan et al. (2012) observed highly significant genotype × tillage practice and genotype × planting method interactions for grain yield and several grain quality attributes in wheat. These results clearly indicate that wheat cultivars with improved adaptation to CA could be developed with more relevant market quality. A QTL analysis identified several loci associated with specific adaptation to tillage regimes, with a major QTL explaining up to 25% of the variation for yield within an individual cross. The authors concluded that it is possible to breed cultivars with specific adaption to CA although genetic control of adaptation appears to be highly complex and greatly influenced by environment, soil type, planting method and crop rotation (Trethowan et al., 2012). In this context, breeding of crop cultivars suitable for CA, with enhanced early vigor and improved canopy architecture could also strengthen weed competitiveness of these cultivars under CA. Research is needed to find genotypes which are not only suitable for CA but can also form an early dense canopy leaving less space and light for weeds. Reduced light availability at the soil surface can restrict germination and subsequent growth of weed seedlings (Chauhan et al., 2006). Developing crop varieties resistant to residue-borne diseases, with a high litter degradation rate or that could grow vigorously in unplowed soil could also be valuable (Farooq et al., 2011). It is well known that CA-based crop management strategies can change the disease spectrum in some environments. Plant breeders have responded by developing cultivars with a broader range of disease resistance (Trethowan et al., 2012). 6. Crop–livestock integration One of the key principles of CA is the maintenance of soil cover by retaining a proportion of crop residue on the field as mulch. Yet
crop residues are often treated as a commodity and are in great demand in smallholding farming systems, especially in dry areas where biomass is generally a limiting factor. Using harvested crop residues as livestock feeds, combined with free grazing during the lean period, keeps the soil surface bare and prone to erosion and degradation. Valbuena et al. (2012) addresses the knowledge gap of potential trade-offs of crop residues used as mulch in mixed crop–livestock systems, based on village surveys at 12 sites in 9 different countries across Sub-Sahara Africa and South Asia. They showed that biomass production tends to be more substantial in high-density sites, covering demands for livestock feed and allowing part of the residues to be used as mulch. In medium-density sites, although population and livestock densities are relatively less, biomass is scarce and pressure on land and feed are high, increasing the pressure on crop residues and their opportunity cost as mulch. In low-density areas, population and livestock densities are relatively low and communal feed and fuel resources exist, resulting in lower potential pressure on residues on an area basis. Yet, biomass production is low and farmers largely rely on crop residues to feed livestock during the long dry season, implying substantial opportunity costs to their use as mulch (Valbuena et al., 2012). These authors concluded that despite the CA potential benefit for smallholder farmers across the density gradient, the introduction of mulching practices appeared potentially easier at sites where biomass production is high enough to fulfill existing demands for feed and fuel. In sites with relatively high feed and fuel pressure, the introduction of CA would probably require increasing biomass production and/or developing alternative sources to alleviate the opportunity costs of leaving some crop residues as mulch (Lahmar et al., 2012). Further research needs to be devoted to better integration of crop–fodder–tree–livestock production systems in dry areas. Possible soil damage by livestock on cropped land can also be a concern for mixed crop-livestock producers (Farooq et al., 2011) but is not a topic considered in this issue. A recent literature review suggested that treading by livestock increases soil strength and bulk density and reduces macro-porosity and infiltration rate (Bell et al., 2011). However, the effects are mostly confined to the soil surface (<0.1 m) and are short-lived due to amelioration through natural processes or tillage. Few studies report yield penalties in subsequent crops, possibly because effects are too small in magnitude or depth to influence plant growth significantly, or because of the highly season-dependent nature of the crop response. Overall, most experimental evidence shows that grazing livestock has little effect on subsequent crop yields, which would only be possible on structurally degraded soils where surface cover falls below critical levels (Bell et al., 2011). However, the integration of livestock into CA systems requires further research to quantify the potential effects on soil properties and water infiltration.
