P. Mapfumo

Pigeonpea rhizobia prevalence and crop response to inoculation in Zimbabwean smallholder-managed soils.

A study was conducted to determine the population sizes of indigenous pigeonpea (Cajanus cajan)-nodulating rhizobia and responses of the crop to rhizobial inoculation in soils under smallholder management. Rhizobia populations were determined in 21 soils from three different agro-ecological regions of Zimbabwe using the plant infection most-probable-number technique. Pigeonpea response to rhizobial inoculation was tested in five soils representative of low, medium and high rhizobia populations. Pigeonpea rhizobia ranged from undetectable to 121 cells per g soil compared with 16 to 159 cells per g soil for cowpea (Vigna unguiculata) which was used for reference. Soils with high cowpea rhizobia counts had relatively low counts of pigeonpea rhizobia and vice versa, showing that the two legumes associate with different subgroups of rhizobia. Poor soil organic matter, low soil moisture at sampling, low pH and low clay content of the soils had a significant negative effect on rhizobial counts. Organic matter appeared critical for maintenance of high populations of indigenous rhizobia in the mostly sandy soils sampled. Lack of pigeonpea response to inoculation in all the soils tested despite the low initial rhizobial populations could be the result of within-season proliferation of indigenous populations which are competitive and effective. There was evidence of rapid build-up of pigeonpea-compatible rhizobia within one growing season when the crop was first introduced. It was concluded that effective pigeonpea rhizobia occur in many arable soils of Zimbabwe. However, to fully exploit biological nitrogen fixation and maximize yields of pigeonpea, highly efficient, adapted and competitive indigenous rhizobial isolates must be identified and evaluated.

Climate change and maize yield in southern Africa: what can farm management do?

There is concern that food insecurity will increase in southern Africa due to climate change. We quantified the response of maize yield to projected climate change and to three key management options - planting date, fertilizer use and cultivar choice - using the crop simulation model, agricultural production systems simulator (APSIM), at two contrasting sites in Zimbabwe. Three climate periods up to 2100 were selected to cover both near- and long-term climates. Future climate data under two radiative forcing scenarios were generated from five global circulation models. The temperature is projected to increase significantly in Zimbabwe by 2100 with no significant change in mean annual total rainfall. When planting before mid-December with a high fertilizer rate, the simulated average grain yield for all three maize cultivars declined by 13% for the periods 2010-2039 and 2040-2069 and by 20% for 2070-2099 compared with the baseline climate, under low radiative forcing. Larger declines in yield of up to 32% were predicted for 2070-2099 with high radiative forcing. Despite differences in annual rainfall, similar trends in yield changes were observed for the two sites studied, Hwedza and Makoni. The yield response to delay in planting was nonlinear. Fertilizer increased yield significantly under both baseline and future climates. The response of maize to mineral nitrogen decreased with progressing climate change, implying a decrease in the optimal fertilizer rate in the future. Our results suggest that in the near future, improved crop and soil fertility management will remain important for enhanced maize yield. Towards the end of the 21st century, however, none of the farm management options tested in the study can avoid large yield losses in southern Africa due to climate change. There is a need to transform the current cropping systems of southern Africa to offset the negative impacts of climate change.

Combating food insecurity on sandy soils in Zimbabwe: The legume challenge

The continued rise in mineral fertilizer costs has demanded cheaper alternative N sources for resource-constrained smallholder farmers, with N2-fixing legumes presenting a viable option to maintain crop productivity. A study was conducted over two years on a coarse sandy soil (Lixisol with Intermediate farmers (RG2) > Resource-constrained farmers (RG3). Although overall biomass productivity for the grain legumes was generally low, 500% (RG3) over the two seasons. However, there was a general preference for bambara nut by RG3 farmers who cited low cash demands in terms of seed and external inputs, and pest-resistance compared with other grain legumes. Increased maize grain yields following legumes, and which exceeded 7 Mg ha−1 for RG1 under green-manure, was apparently due to an increase in soil available N. The results showed scope for enhancing the contribution of legumes to both soil fertility and household nutrition within smallholder farming systems if targeted according to farmers’ resource endowment. The challenge is availing the minimum level of external inputs to RG3 farmers to achieve significant yield benefits on poor soils. The paper presents three main scenarios constituting major challenges for integrating legumes into the current farming systems.

