Productivity of intensified crop—fallow rotations in the Trenbath model

Management of crop—fallow rotations should strike a balance between exploitation, during cropping, and restoration of soil fertility during the fallow period. The Trenbath model describes build-up of soil fertility during a fallow period by two parameters (a maximum level and a half-recovery time) and decline during cropping as a simple proportion. The model can be used to predict potential crop production for a large number of management options consisting of length of cropping period and duration of fallow. In solving the equations, the model can be restricted to sustainable systems, where fallow length is sufficient to restore soil fertility to its value at the start of the previous cropping period. The model outcome suggests that the highest yields per unit of land can be obtained by starting a new cropping period after soil fertility has recovered to 50–60% of its maximum value. This prediction is virtually independent of the growth rate of the fallow vegetation. The nature of the fallow vegetation (natural regrowth, planted trees, or cover crops) mainly influences the crop yield by modifying the required duration of fallow periods. Intensification of land use by shortening fallow periods will initially increase returns per unit land at the likely costs of returns per unit labor. When fallows no longer restore soil fertility to 50% of the maximum, overall productivity will decline both per unit land and per unit labor, unless external inputs replace the soil fertility restoring functions of a fallow.This revised version was published online in November 2005 with corrections to the Cover Date.     

Sesbania sesban improved fallows in eastern Zambia: Their inception, development and farmer enthusiasm

In eastern Zambia, nitrogen deficiency is a major limiting factor for increased food production. Soil fertility has been declining because of nearly continuous maize (Zea mays) cultivation with little or no nutrient inputs. The use of short-duration tree fallows was one of several agroforestry options hypothesized to restore soil fertility. Sesbania sesban, an indigenous N₂-fixing tree was the most promising among species tested in screening trials. Several studies since 1987 have demonstrated the dramatic potential of two- or three-year sesbania fallows in restoring soil fertility and increasing maize yields. Analyses showed that these improved fallow systems were feasible, profitable, and acceptable to farmers. Results suggest that high maize yields following fallows are primarily due to improved N input and availability by the fallows. The potential to increase maize production without applying mineral fertilizers has excited thousands of farmers who are enthusiastically participating in the evaluation of this technology. The number of farmers who are testing a range of improved fallow practices has increased from 200 in 1994 to over 3000 in 1997. Presently, a strong network of institutions comprising government, NGOs, development projects, and farmer organizations is facilitating the adaptive research and expansion of improved fallow technology in eastern Zambia. Key elements in the research process that contributed to the achievements are effective diagnosis of farmers' problems, building on farmers' indigenous knowledge, generating several different fallow options for farmers to test, ex-ante economic analysis, farmer participation in on-farm trials, and development of a network for adaptive research and dissemination.This revised version was published online in November 2005 with corrections to the Cover Date.     

Food-crop-based production systems as sustainable alternatives for Imperata grasslands?

Purely annual crop-based production systems have limited scope to be sustainable under upland conditions prone to infestation by Imperata cylindrica if animal or mechanical tillage is not available. Farmers who must rely on manual cultivation of grassland soils can achieve some success in suppressing Imperata for a number of years using intensive relay and intercropping systems that maintain a dense soil cover throughout the year, especially where leguminous cover crops are included in the crop cycle. However, labour investment increases and returns to labour tend to decrease in successive years as weed pressure intensifies and soil quality declines.Continuous crop production has been sustained in many Imperata-infested areas where farmers have access to animal or tractor draft power. Imperata control is not a major problem in such situations. Draft power drastically reduces the labour requirements in weed control. Sustained crop production is then dependent more solely upon soil fertility management. Mixed farming systems that include cattle may also benefit from manure application to the cropped area, and the use of non-cropped fallow areas for grazing. In extensive systems where Imperata infestation is tolerated, cassava or sugarcane are often the crops with the longest period of viable production as the land degrades.On sloping Imperata lands, conservation farming practices are necessary to sustain annual cropping. Pruned tree hedgerows have often been recommended for these situations. On soils that are not strongly acidic they may consistently improve yields. But labour is the scarcest resource on small farms and tree-pruning is usually too labour-intensive to be practical. Buffer strip systems that provide excellent soil conservation but minimize labour have proven much more popular with farmers. Prominent among these are natural vegetative strips, or strips of introduced fodder grasses.The value of Imperata to restore soil fertility is low, particularly compared with woody secondary growth or Compositae species such as Chromolaena odorata or Tithonia diversifolia. Therefore, fallow-rotation systems where farmers can intervene to shift the fallow vegetation toward such naturally-occurring species, or can manage introduced cover crop species such as Mucuna utilis cv. cochinchinensis, enable substantial gains in yields and sustainability. Tree fallows are used successfully to achieve sustained cropping by some upland communities. A variation of this is rotational hedgerow intercropping, where a period of cropping is followed by one or more years of tree growth to generate nutrient-rich biomass, rehabilitate the soil, and suppress Imperata. These options, which suit farmers in quite resource-poor situations, should receive more attention.     