7. Future thrusts for CA adoption in dry areas Global efforts for the development and dissemination of CA technologies have taken several decades and adoption is still increasing worldwide, including in many of the arid and semiarid areas (Kassam et al., 2012; Kienzler et al., 2012; Mrabet et al., 2012; Wang et al., 2012; Hansen et al., 2012; Llewellyn et al., 2012). These reports are also confirmed by recent evidence for higher crop yields with CA relative to conventional systems in drier climates, as opposed to wet climates (Farooq et al., 2011). However, the lack of precise knowledge on the effects and interactions of minimal soil disturbance, permanent residue cover, planned crop rotations, integrated weed management, and socioeconomic factors, which are all key CA components, can seriously hinder its adoption (Giller
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et al., 2011; Tittonell et al., 2012; Lahmar et al., 2012; Valbuena et al., 2012). Llewellyn et al. (2012) analyzed enabling factors and drivers that have influenced the adoption of CA-related practices, including demand-induced innovation by farmers and agricultural engineers, enabling agronomic technologies such as herbicides and crop disease resistance, extension, and economic forces. Drawing upon existing studies from various regions of the world where CA-related practices have been widely adopted and detailed quantitative analyses of CA adoption in Australia, the authors concluded that the same extension and diffusion processes that led to the current high proportion of farmers in some regions are not likely to lead to highly extensive use of CA in all regions (Llewellyn et al., 2012). As pointed out by Kienzler et al. (2012), in the case of Central Asia, the existence of a general blueprint for a CA approach is highly unlikely because of diverse political, socio-economic and agroecological contexts. Therefore, the adaptation of CA practices to various farming systems requires a shift in research paradigms and current mainstream thinking in favor of cost-effective technologies and agricultural implements specifically suited for small-scale farm enterprises and resource conserving practices. Moreover, successful implementation of site-specific CA practices demands strong support for farmers and awareness raising activities among policymakers.
8. Conclusions The principles that underlie CA are well known and have been tested and validated in various regions around the world, but CA components for crop and agro-ecological resource management are complex and location-specific, including crop residue management, cultivar selection and crop choice for rotation, strategies for nutrient management, tactics for weed management, disease and pest management, and soil water management practices. These components must be managed and fine-tuned for relevant cropping systems, so that the practices are compatible with the principles of CA and most important meet the requirements and are feasible for the farmers. Research and development should focus on better understanding the effects and interaction among all these systems components and to develop site-specific CA options. More importantly, there is a critical need for an active farmer participatory approach in the comprehensive assessment of ecological and socio-economic conditions in which CA could be adapted especially under smallholder farming systems. This should be carried out in the target region in order to identify the most relevant and compatible crop management component technologies for that area. The positive impacts of CA can be harnessed through innovative systems research and development, to enhance agricultural systems productivity, profitability and sustainability for the future.
References Araya, T., Cornelis, W.M., Nyssen, J., Govaerts, B., Getnet, F., Bauer, H., Raes, D., Aregay, N., Haile, M., Deckers, J., 2012. Medium-term effects of Conservation Agriculture based cropping systems for sustainable soil and water management and crop productivity in the Ethiopian highlands. Field Crop Res. 132, 53– 62. Baraibar, B., Westerman, P.R., Carrión, E., Recasens, J., 2009. Effects of tillage and irrigation in cereal fields on weed seed removal by seed predators. J. Appl. Ecol. 46, 380–387. Baudron, F., Tittonell, P., Corbeels, M., Letourmy, P., Giller, K.E., 2012. Comparative performance of conservation agriculture and current smallholder farming practices in semi-arid Zimbabwe. Field Crop Res. 132, 117– 128. Bell, L.W., Kirkegaard, J.A., Swan, A., Hunt, J.R., Huth, N.I., Fettell, N.A., 2011. Impacts of soil damage by grazing livestock on crop productivity. Soil Till. Res. 113, 19–29.
