Zinc fertilization increases productivity and grain nutritional quality of cowpea (Vigna unguiculata [L.] Walp.) under integrated soil fertility management

Cowpea (Vigna unguiculata [L.] Walp.) is an important but under-studied grain legume which can potentially contribute to improved dietary zinc (Zn) intake in sub-Saharan Africa. In this study, surveys were conducted on smallholder farms in Zimbabwe during 2014/15 to determine the influence of diverse soil fertility management options on cowpea grain productivity and nutrition quality. Guided by the surveys, field experiments were conducted to investigate the influence of Zn fertilizer on the productivity and quality of cowpea under integrated soil fertility management (ISFM). Experiments were conducted on two soil-types, namely, sandy (6% clay) and red clay (57% clay) in 2014/15 and 2015/16 where cowpea was grown in rotation with staple maize (Zea mays L.) and fertilized with combinations of Zn, nitrogen (N), phosphorus (P) and two organic nutrient resources, cattle manure and woodland leaf litter. Cowpea grain yields on surveyed farms ranged from 0.3 to 0.9 t ha−1, with grain Zn concentration ranging from 23.9 to 30.1 mg kg−1. The highest grain Zn concentration was on fields where organic nutrient resources were applied in combination with mineral N and P fertilizers. Within the field experiments, mean grain yields of cowpea increased by between 12 and 18% on both soil types when Zn fertilizer was applied, from a baseline of 1.6 and 1.1 t ha−1 on red clay and sandy soils, respectively. When Zn fertilizer was co-applied with organic nutrient resources, grain Zn concentrations of cowpea reached 42.1 mg kg−1 (red clay) and 44.7 mg kg−1 (sandy) against grain Zn concentrations of 35.9 mg kg−1 and 31.1 mg kg−1 measured in cowpea grown with no Zn fertilizer on red clay and sandy soils, respectively. Agronomic biofortification of legumes is feasible and has the potential to contribute significantly towards increasing dietary Zn intake by humans. A greater increase in grain Zn of cowpea grown on sandy than red clay soils under Zn fertilization illustrates the influence of soil type on Zn uptake, which should be explored further in agronomic biofortification programs

Can nitrogen fertilizer management improve grain iron concentration of agro-biofortified crops in Zimbabwe?

Improving iron (Fe) concentration in staple grain crops could help reduce Fe-deficiency anaemia in communities dependent on plant-based diets. Co-application of nitrogen (N) and zinc (Zn) fertilizers has been reported to improve both yield and grain Zn concentration of crops in smallholder farming systems. This study was conducted to determine if similar effects are observed for grain Fe concentration. Field experiments were conducted in two years, in two contrasting agro-ecologies in Zimbabwe, on maize (Zea mays L.), cowpea (Vigna unguiculata [L.] Walp) and two finger millet (Eleusine coracana (L.) Gaertn.) “seed pools”. The two finger millet “seed pools” were collected during previous farmer surveys to represent “high” and “low” Fe concentrations. All plots received foliar Fe-ethylene diamine tetra-acetic acid (EDTA) fertilizer and one of seven N treatments, representing mineral or organic N sources, and combinations thereof. Higher grain yields were observed in larger N treatments. Grain Fe concentration increased according to species: maize < finger millet < cowpea but varied widely according to treatment. Significant effects of N-form on grain Fe concentration were observed in the low finger millet “seed pool”, for which mineral N fertilizer application increased grain Fe concentration to a greater extent than other N forms, but not for the other species. Whilst good soil fertility management is essential for yield and grain quality, effects on grain Fe concentration are less consistent than reported previously for Zn

