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There is a need to quantify agriculture’s potential to sequester carbon (C) to inform global approaches aimed at mitigating climate change effects. Many factors including climate, crop, soil manage...

On low-input smallholder farms of Kenyan upland landscapes, erosion of nutrient-rich topsoil strongly affects crop yields. Where maize (Zea mays) is intercropped on erosion-prone slopes, intercropping can potentially reduce soil erosion. The objective of this research was to quantify the contribution of crops and crop mixtures of different growth habits to erosion control and their influence on above-ground biomass and earthworm abundance as indicators of soil function in smallholder farming systems under a bimodal rainfall pattern in Western Kenya. The experiment involved five treatments, namely maize (Z. mays)/common bean (Phaseolus vulgaris) intercrop (maize intercrop), maize/common bean intercrop plus Calliandra (Calliandra calothyrsus) hedgerows and Calliandra mulch (Calliandra), sole Lablab (Lablab purpureus), sole Mucuna (Mucuna pruriens) and groundnut (Arachis hypogaea) intercropped with maize (during the short rains). The experiment was conducted over three consecutive cropping seasons and the cropping system had significant effects on soil loss, runoff, water infiltration, earthworm abundance and above-ground biomass and crop grain yield. The Calliandra treatment had the lowest runoff (11.6–17.2 mm ha−1) and soil erosion (31–446 kg ha−1 per season) in all the seasons, followed by the Mucuna treatment. Lablab was affected by disease and showed the highest soil erosion in the last two seasons. Infiltration was highest in Calliandra treatment, and earthworm abundance was higher under Mucuna and Calliandra treatments (229 and 165 earthworms per square metre, respectively) than under other crops. Our results suggest that including sole crops of herbaceous species such as Mucuna, or tree hedgerows with mixtures of maize and grain legumes has the potential to reduce runoff and soil erosion in smallholder farming. Additionally, these species provide a suitable habitat for earthworms which stabilise soil structure and macropores and thus potentially increase infiltration, further reducing soil erosion.

Macro-organisms contribute significantly to soil fertility improvement. The influence of conservation agriculture (CA) in southern Africa on their populations has not yet been fully understood. Thus, the objective of the current study was to evaluate the effects of CA and conventional tillage on below ground biological activity in a CA long-term trial in Monze, Zambia from 2011 to 2013. The study had ten treatments which differed by tillage systems (conventional ploughing, planting basins and direct seeding) and crop diversification intensity (sole cropping, 2- or 3-year crop rotations) involving maize, cotton and sunn hemp. These factors were combined to create rotation-tillage (RotTill) treatments. Sampling of macrofauna was done once per year using a metal frame measuring 0.25 m2, hand-sorted to 30-cm depth. RotTill treatments had a significant effect on earthworms (Lumbricus terrestris), termites (Coptotermes formosanus), dung beetles (Scarabaeus viettei) and centipedes (Lithobius forficatus). Earthworms and termites were more abundant in CA treatments than in conventionally ploughed (CP) treatments. Biota diversity was generally higher in CA treatments than in CP controls. Conventional mouldboard ploughing generally reduced macrofauna, thus affecting biological soil fertility and the beneficial effect of the interactions of these organisms with the soil. CA treatments had the highest maize grain yields throughout the study period. Based on the results, reduced tillage systems and crop rotations increase biological activity shown by increased densities of termites and earthworms.

Legumes play an important role in sub-Saharan Africa (SSA) farming systems through provision of food, feed, fuel, income and a range of biophysical benefits, such as soil fertility enhancement and erosion control. However, their full potential is not being realized. The purpose of this study was to assess farmers’ perceptions and knowledge towards legumes and the rationale of farmers for current legume production practices using a survey of 268 farmers in the Democratic Republic of Congo and Kenya. Most of the farmers had some knowledge of legumes and their characteristics. However, they had little knowledge of some key functions, including soil erosion control and soil fertility improvement. Most farmers relied on radio and other farmers for legume-related information. Farmers with relatively large livestock holdings ranked provision of livestock feed as an important legume function. We conclude that farmers put more value on short-term benefits of legumes including food and income than long-term benefits such as natural resource management and thus grain legumes are more readily identified by farmers than forage species. Also, we conclude that farmers require more than just information about legumes to increase uptake, they also require improved market access to procure inputs and sell products to realize other benefits that are associated with growing legumes.