Farmer assessment and economic evaluation of shrub fallows in the Humid Lowlands of Cameroon

Food crop production in highly populated areas around major cities of the humid lowlands of Cameroon is highly dependent on a fallow system (two–four years duration) mainly of Chromolaena odorata. Where such fallows have been in use for some time, problems of soil fertility with declining crop yields and higher incidence of weeds were reported. Although improved fallows have been widely adopted in sub-humid zones, there is no evidence of successful adoption of agroforestry-based technologies for soil fertility improvement in the humid forest areas. In response, ICRAF has developed a short fallow system with Cajanus cajan for soil fertility improvement in the humid lowlands of West Africa. Farmers' response to these cajanus fallows is positive. Benefits reported are higher crop yields after cajanus fallows compared to natural fallows, clearing of cajanus is easier and the shrubs shade out the weeds. Women particularly appreciate the technology for its low labour demand and for the fact that these shrubs can be planted on land with less secure tenure. Economic analysis of cajanus fallows compared to natural fallow over six years shows that cajanus fallows are profitable under most tested scenarios, both in terms of returns to land and to labour. It seems that improved fallows with Cajanus cajan are a good response to shortening natural fallows for households in the humid lowlands of Cameroon with land constraints. However, wider dissemination of the technology requires a targeted extension approach and adequate seed supply strategies, which should be based on joint efforts between farmers, extension services and research.This revised version was published online in November 2005 with corrections to the Cover Date.     

Biophysical interactions in tropical agroforestry systems

The rate and extent to which biophysical resources are captured and utilized by the components of an agroforestry system are determined by the nature and intensity of interactions between the components. The net effect of these interactions is often determined by the influence of the tree component on the other component(s) and/or on the overall system, and is expressed in terms of such quantifiable responses as soil fertility changes, microclimate modification, resource (water, nutrients, and light) availability and utilization, pest and disease incidence, and allelopathy. The paper reviews such manifestations of biophysical interactions in major simultaneous (e.g., hedgerow intercropping and trees on croplands) and sequential (e.g., planted tree fallows) agroforestry systems. In hedgerow intercropping (HI), the hedge/crop interactions are dominated by soil fertility improvement and competition for growth resources. Higher crop yields in HI than in sole cropping are noted mostly in inherently fertile soils in humid and subhumid tropics, and are caused by large fertility improvement relative to the effects of competition. But, yield increases are rare in semiarid tropics and infertile acid soils because fertility improvement does not offset the large competitive effect of hedgerows with crops for water and/or nutrients. Whereas improved soil fertility and microclimate positively influence crop yields underneath the canopies of scattered trees in semiarid climates, intense shading caused by large, evergreen trees negatively affects the yields. Trees in boundary plantings compete with crops for above- and belowground resources, with belowground competition of trees often extending beyond their crown areas. The major biophysical interactions in improved planted fallows are improvement of soil nitrogen status and reduction of weeds in the fallow phase, and increased crop yields in the subsequent cropping phase. In such systems, the negative effects of competition and micro-climate modification are avoided in the absence of direct tree–crop interactions. Future research on biophysical interactions should concentrate on (1) exploiting the diversity that exists within and between species of trees, (2) determining interactions between systems at different spatial (farm and landscape) and temporal scales, (3) improving understanding of belowground interactions, (4) assessing the environmental implications of agroforestry, particularly in the humid tropics, and (5) devising management schedules for agroforestry components in order to maximize benefits. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ecosystem fertility and fallow function in the humid and subhumid tropics