Good soil management can reduce dietary zinc deficiency in Zimbabwe

Dietary zinc (Zn) deficiency is widespread in sub-Saharan Africa (SSA) with adverse impacts on human health. Agronomic biofortification with Zn fertilizers and improved soil fertility management, using mineral and organic nutrient resources, has previously been shown to increase Zn concentration of staple grain crops, including maize. Here, we show the potential of different soil fertility management options on maize crops to reduce dietary Zn deficiency in Zimbabwe using secondary data from a set of survey and field experiments. An ex-ante approach was used, informed by published evidence from studies in three contrasting smallholder production systems in Zimbabwe. To estimate current Zn deficiency in Zimbabwe, data on dietary Zn supply from non-maize sources from the Global Expanded Nutrient Supply (GENuS) data set were linked to maize grain Zn composition observed under typical current soil fertility management scenarios. A baseline dietary Zn deficiency prevalence of 68% was estimated from a reference maize grain Zn composition value of 16.6 mg kg-1 and an estimated dietary Zn intake of 9.3 mg capita-1 day-1 from all food sources. The potential health benefits of reducing Zn deficiency using different soil fertility management scenarios were quantified within a Disability Adjusted Life Years (DALYs) framework. A scenario using optimal mineral NPK fertilizers and locally available organic nutrient resources (i.e. cattle manure and leaf litter), but without additional soil Zn fertilizer applications, is estimated to increase maize grain Zn concentration to 19.3 mg kg-1. This would reduce the estimated prevalence of dietary Zn deficiency to 55%, potentially saving 2,238 DALYs year-1. Universal adoption of optimal fertilizers, to include soil Zn applications and locally available organic leaf litter, is estimated to increase maize grain Zn concentration to 32.4 mg kg-1 and reduce dietary Zn deficiency to 16.7%, potentially saving 9,119 DALYs year-1. Potential monetized yield gains from adopting improved soil fertility management range from 49-to 158-fold larger than the potential reduction in DALYs, if the latter are monetized using standard methods. Farmers should be incentivized to adopt improved soil fertility management to improve both crop yield and quality

Fertilizer management and soil type influence grain zinc and iron concentration under contrasting smallholder cropping systems in Zimbabwe

Micronutrient deficiencies remain prevalent in food systems of southern Africa, although advances in biofortification through crop breeding and agronomy provide opportunities to address these. We determined baseline soil availability of zinc (Zn) and iron (Fe) and the effects of soil type and farmer management on extractable soil Zn and Fe and subsequent concentration in cereal and legume grains under two contrasting agro-ecologies in Zimbabwe. Soil and crop surveys were conducted in Hwedza and Mutasa Districts of Zimbabwe in 2015–16 on 350 locations over different soil types. Fields with different levels of productivity (designated as “most” and “least” productive fields) were sampled using an inherited hierarchical randomized sampling design. Grain Zn and Fe concentration in maize (Zea mays), sorghum (Sorghum bicolor), finger millet (Eleusine coracana) and cowpea (Vigna unguiculata) were generally insufficient for adequate human nutrition. A Linear Mixed Effects (LME) model revealed that diethylene triamine penta-acetic acid- (DTPA) extractable soil Zn concentration and grain Zn concentration were affected primarily by field productivity level. DTPA-extractable soil Zn concentration was more than two-fold greater on the most productive fields (mean 0.8 mg kg−1) than on the least productive fields, with mean grain Zn concentration of 25.2 mg grain Zn kg−1 which was 13% greater than seen on the least productive fields. An interaction effect of field productivity level and total soil Zn concentration on DTPA-extractable soil Zn concentration suggests potential contribution of organic matter management to unlocking unavailable forms of soil Zn. DTPA-extractable soil Fe and grain Fe concentration were primarily affected by soil type and crop type, respectively. The LME modelling approach revealed additional soil geochemical covariates affected DTPA-extractable soil Zn and Fe concentration and grain Zn and Fe concentration within Districts. Future studies can therefore be powered to detect their roles at wider spatial scales for sustainable management of crop Zn and Fe nutrition

Nitrogen effect on zinc biofortification of maize and cowpea in Zimbabwean smallholder farms