Comparative Analysis of the Current and Potential Role of Legumes in Integrated Soil Fertility Management in Southern Africa

Food insecurity, arising from low and non-sustainable agricultural production systems under increasing climate variability is one of the major livelihood threats for the majority of households in Southern Africa. Nitrogen (N) is the most limiting nutrient in the predominantly smallholder maize-based cropping systems, with N inputs almost invariably constituting the largest cost of crop production. Traditional methods of soil fertility management are critically short of matching the increasing food demands of a growing human population against the rising cost of living. Limited access to affordable mineral fertilizers by farmers exacerbates inherent problems of a low soil nutrient capital. This paper explores the current and potential contribution of legume biological N2-fixation (BNF) to the N economy of cropping systems in southern African countries in the context of integrated soil fertility management (ISFM) approaches. There is potential for BNF to generate 200–300 kg N ha−1 under farmer management conditions, reducing N fertilizer inputs for subsequent maize by 50–100 kg ha−1 in a single cropping season. Legumes derive 50–90% of their N requirements from N-fixation, nearly eliminating the need for external N fertilization. However, current N inputs from BNF on smallholder farms remain as low as 5 kg N ha−1 year−1 due to factors that include poor choices of legume types/varieties, small areas allocated for legume production, and poor soil fertility (particularly phosphorus [P] deficiency) and rainfall variability that lead to poor biomass accumulation. Application of P containing fertilizers was found to increase legume yields by 20–100% across dominant soils, while combinations of livestock manure and mineral fertilizer increased yields of grain legumes by 50–150% even poor sandy soils. Although legume green manures and agroforestry tree systems resulted in the highest maize yield responses of up to 400%, their lack of short-term food and income benefits severely limits their preference and adoption by farmers. In contrast, grain legumes showed the highest potential in farming systems in Malawi, Mozambique, Zambia and Zimbabwe where they are used in rotations and/or intercrops depending on rainfall conditions. Grain legumes increased yields of subsequent cereals by 60% to >200% over non fertilized controls following addition of mineral fertilizer in modest amounts. There are, however, significant technical knowledge gaps among farmers, agro-input suppliers and extension agencies on management options for optimizing BNF. The study identified lack of suitable mechanisms to deliver smallholder communities out of an existing ‘maize poverty trap’ as an overaching challenge for intensification and diversification of the cereal-legume systems. Prevailing agricultural production circumstances of many smallholders favour maize monocropping. Research and development intitiatives have also tended to ignore the multiple roles of grain legumes as prioritized by communities, particularly women. Opportunities exist for multi-stakeholder platforms that generate technical and market innovations with communities to drive legume-based ISFM technologies for increased productivity and diversification of southern Africa’s current farming systems.

Pathways to transformational change in the face of climate impacts: an analytical framework

Unprecedented impacts of climate change and climate variability in the twenty-first century are likely to require transformational social, organizational and human responses. Yet, little existing empirical work examines how decision-makers can facilitate such responses. This paper suggests that in order to assess whether responses to climate risks and threats are transformational, it is necessary to move away from a focus only on outcomes and scale and towards the multiple dimensions of social responses and the processes through which transformational changes are realized. In so doing, the paper seeks to move the discussion on transformational change towards the processes and sustainability of adaptation interventions, and the changes they trigger. Drawing on the literature on transformational change in organizational theory and social–ecological systems, the paper first develops a framework with which to examine and assess development and adaptation interventions. The framework is then applied to eight interventions made between 2005 and 2011 in diverse socioecological settings across Africa. All interventions were underpinned by participatory action research methodologies. Our analysis shows how a focus on change agents, generalizability of field-scale adaptation mechanisms and pathways, and sustainability of outcomes, combined with attention to the scale and scope of change processes, provides information that can inform policy on the kinds of intervention that are likely to support long-term and sustainable responses to climate impacts. Although several of the cases mainly illustrate incremental adaptations, use of the analytical framework pointed towards the wider processes of systems change that might lead to transformative trajectories.