The regeneration of natural vegetation (fallowing) is a traditional practice for restoring fertility of agricultural land in many parts in the tropics. As a result of increasing human population and insufficient fertilizer inputs, the ecosystem fertility functions of traditional fallows must now be improved upon via the use of managed fallows. Interactions between vegetation and soil determine nutrient losses and gains in crop—fallow systems and are influenced by fallow species, patterns and rates of biomass allocation, and crop and fallow management. Nutrient losses occur through offtake in crop harvests during the cropping phase and through leaching, runoff, and erosion in the cropping phase and the initial stage of fallows $#x2014; when nutrient availability exceeds nutrient demand by vegetation. Gains in nutrient stocks in later stages of fallow are generally more rapid on soils with high than low base status due to greater quantities of weatherable minerals and lack of constraints to N₂ fixation, deep rooting, and retrieval of subsoil nutrients by fallow vegetation. On low base status soils (exchangeable Ca < 1 cmol_c kg^–1), N₂ fixation and atmospheric inputs are likely to be the main sources of nutrient additions. On high base status soils limited by N, gains in N stocks by inputs from N₂ fixation and retrieval of subsoil nitrate can occur relatively rapidly; hence short-term fallows can often improve crop performance. Large losses of Ca associated with soil organic matter (SOM) mineralization and soil acidification during cropping and fallow establishment, combined with chemical barriers to root penetration, suggest that long-duration fallows (> 5 yr) are needed for recovery of cation stocks and crop performance on low base status soils. On both soils, however, residual benefits of fallows on crop yields usually last less than three crops.This revised version was published online in November 2005 with corrections to the Cover Date.     

肯尼亚西部印度田菁改良休耕地对玉米生产力和黄独脚金寄生杂草(Sesbania sesban)侵扰的作用

在撒哈拉以南的非洲地区,黄独脚金寄生杂草（Strigahermonthica）侵扰是限制小农产自给性农业生产的主要因素之一。土壤肥力低加之总体环境退化是寄生杂草侵扰产生的重要原因。引入改良的耕作制度来解决寄生杂草侵扰和土壤肥力下降的问题势在必行。本文对肯尼亚西部双峰高原地区内,用豆科植物--印度田菁改良的休耕地对玉米产量和农田寄生杂草侵扰的作用进行了研究。实验处理分阶段进行,处理包括田菁改良6和18个月的休耕地、未经耕作自然植物再生6和18个月的休耕地、连续种植玉米未施肥的耕地和连续种植玉米同时施加氮和磷肥的耕地。结果表明,与未施肥玉米地相比,田菁改良休耕地明显（p〈0.5）增加玉米产量.除草管理降低了第一季度（428000&#177;63000株&#183;hm-2）、第二季度（51000&#177;1500株&#183;hm-2）玉米地寄生杂草植株种群。实验周期内,除草管理降低玉米地土壤中寄生杂草种子种群数。短期田菁改良休耕地对玉米产量的促进作用明显好于未施肥的玉米地,但是短期杂草休耕地对玉米产量无显著影响。种植玉米和除草控制寄生杂草效果要好于休耕。     