Agronomic biofortification of crops with zinc (Zn) can be enhanced under increased nitrogen (N) supply. Here, the effects of N fertilizer on grain Zn concentration of maize (Zea mays L.) and cowpea (Vigna unguiculata L.) were determined at two contrasting sites in Zimbabwe over two seasons. All treatments received soil and foliar zinc‐sulphate fertilizer. Seven N treatments, with three N rates (0, 45, and 90 kg ha−1 for maize; 0, 15, and 30 kg ha−1 for cowpea), two N forms (mineral and organic), and combinations thereof were used for each crop in a randomized complete block design (n = 4). Maize grain Zn concentrations increased from 27.2 to 39.3 mg kg−1 across sites. At 45 kg N ha−1, mineral N fertilizer increased maize grain Zn concentration more than organic N from cattle manure or a combination of mineral and organic N fertilizers. At 90 kg N ha−1, the three N fertilizer application strategies had similar effects on maize grain Zn concentration. Co‐application of N and Zn fertilizer was more effective at increasing Zn concentration in maize grain than Zn fertilizer alone. Increases in cowpea grain Zn concentration were less consistent, although grain Zn concentration increased from 39.8 to 52.7 mg kg−1 under optimal co‐applications of N and Zn. Future cost/benefit analyses of agronomic biofortification need to include information on benefits of agro‐fortified grain, complex farmer management decisions (including cost and access to both N and Zn fertilizers), as well as understanding of the spatial and site‐specific variation in fertilizer responses

A meta-analysis of long-term effects of conservation agriculture on maize grain yield under rain-fed conditions

Conservation agriculture involves reduced tillage, permanent soil cover and crop rotations to enhance soil fertility and to supply food from a dwindling land resource. Recently, conservation agriculture has been promoted in Southern Africa, mainly for maize-based farming systems. However, maize yields under rain-fed conditions are often variable. There is therefore a need to identify factors that influence crop yield under conservation agriculture and rain-fed conditions. Here, we studied maize grain yield data from experiments lasting 5 years and more under rain-fed conditions. We assessed the effect of long-term tillage and residue retention on maize grain yield under contrasting soil textures, nitrogen input and climate. Yield variability was measured by stability analysis. Our results show an increase in maize yield over time with conservation agriculture practices that include rotation and high input use in low rainfall areas. But we observed no difference in system stability under those conditions. We observed a strong relationship between maize grain yield and annual rainfall. Our meta-analysis gave the following findings: (1) 92% of the data show that mulch cover in high rainfall areas leads to lower yields due to waterlogging; (2) 85% of data show that soil texture is important in the temporal development of conservation agriculture effects, improved yields are likely on well-drained soils; (3) 73% of the data show that conservation agriculture practices require high inputs especially N for improved yield; (4) 63% of data show that increased yields are obtained with rotation but calculations often do not include the variations in rainfall within and between seasons; (5) 56% of the data show that reduced tillage with no mulch cover leads to lower yields in semi-arid areas; and (6) when adequate fertiliser is available, rainfall is the most important determinant of yield in southern Africa. It is clear from our results that conservation agriculture needs to be targeted and adapted to specific biophysical conditions for improved impact

Transformation, adaptation and development: relating concepts to practice

In recent years there has been a growing number of academic reviews discussing the theme of transformation and its association with adaptation to climate change. On the one hand this has stimulated exchange of ideas and perspectives on the parameters of transformation, but it has also given rise to confusion in terms of identifying what constitutes a non-incremental form of adaptation on the ground. What this article aims to do instead is help researchers and practitioners relate different interpretations of transformation to practice by proposing a typological framework for categorising forms of change that focuses on mechanisms and objectives. It then discusses how these categorisations link to the broader conceptions and critiques noted above, with the idea that this will enable those who seek to analyse or plan adaptation to better analyse what types of action are potentially constitutive of transformation. In doing so, it should equally assist in the identification and specification of critical questions that need to be asked of such activity in relation to issues of sustainability and equity. As the term transformation gains ground in discussions of climate change adaptation, it is necessary to take a step back, review quite what commentators mean when they use the word, and consider the implications on people, especially the most vulnerable and marginalised, of “doing” or promoting transformation in its different forms