Creating food self reliance among the smallholder farmers of eastern Zimbabwe: exploring the role of integrated agricultural research for development

The failure of the linear and non-participatory Agricultural Research and Development (ARD) approaches to increase food security among smallholder farmers in sub-Saharan Africa has prompted researchers to introduce an Integrated Agricultural Research for Development (IAR4D) concept. The IAR4D concept uses Innovation Platforms (IPs) to embed agricultural research and development organizations in a network to undertake multidisciplinary and participatory research. This paper uses Zimbabwe as a case study to analyze the relevance of the technologies and innovations that are being promoted by IPs in Zimbabwe to improve food security. Using data collected through the Sub-Saharan Africa Challenge Programme, the paper shows that access to inputs, social capital, productivity enhancing technologies and market information are critical in addressing food security issues among smallholder farmers. The multi-stakeholder partnership forged through IPs should adopt a coordinated approach to provide smallholders with access to these prerequisites for food security. The paper argues that more emphasis should be put on these issues rather than on farm research initiatives whose contribution to food security appears to be less significant.

Ecological Intensification: Local Innovation to Address Global Challenges

The debate on future global food security is centered on increasing yields. This focus on availability of food is overshadowing access and utilization of food, and the stability of these over time. In addition, pleas for increasing yields across the board overlook the diversity of current positions and contexts in which local agriculture functions. And finally, the actual model of production is based on mainstream agricultural models in industrialized societies, in which ecological diversity and benefits from nature have been ignored or replaced by external inputs. The dependence upon external inputs should exacerbate the negative impacts on the environment and on social equity. Strategies to address future global food security thus require local innovation to increase agricultural production in a sustainable, affordable way in the poorest regions of the world, and to reduce the environmental impact of agriculture and its dependence on non-renewable resources. Ecological intensification, the smart use of biodiversity-mediated ecosystem functions to support agricultural production, is portrayed as the most promising avenue to achieve these goals.

Assessing the potential benefits of organic and mineral fertiliser combinations on legume productivity under smallholder management in Zimbabwe

Productivity of grain legumes on sandy soils of southern Africa is critically limited by marginal fertilisation. Effects of co-applying phosphorus (P)-based mineral fertilisers and organic nutrient resources to cowpea (Vigna unguiculata (L.) Walp.) and soyabean (Glycine max L.) were investigated on smallholder farms in eastern Zimbabwe over two years. Over 70% of the surveyed farmers grew cowpea without fertilisation. Fertilisation of legumes with one or more nutrient resources increased shoot biomass productivity by between 20% and 300% relative to the non-fertilised control. Fertilised soyabean and cowpea yielded 2.2 t grain ha−1 and 2.5 t grain ha−1, respectively, translating to more than double the yields of unfertilised controls. In contrast, sole application of either mineral P-containing fertilisers or organic nutrient resources yielded less than 1 t ha−1 legume grain. The effects of combined organic and mineral fertilisation were also reflected in increased C〇2-carbon evolution from soils following growth of the legumes. Under the same soils, net nitrogen (N) mineralisation was highest where cattle manure was co-applied with an NP-containing fertiliser, with at least 85 mg N kg−1 soil released within six weeks. Co-application of organic and NP-containing fertilisers significantly enhance legume grain yields and residual soil N availability, but most smallholder farmers do not currently use this fertilisation strategy.