Weed management through short-term improved fallows in tropical agroecosystems

Weeds in tropical agricultural systems can cause serious economic damage, and their control often requires the commitment of substantial resources in the way of labor, capital, or pesticides. Tropical shifting cultivation systems employ a fallow period to help overcome weed infestations in addition to improving soil productivity and reducing other pest populations. Short-term, intensive fallow systems that employ herbaceous and woody species to facilitate rapid restoration of soil productivity have evolved as an alternative to long-term fallows. Short-term fallows can impact weeds at all growth stages and play a role in the integrated management of weeds and crops. Fallow management that promotes vegetative soil cover may reduce weed recruitment due to attenuation of soil temperature and/or shift in light quality at the soil surface. Residues or litterfall from fallow species may alter the chemical and microbial ecology of the soil to favor losses from the weed seed bank due to germination, loss of seed vigor, or seed decay. In addition, fallow vegetation can influence weed seed predation. Enhancement of soil productivity should increase the vigor of crop growth and enable crops to better compete with weeds. The burning of fallow species residues may result in weed seed death due to extreme temperatures or may induce seed germination by the release of mechanical dormancy or chemical germination cues. Certain weeds may serve as improved fallow species due to their high nutrient scavenging efficiency in low-fertility environments and their ease of establishment. Short-term improved fallows can be an important component of integrated weed management, particularly by promoting the prevention and tolerance of weeds in crops.This revised version was published online in November 2005 with corrections to the Cover Date.     

Potential of improved fallows to increase household and regional fuelwood supply: evidence from western Kenya

Fuelwood is the main energy source for households in rural Africa, but its supply is rapidly declining especially in the densely populated areas. Short duration planted tree fallows, an agroforestry technology widely promoted in sub-Sahara Africa for soil fertility improvement may offer some remedy. Our objective was to determine the fuelwood production potential of 6, 12 and 18 months (the common fallow rotation periods) old Crotalaria grahamiana, Crotalaria paulina, Tephrosia vogelli and Tephrosia candida fallows under farmer-managed conditions in western Kenya. Based on plot-level yields, we estimated the extent to which these tree fallows would meet household and sub-national fuelwood needs if farmers planted at least 0.25 hectares, the proportion of land that is typically left under natural fallows by farmers in the region. Fuelwood yield was affected significantly (P < 0.05) by the interaction between species and fallow duration. Among the 6-month-old fallows, T. candida produced the highest fuelwood (8.9 t ha⁻¹), compared with the rest that produced between 5.6 and 6.2 t ha⁻¹. Twelve months old T. candida and C. paulina also produced significantly higher fuelwood yield (average, 9.6 t ha⁻¹) than T. vogelli and C. grahamiana of the same age. Between the fallow durations, the 18-month fallows produced the most fuelwood among the species evaluated, averaging 14.7 t ha⁻¹. This was 2–3 times higher than the average yields of 6 and 12-month-old fallows whose yields were not significantly different. The actual fuelwood harvested from the plots that were planted to improved fallows (which ranged from 0.01 to 0.08 ha) would last a typical household between 11.8 and 124.8 days depending on the species and fallow duration. This would increase to 268.5 (0.7 years) and 1173.7 days (0.7–3.2 years) if farmers were to increase area planted to 0.25 ha. Farmers typically planted the fallows at high stand densities (over 100,000 plants ha⁻¹ on average) in order to maximize their benefits of improving soil fertility and providing fuelwood at the same time. This potential could be increased if more land (which fortunately exists) was planted to the fallows within the farms in the region. The research and development needs for this to happen at the desired scale are highlighted in the paper.     

