•We retrieved 44 publications and 199 observations comparing continuous cereal with grain legume-cereal rotation in SSA.•Cereal after legume yielded on average 0.49t grain ha−1 or 41% more than ...continuous cereal.•Sustained residual benefits of legumes with large N applications indicate the importance of non-N effects.•Relevant non-N effects include improved P availability, changes in SOM, and in pest, disease and striga pressure.
We conducted a systematic review of literature on the residual effects of grain legumes in cereal-based systems of sub-Saharan Africa (SSA) to quantify the magnitude and variability of rotational effects, to explore the importance of environmental and management factors in determining variability and to evaluate the evidence of the different mechanisms that explain rotational effects. We retrieved 44 unique publications providing 199 observations comparing continuous cereal performance with that of a grain legume-cereal rotation. The overall mean yield increase of 0.49t grainha−1, equal to an increase of 41% of the continuous cereal yield, is highly significant, but the variability in residual effects is large. Effects were more pronounced in southern Africa, the highlands of East Africa and the Guinea savannah, and less in the humid forest/derived savannah of West Africa and the Sudano-Sahelian zone. Maize showed stronger yield responses after a legume than millet and sorghum. Agro-ecological zone and cereal type were however confounded. All grain legume types significantly improved cereal yields, with stronger residual effects observed after soybean and groundnut than after cowpea. Fertiliser N application to cereals reduces the residual effects of legumes, but the response at 60–120kgNha−1 still equalled 0.32tha−1 or 59% of the response when no N is applied. The sustained benefits with large N applications indicate the importance of non-N effects. While mechanisms for improved soil P availability after grain legumes have been studied in some detail, it remains uncertain how important these are in farmers’ fields. Grain legumes are unlikely to have a major influence on the availability of nutrients other than N and P, or on soil pH. Beneficial impacts of grain legumes on soil organic matter content can occur if legumes contribute to a greater overall cropping productivity, but studies generally report no such impacts. Evidence of impacts of grain legumes on weeds is limited to striga. Studies on the impacts on nematode pressure in cereals are inconclusive, probably because legumes act as a host for some of the key nematode genera that harm maize. The impact on the pressure of other pests and diseases in cereals is probably important, but evidence on this from SSA is lacking. Future research on N2-fixation by grain legumes and residual N benefits should focus on explaining the wide variability observed among sites. There is a clear need for more detailed mechanistic studies to assess the occurrence and relevance of non-N effects of grain legumes, particularly in relation to common pests and diseases in cereals.
•Cropping system and soil fertility effects on N2-fixation were tested in northern Ghana.•More N2 is fixed in sole cropping than intercropping despite comparable %Ndfa.•Poorly fertile fields give ...limited grain legume benefits despite enhanced %Ndfa.•Partial N balances are unreliable indicators of cropping system sustainability.•Different grain legumes should be targeted to different sites in the Guinea savanna.
Continuous cereal-based cropping has led to a rapid decline in soil fertility in the Guinea savanna agro-ecological zone of northern Ghana with corresponding low crop yields. We evaluated the effects of cropping system and soil fertility status on grain yields and N2-fixation by grain legumes and net N contribution to soil fertility improvement in contrasting sites in this agro-ecological zone. Maize was intercropped with cowpea, soybean and groundnut within a row, with a maize stand alternated with two equally spaced cowpea or groundnut stands and in the maize-soybean system, four equally spaced soybean stands. These intercrops were compared with sole crops of maize, cowpea, soybean and groundnut in fertile and poorly fertile fields at sites in the southern (SGS) and the northern (NGS) Guinea savanna. The proportion of N derived from N2-fixation (%Ndfa) was comparable between intercrops and sole crops. However, the amount of N2-fixed was significantly larger in sole crops due to a greater biomass accumulation. Legumes in poorly fertile fields had significantly smaller shoot δ15N enrichment (−2.8 to +0.7‰) and a larger %Ndfa (55–94%) than those in fertile fields (−0.8 to +2.2‰; 23–85%). The N2-fixed however was larger in fertile fields (16–145kgNha−1) than in poorly fertile fields (15–123kgNha−1) due to greater shoot dry matter and N yields. The legumes grown in the NGS obtained more of their N requirements from atmospheric N2-fixation (73–88%) than legumes grown in the SGS (41–69%). The partial soil N balance (inkgha−1) was comparable between intercrops (−14 to 21) and sole legumes (−8 to 23) but smaller than that of sole maize receiving N fertiliser (+7 to +34). With other N inputs (aerial deposition) and outputs (leaching and gaseous losses) unaccounted for, there is uncertainty surrounding the actual amount of soil N balances of the cropping systems, indicating that partial N balances are not reliable indicators of the sustainability of cropping systems. Nevertheless, the systems with legumes seem more attractive due to several non-N benefits. Our results suggest that soybean could be targeted in the SGS and cowpea in the NGS for greater productivity while groundnut is suited to both environments. Grain legumes grown in poorly fertile fields contributed more net N to the soil but growing legumes in fertile fields seems more lucrative due to greater grain and stover yields and non-N benefits.