Nitrogen fixation and biomass productivity of indigenous legumes for fertility restoration of abandoned soils in smallholder farming systems

Abstract Most legume-based soil fertility technologies often fail to make the desired impact on nutrient-depleted soils partly due to low N2-fixation rates and poor biomass productivity. A study was conducted in the 2004/05 and 2005/06 rainfall seasons to evaluate biomass productivity and N2-fixation of indigenous legumes on nutrient-depleted fields under low (450- 650 mm yr−1) to high (> 800 mm yr−1) rainfall areas of Zimbabwe. Legume species, mostly of Crotalaria, Indigofera and Tephrosia genera, were sown in mixtures on disturbed soil. Indigenous legume fallows (indifallows) produced 2–5 t biomass ha−1 under low and 5–15 t biomass ha−1 under high rainfall. They significantly (P<0.05) out-yielded natural fallows by 84% and a sunnhemp (Crotalaria juncea) green manure fallow by 32% over a growth period of six months. Cumulatively, indifallows produced more biomass (~29 t ha−1) than natural fallows (~ 8 t ha−1) over two seasons. Indigenous legumes derived 61–90% of their N from the atmosphere with amounts fixed ranging from 2–57 kg N ha−1 under medium rainfall conditions to 1–173 kg N ha−1 under high rainfall. Application of P increased indifallow biomass productivity by 15% under low rainfall, and N2-fixation by 32% and 18% under low and high rainfall, respectively. These results demonstrated that indigenous legumes generate sufficiently high biomass and fix nitrogen on nutrient-depleted fields where most conventional green manure and grain legume cultivars often fail to establish.

Sequencing integrated soil fertility management options for sustainable crop intensification by different categories of smallholder farmers in Zimbabwe

SUMMARY Research has proved that integrated soil fertility management (ISFM) can increase crop yields at the field and farm scales. However, its uptake by smallholder farmers in Africa is often constrained by lack of technical guidelines on effective starting points and how the different ISFM options can be combined to increase crop productivity on a sustainable basis. A 4-year study was conducted on sandy soils (<10% clay) on smallholder farms in eastern Zimbabwe to assess how sequencing of different ISFM options may lead to incremental gains in soil productivity, enhanced efficiency of resource use, and increase crop yields at field scale. The sequences were primarily based on low-quality organic resources, nitrogen-fixing green manure and grain legumes, and mineral fertilizers. To enable comparison of legume and maize grain yields among treatments, yields were converted to energy (kilocalories) and protein (kg) equivalents. In the first year, ‘Manure-start’, a cattle manure-based sequence, yielded 3.4 t ha −1 of maize grain compared with 2.5 and 0.4 t ha −1 under a woodland litter-based sequence (‘Litter-start’) and continuous unfertilized maize control, respectively. The ‘Manure-start’ produced 12 × 10 6 kilocalories (kcal); significantly (p < 0.05) out-yielding ‘Litter start’ and a fertilizer-based sequence (‘Fertilizer-start’) by 50%. A soyabean-based sequence, ‘Soyastart’, gave the highest protein production of 720 kg against <450 kg for the other sequencing treatments. In the second year, the sequences yielded an average of 5.7 t ha −1 of maize grain, producing over 19 × 10 6 kcal and 400 kg of protein. Consequently, the sequences significantly out-performed farmers’ designated poor fields by fivefold. In the third year, ‘Soya-start’ gave the highest maize grain yield of 3.7 t ha −1 ; translating to 1.5 and 3 times more calories than under farmers’ designated rich and poor fields, respectively. In the fourth year, ‘Fertilizer-start’ produced the highest calories and protein of 14 × 106 kcal and 340 kg, respectively. Cumulatively over 4 years, ‘Manure-start’ and ‘Soya-start’ gave the highest calories and protein, out-performing farmers’ designated rich and poor fields. Sunnhemp (Crotalaria juncea L.)based sequences, ‘Green-start’ and ‘Fertilizer-start’, recorded the highest gains in plant available soil P of 4m g kg −1 over the 4-year period. Assessment of P agronomic efficiencies showed significantly more benefits under the ISFM-based sequences than under farmers’ designated rich and poor fields. Based on costs of seed, nutrients and labour, ‘Soya-start’ gave the best net present value over the 4 years, while ‘Fertilizer-start’ was financially the least attractive. Overall, the ISFM-based sequences were more profitable than fields designated as rich and poor by farmers. We concluded that ISFM-based sequences can provide options for farm-level intensification by different categories of smallholder farmers in Southern Africa.