Optimal tree fallow rotations: some principles revealed by modelling

A model (ROTATE) of the nitrogen (N) cycle during the tree and crop phases of fallow systems [Robertson, 1994] was used to determine the primary factors influencing longterm crop yields. The model simulated the expected patterns of increase in old (recalcitrant) soil organic N during tree rotations and their decrease under continuous cropping. After 3–4 fallow cycles an equilibrium soil organic N content is reached, where N losses by crop removal are balanced by N gains by the trees (either by fixation or pumping from depth) plus small inputs in rain. The rotation period has two variable components; the cycle length (tree plus crop period) and the fraction of years in each cycle occupied by trees (1 = sole trees, 0 = sole crops). Both components have optima determined by the time taken for the trees to increase the old soil organic N pool to an optimal (but not maximal) size. This optimum exists because the rate of increase in old organic N slows as the tree fallow progresses and a time is reached (often soon after the trees reach full size) when the benefits of further improvements in soil fertility are outweighed by crop yield foregone. In the example chosen of Acacia/sorghum in the Sahel, the optimum cycle seemed to be about 50 years with half of the time in trees. The optimum fallow period is shortened by growing fast-growing trees, and the benefit of fallow periods are greatest when (i) a large proportion of the N in litter (above and belowground) is transferred to the recalcitrant soil pool, and (ii) the trees attain a large size with correspondingly large annual additions of N to the soil.     

Potential of natural and improved fallow using indigenous trees to facilitate cacao replanting in Ghana

Poor establishment, due to loss of soil fertility, weeds and lack of appropriate shade, is a major constraint to replanting cacao on previously used land. Spathodea campanulata, Newbouldia laevis and Ricinodendron heudelotii planted as monospecific improved fallow and Terminalia ivorensis, T. superba and Antiaris toxicaria planted as a multispecies improved fallow and a natural tree fallow were assessed for their potential to facilitate cacao replanting in a randomized complete block design experiment. Simpson and Shannon diversity indices and species richness in the natural tree fallow were 0.6, 1.6 and 20, respectively, at 4 years after trial inception. The Multispecies and the R. heudelotii improved fallows had better height growth, crown development and light transmission characteristics, which are desirable for cacao shade. However, these were not comparable to S. campanulata or the natural tree fallow in terms of improving microsite topsoil pH, % organic carbon and % total nitrogen and site capture. Since optimum fallow period is shortened by growing fast-growing trees, the height growth rate >2.0 m per annum in all the treatments except N. laevis indicates the suitability of these species for improved fallow. The trees species showed different and complementary characteristics and from a standpoint of biodiversity conservation and the future floristic composition of the landscape the natural tree fallow with its diversity of tree species may be recommended as a rehabilitation technique to facilitate the replanting of cacao with a diverse overhead shade.     

Degrading uplands in the rainforest region of Madagascar: Fallow biomass,nutrient stocks,and soil nutrient availability

Soil fertility restoration depends on natural fallows in the slash-and-burn system of eastern Madagascar. In the Beforona-Vohidrazana study zone, none of the fallow species are able to withstand the slashing, burning and cropping frequencies of 3–5 years. Eventually soils are abandoned for agriculture. Along the degradation sequence, this study quantifies fallow biomass, nutrient stocks and soil nutrient availability of four dominant fallow species Trema orientalis, Psiadia altissima, Rubus moluccanus, and Imperata cylindrica. At 3 years, the shrubs Psiadia and Rubus were more productive (11–14.4 t/ha aboveground biomass or AGB) than the tree species Trema (8.5 t/ha). Only after 5 years did tree productivity (24.7 t/ha) exceed that of shrubs (17–20 t/ha). Imperata’s biomass stagnated at 5.5 t/ha after 3 years. A sharp decline in fallow productivity was observed with advancing fallow cycles after deforestation. While Psiadia produced highest AGB in the second fallow cycle (C2) being 100%, C1 achieved 89% of that, C3 74%, and C4 only 29%. With the ability to propagate vegetatively and to accumulate important amounts of nutrients in roots, Rubus and Imperata, both exotic and invasive species, showed improved adaptation mechanisms towards frequent disturbances compared to the two indigenous species Trema and Psiadia. Available soil nutrients P, K, Mg were highest under forest and declined rapidly with increasing fallow cycles. Ca and pH rose momentarily in the first fallow cycle before declining with advanced soil use. Al concentrations increased steadily with time. As lengthened fallow periods are not practical, there is a need to intensify upland systems based on improved nutrient cycling, targeted inputs, fire-less land management, and land use diversification. Allowing regrading tree and bush fallows to accumulate biomass (above- and belowground carbon) will significantly improve Madagascar's greenhouse gas mitigation contribution.     