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Chauhan, B.S., Gill, G., Preston, C., 2006. Tillage system effects on weed ecology, herbicide activity and persistence: a review. Aust. J. Exp. Agr. 46, 1557– 1570. Farooq, M., Flower, K.C., Jabran, K., Wahid, A., Siddique, K.H.M., 2011. Crop yield and weed management in rainfed conservation agriculture. Soil Till. Res. 117, 172–183. Flower, K.C., Cordingley, N., Ward, P.R., Weeks, C., 2012. Nitrogen, weed management and economics 1 with cover crops in conservation agriculture in a Mediterranean climate. Field Crop Res. 132, 63–75. Giller, K.E., Witter, E., Corbeels, M., Tittonell, P., 2009. Conservation agriculture and smallholder farming in Africa: the heretics’ view. Field Crop Res. 114, 23– 34. Giller, K.E., Corbeels, M., Nyamangara, J., Triomphe, B., Affholder, F., Scopel, E., Tittonell, P., 2011. A research agenda to explore the role of conservation agriculture in African smallholder farming systems. Field Crop Res. 124, 468– 472. Govaerts, B., Sayre, K.D., Goudeseune, B., Corte, P.D., Lichter, K., Dendooven, L., Deckers, J., 2009. Conservation agriculture as a sustainable option for the central Mexican highlands. Soil Till. Res. 103, 222–230. Guto, S.N., De Ridder, N., Giller, K.E., Pypers, P., Vanlauwe, B., 2012. Minimum tillage and vegetative barrier effects on crop yields in relation to soil water content in the Central Kenya Highlands. Field Crop Res. 132, 129–138. Hansen, N.C., Allen, B.L., Baumhardt, R.L., Lyon, D.J., 2012. Research achievements and adoption of no-till, dryland cropping in the Semi-Arid US Great Plains. Field Crop Res. 132, 196–203. Hobbs, P.R., Sayre, K., Gupta, R., 2008. The role of conservation agriculture in sustainable agriculture. Philos. Trans. R. Soc. London Biol. 363, 543– 555. Johansen, C., Haque, M.E., Bell, R.W., Thierfelder, C., Esdaile, R.J., 2012. Conservation agriculture for small holder rainfed farming: opportunities and constraints of new mechanized seeding systems. Field Crop Res. 132, 18–32. Kassam, A., Friedrich, T., Derpsch, R., Lahmar, R., Mrabet, R., Serraj, R., Basch, G., González-Sánchez, E.J., 2012. Conservation agriculture in the dry Mediterranean climate. Field Crop Res. 132, 7–17. Kienzler, K.M., Lamers, J.P.A., McDonald, A., Mirzabaev, A., Ibragimov, N., Egamberdiev, O., Ruzibaev, E., Akramkhanov, A., 2012. Conservation agriculture in Central Asia – What do we know and where do we go? Field Crop Res. 132, 95–105. Lahmar, R., Bationo, A.B., Lamso, N.D., Guéro, Y., Tittonell, P., 2012. Tailoring conservation agriculture technologies to West Africa semi-arid zones: building on traditional local practices for soil restoration. Field Crop Res. 132, 158– 167. Lal, R., 1985. Soil surface management. In: Muchow, R.C. (Ed.), Agro-Research for the Semi-Arid Tropics: North-West Australia. University of Queensland Press, pp. 273–300. Lal, R., 2007. Constraints to adopting no-till farming in developing countries. Soil Till. Res. 94, 1–3. Llewellyn, R.S., D’Emden, F.H., Kuehne, G., 2012. Extensive use of no-tillage systems. Field Crop Res. 132, 204–212. Mrabet, R., Moussadek, R., Fadlaoui, A., Van Ranst, E., 2012. Conservation agriculture 1 in dry areas of Morocco. Field Crop Res. 132, 84–94. Mupangwa, W., Twomlow, S., Walker, S., 2012. Reduced tillage, mulching and rotational effects on maize (Zea