•N2-fixation depends on genetic, environmental, management and socio-economic factors.•More productive and wealthier farmers benefit most from soybean-maize crop rotation.•Soybean-maize rotations ...should be promoted with integrated soil and crop management.
Soybean production can contribute to the nitrogen economy of smallholder farming systems, but our understanding of factors explaining variability in nitrogen fixation and rotational benefits across farms and regions is limited. Biological nitrogen fixation (BNF) was quantified with the natural abundance method in 150 farmer-managed soybean plots under different varieties and inputs in Dowa, Mchinji and Salima districts of Malawi. Soybean yielded on average 1.2tha−1 grain and the above-ground biomass at mid pod filling (R5.5) was 2.8tha−1 and contained in total 63kgha−1 nitrogen derived from the atmosphere (Ndfa). Locally sourced varieties obtained a larger %Ndfa (65%) than the ‘improved’ variety Nasoko (53%). The %Ndfa was positively associated with soil sand content, sowing date, plant population and biomass accumulation, but it was not affected by inoculation with rhizobia or the combination of inoculation and NPK fertiliser application. Quantities of N2 fixed differed between regions and years, and was enhanced by applying inoculant and fertiliser together, leading to more biomass accumulation and larger grain yields. Soil available P and exchangeable K contents also increased the total amount of N2 fixed. In a related trial, continuous maize yields were compared with maize following soybean in 53 farmer-managed fields. Average yield in continuous maize was 2.5tha−1, while maize after soybean produced 3.5tha−1 (139% of continuous maize). Farmers with higher maize yields, who applied external nutrient inputs, and with a larger value of household assets achieved greater yield responses to rotation with soybean. A relative yield increase of more than 10% was observed on 59, 90 and 77% of the fields in Dowa, Mchinji and Salima respectively. We conclude that fields of soybean and maize that receive adequate nutrient inputs and good management to ensure good yields benefit most in terms of quantities of N2 fixed by the legume and the yield response of the following maize crop. The results suggest that the promotion of soybean-maize rotations should be done through an integrated approach including the promotion of appropriate soil and crop management techniques. Furthermore, they suggest that wealthier households are more likely to apply adequate nutrient inputs and good crop management practices and are likely to receive larger maize yield responses to the incorporation of soybean.
This paper provides a systematic review of studies assessing the impact of climate change on crop yields in southern Africa. Moreover, it synthesises the current knowledge of the impact of elevated ...ambient CO
2
levels (eCO
2
) and temperatures on physiological processes, and the application of this knowledge in mechanistic crop models. While eCO
2
evidently has a strong impact on photosynthesis and crop water use, it is uncertain how this will work out for the climatic and crop management conditions prevailing in southern Africa. The impact of heat stress on crop reproductive processes and the process of transpiration cooling mitigating heat stress are poorly represented in models, while both process are relevant given the climatic conditions prevailing in southern Africa. Twenty studies assessing the impact of climate change on future yields of crops, mostly maize, have been retrieved. The results suggest that potato, Bambara groundnut and sugarcane yields may improve. No consistent trends for maize and sorghum could be identified. While yield predictions are obviously context-specific, large uncertainties related to climate predictions and crop models imply results should be treated with caution. Suggestions are made for field experimentation and the improved application of crop models for climate change research in the region.