Legumes form part of an ecological based solution to intensification in areas with limited access to external inputs or to support increased efficiency of available fertiliser nutrients. Despite a number of decades of intervention, uptake of legumes has been slow within smallholder farming systems in Sub-Saharan Africa. This paper explores the drivers behind adoption of legumes by developing an indicator of household legume cultivation (HLC) from a bespoke survey of 274 small-scale farm households in Kenya and the Democratic Republic of the Congo.We decompose this indicator using a beta regression framework and find a range of intensities across sites and farms, indicating limited influence of agro-ecological zones and formal institutions on uptake. There was some commonality in drivers across sites, though age, income and gender have positive but very marginal effects. Farm households with more intense legume cultivation were less driven by commercial growth objectives and have limited access to markets. There was little interest in expanding farm area which reflects the lack of assets available to these farmers and, as a consequence, promotes the use of legumes in providing home nutrition, or supporting farm fertility and provision of livestock feed. Further development of this HLC metric would be enabled by consistent data gathering across regions, or at least equally detailed studies of legume uptake. Overcoming constraints to increasing use of legumes should be a significant component of local and international agricultural intervention as countries experience increasing environmental and social pressures and the need to commercialise as farming develops.

Conservation agriculture (CA) and no-till (NT)-based cropping systems could address soil degradation and fertility decline in southern Africa. A multi-location and multi-year experiment was carried out between 2008 and 2014 to assess the effects of different levels of maize residue biomass (0, 2, 4, 6 and 8 t ha−1) and nitrogen (N) fertilizer (0, 30, 90 kg ha−1) on maize performance under no-tillage. In some sites, different (N) fertilizer levels were superimposed to test their effects on maize grain yield and leaf chlorophyll content under different maize residue biomass levels. The different residue levels had no significant effect on maize yield in most growing seasons. Maize residue cover increased grain yield in eight out of 39 site-years across the sites used. However, in some sites, maize yield decreased with increases in residue level in cropping seasons that had average to above average rainfall. At a few sites maize yield increased with increase in residue level. Seasonal rainfall pattern influenced the effect of different residue levels on grain yield at most sites. Nitrogen fertilizer increased maize yield regardless of the residue level applied. This study demonstrates that mulching with maize residues in CA/NT systems results in limited maize yield gains – at least within the first 6 years in different agro-ecological conditions of southern Africa.

Arthropod pest control is one of the most critical agronomic practices in cereal-based cropping systems. Cropping systems that preserve or promote biological pest control agents are more desirable. Data collected from a long-term trial at the Monze Farmer Training Centre (MFTC) from 2009 to 2012 was used to compare effects of different Conservation Agriculture (CA) systems with conventional practices (CP) on arthropod species diversity and populations with specific emphasis on beneficial and detrimental aerial arthropods. Up to 13 arthropod orders comprising of 40 species were identified in the cropping systems and their population density differed in years and cropping systems. Higher diversity was observed in CA systems in all years based on the Shannon-Weiner indices ranging from 0.4 to 2. However, a CP system with no rotations (CP-M) showed comparable results in the year with low rainfall. Community evenness increased in the same year with a value of 0.46. More beneficial and detrimental arthropods were recorded in a CA system with a three-year rotation of maize, cotton and sunnhemp reaching up to 9533 individuals ha−1. The CP-M recorded the highest increase in detrimental arthropods from 2009 to 2012. A canonical correspondence analysis did not show a clear-cut association of both CA and CP systems with either beneficial or detrimental arthropods. However, associations were more driven by seasonal effects. Reduced soil disturbance, crop residue retention and crop diversity in CA systems preferentially attracted beneficial arthropods while preserving existing favourable arthropods. Conventional tillage systems often disturbed arthropod habitats, disrupted their life cycles leading to lower biological activity. Increased biodiversity from CA systems may therefore support biological control.