Residual effects of fallows on selected soil hydraulic properties in a kaolinitic soil subjected to conventional tillage (CT) and no tillage (NT)

Improved fallows have been used to reduce time required for soil fertility regeneration after cropping in low input agricultural systems. In semi-arid areas of Southern Africa, Acacia angustissima and Sesbania sesban are among some of the more widely used improved fallow species. However the residual effects of improved fallows on soil hydraulic properties during the cropping phase is not known. The aim of this study was to quantify the residual effects of fallows and tillage imposed at fallow termination on soil hydraulic properties (infiltration rates, hydraulic conductivity and soil porosity) during the cropping phase. Treatments evaluated were planted fallows of Acacia angustissima, Sesbania sesban and natural fallow (NF) and continuous maize as a control. Steady state infiltration rates were measured using a double ring infiltrometer and porosity was calculated as the difference between saturated infiltration rates and tension infiltration measurements on an initially saturated soil. Unsaturated hydraulic conductivity (K_o) and mean pore sizes of water conducting pores were measured using tension infiltrometer at tensions of 5 and 10 cm of water on an initially dry soil. While there was no significant difference in steady state infiltration rates from double ring infiltrometer measurements among the fallow treatments, these were significantly higher than the control. The steady state infiltration rates were 36, 67, 59 and 68 mm h^-1 for continuous maize, A. angustissima, S. sesban and NF respectively. Tillage had no significant effect on steady state infiltration rate. Pore density at 5 cm tension was significantly higher in the three fallows than in maize and varied from 285–443 m⁻² in fallows, while in continuous maize the pore density was less than 256 m⁻². At 10 cm tension pore density remained significantly higher in fallows and ranged from 4,521–8,911 m⁻² compared to 2,689–3,938 m⁻² in continuous maize. Unsaturated hydraulic conductivities at 5 cm tension were significantly higher in fallows than in continuous maize and were 0.9, 0.7, 0.8 cm and 0.5 cm h⁻¹ for A. angustissima, S. sesban, NF and continuous maize, respectively. However there were no significant treatment differences at 10 cm tension. Fallows improved infiltration rates, hydraulic conductivity and soil porosity relative to continuous maize cropping. Through fallowing farmers can improve the soils hydraulic properties and porosity, this is important as it affects soil water recharge, and availability for plant growth     

From alley cropping to rotational fallow: Farmers' involvement in the development of fallow management techniques in the humid forest zone of Cameroon

Alley cropping was introduced in the humid forest zone of Cameroon to increase soil fertility in 1987, but until 1992 the adoption rate had remained low. To better understand the reasons behind this, three types of on farm trials were established from fully researcher controlled to fully farmer controlled. During the evaluation of the technology with farmers a number of modifications were registered (1) Pruning height and frequency: Because of the difficulties to consistently cut back at 30 cm, farmers decided to slash at ground level as they normally do while slashing the natural fallow vegetation. This equally allowed for more flexibility in time of pruning. (2) Cropping intensity and pattern: Initially, alleys were cropped each year. This however had several shortcomings. Having observed the positive impact of incidental fallow period in a farmer's field, it was decided to introduce a fallow phase of at least one year. During the fallow period the plot can be used for fodder production, bee farming and production of stakes. (3) Residue management: Fire went incidentally in a farmer's tree plot after slashing, and the trees were not affected. This gave farmers an alternative way to manage the residue, by controlling the fire, before bringing in other crops such as groundnut and cassava. (4) Agroforestry species: Because Leucaena leucocephala, in spite of it soil fertility restoration potential, rapidly became a noxious weed, farmers have asked for a less invasive species. Calliandra calothyrsus was introduced for this purpose and became a good bee forage. With these modifications, the original alley cropping system has evolved into a rotational tree fallow with higher adoption potential. From about 15 farmers who were testing the technology in 1992, the number increased to 52 in 1996, 120 in 1997 and 236 in 1998. This revised version was published online in June 2006 with corrections to the Cover Date.     