The South African policy sphere hails the commercialisation of small-scale farming as the answer to a myriad of socio-economic, ecological and political challenges of rural livelihoods. Yet the low ...agricultural productivity of this sector challenges the realisation of this pathway. Through comparison with large-scale farmers, this review sought to elicit the main reasons for differences in productivity and explore the prospects of small-scale farming. It highlights that low productivity of small-scale farming cannot be solely ascribed to biophysical constraints and that differences rather arise at farm and regional level. Therefore, intervention strategies should not be solely sought at field scale, which seems to be the norm. While the prospects of small-scale farming may seem gloomy at first glance, opportunities such as investing in horticulture exist. Prospects for small-scale farming are limited by the country's very competitive and thriving large-scale farming that saturate most agricultural markets. A key conclusion from this review is that we still do not know enough about small-scale farming systems in South Africa. For example, what is the contribution of small-scale farming to the living income of households? Are farming households food and nutrition secure? In particular, the role of agriculture in improving rural livelihoods is poorly understood. Farming is likely to remain an important supplementary livelihood opportunity for the majority of rural households. As such, small-scale farming needs to be rethought as part of a broader livelihood strategy by all stakeholders while continuously seeking alternative entry points towards thriving rural livelihoods. This means provision of support for transition to more commercial farming activities for those with interest and sufficient resources, while alternative employment or social protection is provided for others. A key question for research is what types of farming (crops and livestock) and what scale of operation is needed to achieve commercial success in different regions.
In South Africa, the potato is produced in regions with different climates. Climate change is expected to result in higher temperatures, thus increasing the incidence of heat stress, but lowering the ...risk of frosts in mid-altitude areas. Increasing ambient carbon dioxide levels can enhance photosynthetic rates and reduce water use of potato. This study assessed the impact of climate change on potential yield and water use efficiency (WUE) in twelve potato production regions of South Africa using the LINTUL-POTATO crop model. With current planting times, simulated yields between 1 961 and 2 050 increased by 0.02 and 0.40 t ha
−1
y
−1
and WUE by 0.00 and 0.08 g l
−1
y
−1
evapotranspired. Improvements in yields and WUE were close to zero when the crop was grown in hot periods, as an increasing incidence of heat stress and higher evapotranspiration largely discounted benefits of higher CO
2
levels. This was particularly the case in the interior production regions, where expected temperature increases were most severe. In many regions, potato growers are likely to respond to climate change by advancing planting time. Often a trade-off existed between maximising yield and WUE. A compromise could be achieved by planting as early as possible in the optimum spring planting window for summer crops.
Summary
Intra‐ and inter‐sample similarities for four populations of the annual grass weed Phalaris minor from Haryana state, India, were examined using inter‐simple sequence repeat (ISSR) DNA ...markers. Levels of polymorphism within and between populations were low in comparison with values reported from other grassy weed species. Analysis of inter‐population similarities allowed a partial differentiation of the four populations and of pairs of populations classified by cropping system. Analysis of the intra‐population similarity data showed a weak but consistent and statistically significant negative correlation between the molecular similarity of seedlings and the physical distance between their mother plants over distances up to 40 m (the maximum separation tested) in all four populations. The consistency of the observed relationship between molecular similarity and physical separation, and the differences in cultivation practices at the four sites, suggested that the relationship may be a result of localized out‐crossing, rather than an effect of localized seed rain. The results of the analyses are discussed in relation to the potential for evolution of multiple traits in the weed in response to changes in the wheat production system in the region.
Summary This paper tests the hypothesis that increased soil nitrogen supply reduces the growth of late‐emerging weeds in wheat and potato by enhancing canopy leaf area development and thereby ...reducing the availability of light for weed growth. Two series of experiments were conducted: one in spring wheat (1997, 1999) with sown Stellaria media and one in potato (1998, 1999) with naturally emerged weeds, including S. media. For each crop, two cultivars were grown at three levels of nitrogen supply. In wheat, as in a monoculture of S. media, total dry weight and seed number of the weed increased with soil nitrogen supply, whereas in potato the opposite was found. Increased soil nitrogen supply increased the nitrogen uptake of S. media in wheat, despite the reduced light availability, indicating that S. media in wheat was limited by nitrogen. In potato, on the other hand, growth of S. media was limited by light availability, which decreased with increased soil nitrogen supply. We conclude that the differences in response of S. media in wheat and potato to additional nitrogen supply are attributable to the dual influence of soil nitrogen supply on light and nitrogen availability, which are mediated by the crops.
A combination of agro-ecological and socioeconomic factors gave rise to the current weed epidemic of Phalaris minor, threatening the sustainability of wheat (Triticum aestivum) production and rural ...livelihoods across a large area of the Indo-Gangetic plains in northern India. This paper presents the results of a system audit describing the current status of the P. minor epidemic in Haryana State, India, in a broad agronomic and socioeconomic context. The audit showed that the P. minor epidemic is a major production constraint for many farmers, and seriously affects farm profitability. It suggests that the epidemic is closely entangled with geographic factors, as well as farm management systems and farmers' socioeconomic positions. Since the farmer community is not a homogeneous group with respect to P. minor management, nor in ability to change farm management practices, farmers are not equally receptive to technical innovations aimed at improving sustainability of wheat production.
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