Soil erosion and land fragmentation threaten agricultural production of sub-Saharan African highlands. At our study site in Western Kenya, farm size is mostly

Smallholder farmers in eastern and southern Africa face various food production constraints that have resulted in high food insecurity problems. The smallholder cropping systems are affected by poor rainfall distribution, high drought frequencies, poor soil fertility status, limited access to adequate inputs and lack of knowledge among others. The situation has been further worsened by increasing variability in climate. To improve food security, enhance soil conservation and fertility and boost livestock feed in different cropping systems, use of legumes such as velvet bean has been recommended. A literature study has been carried out aiming to review research findings on the roles of velvet bean (Mucuna pruriens L.) in African smallholder farming systems with emphasis on food and feed provision, soil fertility improvement and soil erosion control. The review found that velvet bean adapt very well within the tropical climate, but its uptake in smallholder farming systems in the region remains low. Velvet bean provides food in the form of seed grain after careful processing using different methods including soaking the seeds in running water for 72 h or soaking in water for 48 h with 12 h water replacements, among other methods. It can be integrated into smallholder farming systems through intercropping with other crops, crop rotations, relay cropping and as a sole crop. Velvet bean has high protein concentration (23–35%), which can help reduce the protein deficiency gaps for resource-poor farmers. It has also been used as a medicinal plant. The crop can be used as green manure cover crop to improve soil fertility and as source of soil cover which helps reduce the impact of raindrops which reduces runoff and soil erosion. Velvet bean fixes more nitrogen when compared to other legumes, 34–108 kg atmospheric nitrogen, which helps improve soil fertility. The crop may also be utilised as livestock feed through use of grain seed and the crop residues which both increase the protein availability to the animals. Velvet bean also smooths weeds due to high growth rate and biomass production, which may reduce weed density up to 92% when rotated with maize. In summary, velvet bean has multiple functions which have potential to improve smallholder farmer’s livelihoods through increased livestock and crop production.

Weed management is a challenge to resource constrained smallholder agricultural production in sub Saharan Africa due to insufficient and inadequate management strategies. When weeds are controlled with herbicides or through rotations under conservation agriculture, shifts in weed spectra are expected, increasing the need to adapt to this new situation. Experiments were conducted at four experimental sites namely the University of Zimbabwe farm (UZ) (clay soil), Domboshava Training Centre (DTC) (sand soil) and two contrasting soil types at Henderson Research Station (HRS sand (s), HRS clay (c)) to investigate the responses of weed communities to crop rotations. The trial was carried out from the 2008–09 to the 2013–14 cropping season. Rotations consisted of maize (Zea mays L.) rotating with a range of green manures, and the control treatment was maize monocropping. Herbicides were only applied in the maize phase at seeding, supplemented by hand weeding whenever weeds were 10 cm tall or 10 cm in diameter for weeds with a stoloniferous growth habit. Weed count data was collected between 2011 and 2014. Weed density, the Shannon–Weiner index and its components were used to explain weed community responses to rotations. There was a decrease in weed densities over time at all sites with a percentage decrease as high as 92% (i.e. from 357 to 30 weeds m−2) observed in maize-velvet bean (Mucuna pruriens (L.) DC) rotation. At all sites, some maize–green manure rotations were associated with high weed densities and these included maize-black sunnhemp (Crotalaria juncea L.) and maize-cowpea (Vigna unguiculata Walp) rotations. The two weeds Galinsoga parviflora Cav. and Ricardia scabra L. remained abundant throughout the study at all sites with densities reaching above 1000 plants m−2 per season in some plots. Shannon's E′ index was highest at HRSs and HRSc sites in the maize-velvet bean rotation and maize-common rattle pod (Crotalaria grahamiana Wight & Arn.) rotations respectively suggesting that in these treatments dominant weeds were reduced in numbers. This suggests that rotations with cover crops such as velvet bean may reduce weed numbers and dominance of problematic weeds over time. This can potentially lead to a less intense weeding schedule, which is more cost effective and affordable for smallholder farmers.

Increased challenges of weed control in the smallholder farming sector of southern Africa have often resulted in small yields. The objective of this study was to evaluate the effects of different weed control strategies on weed flora and composition under conservation agriculture (CA) systems in Zimbabwe. This study was conducted at three on-station trial sites namely Domboshawa Training Centre (DTC), University of Zimbabwe farm (UZ farm) and Henderson Research Station (HRS) in a maize–soybean rotation for four seasons from 2009–2010 to 2012–2013 seasons. Hand weeding was done whenever weeds were 10 cm tall or 10 cm in circumference for weeds with a stoloniferous growth habit. Weed identification was done up to the weed species level, and the Shannon–Weiner diversity and evenness index was used to determine the response of weed flora to herbicides. Results showed that there were more weeds in the early years which decreased gradually until the final season. Weed species diversity was not affected by herbicide application and the results indicated that weed species diversity was small in CA systems. Annual weed species constituted a greater proportion of species, and species richness decreased with the duration of the study. Richardia scabra L. and Galinsoga parviflora Cav. were the most common dominant weed species at all sites and in all seasons. Moreover, herbicide application had no effect on the evenness of weeds in the plots but site characteristics had a significant effect on the distribution of weed species (weed species evenness). The results presented in this study suggest that herbicide application facilitates a depletion of weed seed bank/number of weeds over time. Thus, herbicide application in CA has potential to reduce weed density, species richness and species diversity in the long term which may lead to more labour savings and larger yields.