Adoption of improved fallow technology for soil fertility management in Zambia: Empirical studies and emerging issues

In the subsistence-agricultural region of eastern Zambia, less than 10% of the households have adequate supply of maize (Zea mays L.), the staple food, throughout the year. A major constraint to increasing crop production in the region is poor fertility status of the soil. In order to address this problem, improved fallow has been introduced as a technology for improving soil fertility within a short span of two to three years. Farmers have been testing the technology and a number of empirical studies have been undertaken over the years to identify the factors influencing farmers' decision to adopt the technology. This paper presents a synthesis of the results of adoption studies and highlights generic issues on the adoption of improved fallows in Zambia. The synthesis indicates that farmers' decision on technology adoption does not have a simple directed relationship of some technological characteristics only, but constitutes a matrix of factors including household characteristics, community level factors, socioeconomic constraints and incentives that farmers face, access to information, local institutional arrangements and macro policies on agriculture. The adoption of improved fallows is not strictly speaking a binary choice problem but a continuous process in which farmers occupy a position along a continuum in the adoption path. Further, adoption of improved fallows may not take place in a policy vacuum but needs to be facilitated by appropriate and conducive policy and institutional incentives. Several questions and issues that require further study emerge from the synthesis. These include determination of the relative importance of the factors in the adoption matrix, identification of the conditions under which farmers use a combination of inputs and their profitability under changing price scenarios, exact definition to delineate between `non-adopters', `testers' and `adopters' of agroforestry technologies, and understanding the impact of cash crop farming in farmers' adoption decisions of improved fallows (where off farm opportunities exist). Further, there is a need to determine the inter-relationship between household poverty, labor availability and the adoption of improved fallows and, to evaluate a combination of policy interventions at both national and local level to promote the adoption of agroforestry-based soil fertility management. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of improved fallow with <Emphasis Type="Italic">Sesbania sesban</Emphasis> on maize productivity and <Emphasis Type="Italic">Striga hermonthica</Emphasis> infestation in Western Kenya

Striga hermonthica is a major constraint to smallholder subsistence agriculture production in the sub-Saharan African region. Low soil fertility and overall environmental degradation has contributed to the build-up of the parasitic weed infestation. Improved cropping systems have to be introduced to address the interrelated problems of S. hermonthica and soil fertility decline. Thus, the effects of improved fallow with leguminous shrub Sesbania sesban on maize yields and levels of S. hermonthica infestation on farm land in the bimodal highlands of western Kenya were investigated. The experimental treatments were arranged in a phased entry, and randomized complete block scheme were six months Sesbania fallow, 18 months Sesbania fallow, six months natural fallow consisting of regrowth of natural vegetation without cultivation, 18 months natural fallow, continuous maize cropping without fertilizer application, and continuous maize cropping with P and N fertilization. Results show that Sesbania fallows significantly (p<0.05) increase maize yield relative to continuous unfertilized maize. S. hermonthica plant populations decrease in continuous maize between the first season (mean = 428 000 ± 63 000 ha⁻¹) and second season (mean=51 000 ± 15 000 ha⁻¹), presumably in response to good weed management. S. hermonthica seed populations in the soil decrease throughout the duration of the experiment in the continuous maize treatments. Short-duration Sesbania fallows can provide modest yield improvements relative to continuous unfertilized maize, but short-duration weedy fallows are ineffective. Continuous maize cultivation with good weed control may provide more effective S. hermonthica control than fallowing.     