mays L.), cowpea (Vignaunguiculata (Walp) L.) and sorghum (Sorghum bicolor L. (Moench)) yields under semi-arid conditions. Field Crop Res. 132, 139–148. Ngwira, A., Aune, J.B., Mkwinda, S., 2012. On-farm evaluation of yield and economic benefit of short term maize legume intercropping systems under conservation agriculture in Malawi. Field Crop Res. 132, 149–157. Rockstrom, J., Kaumbutho, P., Mwalley, J., Nzabi, A.W., Temesgen, M., Mawenya, L., Barron, J., Mutua, J., Damgaard-Larsen, S., 2009. Conservation farming strategies in East and Southern Africa: yields and rain water productivity from on-farm action research. Soil Till. Res. 103, 23–32. Scopel, E., Da Silva, F., Corbeels, M., Affholder, F., Maraux, F., 2004. Modelling crop residue mulching effects on water use and production of maize under semi-arid and humid tropical conditions. Agronomie 24, 383–395. Sommer, R., Piggin, C., Haddad, A., Hajdibo, A., Hayek, P., Khalil, Y., 2012. Simulating the effects of minimum tillage and crop residue retention 1 on water relations and yield of wheat under rainfed semiarid Mediterranean conditions. Field Crop Res. 132, 40–52. Thierfelder, C., Wall, P.C., 2010. Rotation in conservation agriculture systems of Zambia: effects on soil quality and water relations. Exp. Agric. 46, 309–325. Tittonell, P., Scopel, E., Andrieu, N., Posthumus, H., Mapfumo, P., Corbeels, M., Van Halsema, G.E., Lahmar, R., Lugandu, S., Rakotoarisoa, J., Mtambanengwe, F., Pound, B., Chikowo, R., Naudin, K., Triomphe, B., Mkomwa, S., 2012. Agroecologybased aggradation-1 conservation agriculture (ABACO): targeting innovations to combat soil degradation and food insecurity in semi-arid Africa. Field Crop Res. 132, 168–174. Trethowan, R.M., Mahmood, T., Ali, Z., Oldach, K., Garcia, A.G., 2012. Breeding wheat cultivars better adapted to conservation agriculture. Field Crop Res. 132, 76–83. Valbuena, D., Erenstein, O., Homann-KeeTui, S., Abdoulaye, T., Claessens, L., Duncan, A.J., Gérard, B., Rufino, M., Teufel, N., Van Rooyen, A., Van Wijk, M.T., 2012. Conservation Agriculture in mixed crop-livestock systems: scoping 1 crop residue trade-offs in Sub-Saharan Africa and South Asia. Field Crop Res. 132, 175–184. Wang, X., Wu, H., Dai, K., Dingchen, Z., Feng, Z., Zhao, Q., Wu, X., Jin, K., Cai, D., Oenema, O., Hoogmoed, W.B., 2012. Tillage and crop residue effects on rainfed wheat and maize production in northern China. Field Crop Res. 132, 106–116.
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Preface / Field Crops Research 132 (2012) 1–6
Ward, P.R., Flower, K.C., Cordingley, N., Weeks, C., Micin, S.F., 2012. Soil water balance with cover crops and conservation agriculture in a Mediterranean climate. Field Crop Res. 132, 33–39. Zotarelli, L., Zatorre, N.P., Boddey, R.M., Urquiaga, S., Jantalia, C.P., Franchini, J.C., Alves, B.J.R., 2012. Influence of no-tillage and frequency of a green manure legume in crop rotations for balancing N outputs and preserving soil organic C stocks. Field Crop Res. 132, 185–195.
Rachid Serraj a,∗ Kadambot H.M. Siddique b a International Centre for Agricultural Research in the Dry Areas (ICARDA), PO Box 5466, Aleppo, Syria
b
The UWA Institute of Agriculture, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia ∗ Corresponding author. E-mail address: [email protected] (R. Serraj)
Available online 27 March 2012