Weed management under conservation agriculture (CA), especially when manually controlled is one of the major setbacks for the widespread adoption of CA in southern Africa. This study was conducted at three on-station and three on-farm sites: CIMMYT-Harare, Domboshawa Training Centre and Henderson Research Station (on-station sites), Hereford farm, Madziva communal area and Shamva communal area (on-farm sites). The evaluation focused on the effect of initial herbicide application and succeeding manual weeding whenever weeds were 10 cm tall or 10 cm in length for grasses with stoloniferous–rhizomatous growth habit. Weeds counts, weeding time and grain yields were collected at all on-station sites. At the on-farm sites, weed counts were done before weeding and a number of farmers were timed during weeding. The results showed that herbicides use reduced the weed density and time taken on weeding at all sites. Combining herbicides e.g. atrazine, glyphosate and metalachlor had the lowest weed density and weeding time at all sites. However, the treatments had no effect on maize grain yields suggesting that appropriate and timely manual weeding reduced crop/weed competition. Herbicides treatments had higher input costs than manual weeding due to the additional cost of herbicide but the treatment with manual weeding only had more overall labour days compared to the mixture of three herbicides. In order to achieve economic benefits, smallholder farmers may use the time for value addition e.g. expand cropped land area, use time for value addition, or sell new products on the market. Herbicides use reduces the manual labour needed to control weeds and minimise total crop failure due to untimely weeding hence, herbicides are an important but not the only weed control option under CA systems in Zimbabwe.

Weed management challenges in the early years of conservation agriculture (CA) adoption may require the use of herbicides for farmers to realise the immediate benefits of CA practices. The objective of this study was to assess the effects of atrazine on soybean grown after maize in a maize-soybean rotation under conservation agriculture. Atrazine was applied at 1.8 kg ha-1 active ingredient (a.i), as a pre-emergence herbicide in the maize phase of the rotation. The study was conducted in Zimbabwe at Domboshawa Training Centre (DTC), Henderson Research Station (HRS) and University of Zimbabwe farm (UZ farm) over two seasons (2011/2012 and 2012/2013). Manual weeding was utilised to keep the study area weed free and eliminate interference from weeds. Weed density, weed biomass, soybean germination, soybean biomass and grain yield were measured. The lowest germination of soybean was recorded at 57% in 2011/2012 at DTC under atrazine + glyphosate + metolachlor in combination with manual weeding treatment. Previous atrazine treatment to maize showed no significant differences on soybean biomass accumulation and broadleaf weed density at all sites in both seasons. The highest soybean yields recorded were 3707 kg ha-1 at DTC in 2011/12 season in atrazine + glyphosate + metolachlor plus manual weeding treatment. Based on results obtained in this study it can be concluded that soybean can be grown in plots where atrazine was applied as a pre-emergent herbicide during the maize phase.

Conservation agriculture (CA) is promoted as a cropping system that has potential to alleviate poor crop yields in smallholder farming while protecting the environment. It involves maintenance of permanent soil cover, diverse crop rotations and/or interactions; and minimum soil disturbance. CA is associated with crop residue management challenges due to low crop biomass yields and crop-livestock interactions in Zimbabwean smallholder farming sector. There is competition on crop residue uses causing challenges in retaining adequate crop residues for full benefits of using residues to be realised. Among the crop residues management options fencing fields reduces the chances of crop residues grazing by free roaming cattle during the dry season. Construction of rakes to pile up crop residues where cattle cannot access has been practiced in some communal areas. Farmers have practised the system of taking the crop residue harvest to homesteads into protected areas to reduce risk of grazing. Farmers may use fences around fields to reduce access into fields. However, all these management options require an investment from the farmers who are resource constrained. Farmers may use non-crop residues such as thatch grass and reduce competition for crop residue use where farmers feed them to livestock during the dry season.