Hybrid improved tree fallows: harnessing invasive woody legumes for agroforestry

For several decades, agroforestry specialists have promoted the planting of fallow fields with nitrogen-fixing, fast-growing trees or shrubs to accelerate soil rehabilitation and provide secondary products like woodfuel. Yet, such ‘improved fallows’ have not been widely adopted, in part due to the costs of labour and seedlings. In some situations, however, farmers have developed novel approaches to agroforestry fallows by taking advantage of spontaneous invasions of woody leguminous tree species present in the vicinity of their fields. In this paper, we examine cases from Réunion, highland Madagascar, the Bateke plateau in Congo, and the Palni hills of southern India where farmers have adapted their cultivation practices to take advantage of the invasive characteristics of Australian acacias that were introduced earlier for other reasons. We focus on the key social, economic, and environmental factors that influence farmers in these places to gain opportunistic benefit from these introduced tree species that biologists have been deemed invasive and damaging to local ecosystems and biodiversity. We conclude that opportunistic fallowing of invasives can be viewed as a hybrid strategy combining elements of natural fallows and improved fallows—which we call ‘hybrid improved fallows’—in that it takes advantage of the ‘weedy’ characteristics of introduced leguminous tree species in the landscape and offers a cost-effective and pragmatic strategy for soil and vegetation management for farmers.     

Financial evaluation of smallholder timber-based agroforestry systems in Claveria, Northern Mindanao, the Philippines

In the Philippines, timber production on small farms has become profitable as a result of reduced supplies due to extensive deforestation and increasing demand. In the early 1990s, when the price of timber was high, farmers were promised huge returns from tree farming. However, widespread planting of few species has led to oversupply and a sharp decline in the price of farm-grown timber. Moreover, low intercrop yields as a result of competition from fast-growing trees and low timber yields due to poor tree management, further reduce net economic returns. In spite of this, interest in tree farming remains high. This paper examines the private profitability of two tree-maize systems, namely trees in blocks and trees in hedgerows, compared with the alternative of maize monocropping. The analysis reveals that maize monocropping provides higher returns to land at the current timber price, but considerably lower returns to labour, than the maize-tree systems tested. This suggests that tree farming is a more attractive option for labour and capital-constrained households or those with off-farm opportunities that compete for their labour. These farmers may raise productivity and income by planting trees on the excess land that cannot be devoted to annual crops. The analysis also indicates that wide-spaced tree hedgerows are superior to tree blocks, due to lower establishment and management costs, longer periods of viable intercropping and more rapid tree growth.     

Modelling of planted legume fallows in Western Kenya. (II) Productivity and sustainability of simulated management strategies

Improved fallow is a technology that can help to raise agricultural productivity in systems of poor soil fertility and low financial capital. Models, once calibrated, can be used to investigate a range of improved fallow systems relatively quickly and at relatively low cost, helping to direct experimental research towards promising areas of interest. Six fallow crop rotations were simulated using the WaNuLCAS model in a bimodal rainfall setting in Kenya over a 10 year period: (A) alternating fallow and crop seasons, (B) one season fallow followed by three seasons crop, (C) one season fallow followed by four seasons crop, (D–F) 1–3 seasons fallow periods followed by 3–5 seasons crop. The strategies were tested using a number of fallow growth rates, soil clay contents, and rainfall amounts to determine the interaction of fallow rotation and biophysical variables on maize (Zea mays (L.)) yield and sustainability (organic matter, N₂ fixation, leaching). The best simulated fallow strategies doubled maize yield compared to continuous maize over a 10 year period. Across all biophysical treatments strategy A and B of no more than three consecutive cropping seasons and of one consecutive fallow season yielded the most maize. This was because fallow benefits were largely due to the immediate fallow soil fertility benefit (IFB) rather than the cumulative benefit (CFB). The difference in yield between the two strategies was through a balance between (1) their interaction with the biophysical variables affecting accumulation of organic matter, hence increasing soil fertility and (2) the extra intrinsic soil fertility used for maize productivity by the inclusion of more cropping seasons within the rotation. We propose the following conceptual framework to manage fallows for maximum maize yield: when environmental factors are strongly limiting to fallow and crop growth then fallow strategy A would be the best strategy to employ (less risk but more labour) and when factors are less limiting then strategy B would be the best to employ.