Closing the cassava yield gap: An analysis from smallholder farms in East Africa

Cassava yields in Africa are small and it remains unclear which factors most limit yields. Using a series of farm surveys and on-farm and on-station trials in Uganda and western Kenya, we evaluated the importance of abiotic, biotic and associated crop management constraints for cassava production in a range of socio-economic settings as found in smallholder farms in the region. Average yields under farmer management were 8.6 t ha⁻¹, but these were more than doubled to 20.8 t ha⁻¹ by using improved crop establishment, improved genotypes and 100–22–83 kg ha⁻¹ of single-nutrient N–P–K fertilizers. A farm survey revealed large yield differences between farms. Less endowed farmers harvested less cassava per unit area than better endowed farmers (difference of 5.9 and 9.7 t ha⁻¹ in Kenya and Uganda, respectively); differences were associated with less access to labour, poorer soils, and premature harvesting by less endowed farmers. Analysis of 99 on-farm and 6 on-station trials showed that constraints for cassava production varied strongly between sites and years. Poor soil fertility, early water stress and sub-optimal weed management limited cassava production by 6.7, 5.4 and 5.0 t ha⁻¹, respectively, when improved crop establishment and genotypes were used. Pests and diseases were relatively unimportant, while weed management was particularly important in farmer fields during a dry year in Kenya (yield gap of 11.6 t ha⁻¹). The use of complementary analytical tools such as multiple regression and boundary line analysis revealed that many fields were affected by multiple and interacting production constraints. These should be addressed simultaneously if significant productivity improvements are to be achieved. This will be more difficult for less endowed than for better endowed farm households, since the former lack social and financial capital to improve management.     

Phosphorus efficiency in long-term (15 years) wheat–maize cropping systems with various soil and climate conditions

Long-term (over 15 years) winter wheat (Triticum aestivum L.)–maize (Zea mays L.) crop rotation experiments were conducted to investigate phosphorus (P) fertilizer utilization efficiency, including the physiological efficiency, recovery efficiency and the mass (the input–output) balance, at five sites across different soil types and climate zones in China. The five treatments used were control, N, NP, NK and NPK, representing various combinations of N, P and K fertilizer applications. Phosphorus fertilization increased average crop yield over 15 years and the increases were greater with wheat (206%) than maize (85%) across all five sites. The wheat yield also significantly increased over time for the NPK treatments at two sites (Xinjiang and Shanxi), but decreased at one site (Hunan). The P content in wheat was less than 3.00 g kg⁻¹ (and 2.10 g kg⁻¹ for maize) for the N and NK treatments with higher values for the Control, NP and NPK treatments. To produce 1 t of grain, crops require 4.2 kg P for wheat and 3.1 kg P for maize. The P physiological use efficiency was 214 kg grain kg⁻¹ P for wheat and 240 kg grain kg⁻¹ P for maize with over 62% of the P from P fertilizer. Applying P fertilizer at 60–80 kg P ha⁻¹ year⁻¹ could maintain 3–4 t ha⁻¹ yields for wheat and 5–6 t ha⁻¹ yields for maize for the five study sites across China. The P recovery efficiency and fertilizer use efficiency averaged 47% and 29%, respectively. For every 100 kg P ha⁻¹ year⁻¹ P surplus (amount of fertilizer applied in excess of crop removal), Olsen-P in soil was increased by 3.4 mg P kg⁻¹. Our study suggests that in order to achieve higher crop yields, the long-term P input–output balance, soil P supplying capacity and yield targets should be considered when making P fertilizer recommendations and developing strategies for intensively managed wheat–maize cropping systems.     

Irrigation and nitrogen scheduling as a requirement for optimising wheat yield and quality in Haryana, India

Wheat in Haryana (NW India) is grown as a winter crop in an annual sequence with rice, cotton, pearl millet or cluster bean as the main monsoon crops. Higher wheat yields in Haryana are associated with the use of modern varieties, increase in fertiliser use, improved irrigation practice and conservation tillage, and the recommendation to farmers for N fertiliser rates and timing and irrigation practice have an emphasis on optimising yield and input efficiencies. In India the importance to consumers of product quality does exist and, although the market place presently does not actively reward farmers for better quality wheat, the need for creating suitable and targeted marketing opportunities is now recognised. This paper examines aspects of input efficiencies and focuses on combinations of N-fertiliser and irrigation input in wheat crops grown with these four rotations (rice-wheat, cotton-wheat, pearl millet-wheat and cluster bean-wheat). Management practices that optimise grain production as well targeting grain that achieves best chapatti (Indian flat bread) quality are evaluated within a split-plot experiment where 4 irrigation schedule treatments were split with nitrogen management treatments involving a 2-way or 3-way split of N fertiliser. With the rice-wheat system, there were no differences between different split timings of N with grain yield, however with the 3 other wheat systems the 3 way split of N-fertiliser application, with N applied equally at N-fertiliser applied at seeding, early tillering and first node stage, always gave the highest yield. With all 4 rotations the highest protein level was achieved (range 11.8-12.5%) with this 3-way N application split. Grain yield increased in a step-wise manner as additional irrigation was implemented with all rotations and the highest protein outcomes were achieved with the least irrigations. The apparent recovery of N fertiliser applied was similar and highest with the 3-way split, and the 2-way split that did not include a basal N fertiliser application. Different rates of N fertiliser were included in separate experiments using the 3-way split of N application, and with the rice-wheat rotation the GreenSeeker instrument was used to establish the rate for the third application of N. The application of extra N-fertiliser with the non rice-wheat rotations produced no additional grain yield with an increase in the N-fertiliser input beyond 150 kg N ha⁻¹, although protein and N-content increased incrementally. Grain hardness and chapatti score trended higher with increases in N-fertiliser input but the increases were relatively small. The use of the GreenSeeker instrument with the rice-wheat rotation resulted in N saving of 21-25 kg N ha⁻¹ with similar grain yield, protein and grain hardness to that provided by using the recommended 150 kg N ha⁻¹. Where the GreenSeeker was used the apparent recovery was 70-75% compared with 60% with the wheat receiving the recommended 150 kg N ha⁻¹, suggesting farmers are likely to be over-fertilising their wheat crop. The best yields obtained in these experiments were about 5.5-6.0 t ha⁻¹ and these yields are consistent with a decade-long attainable yield identified for wheat in rice-wheat rotation for Haryana. If farmers can achieve market recognition for chapatti quality, and with the use of appropriate varieties, then farmers can assume that the best practice outlined here for optimising grain yield with integrated nutrient and soil management will be the same practice that optimises chapatti quality.     

Soil type,management history and current resource allocation: Three dimensions regulating variability in crop productivity on African smallholder farms

Soil fertility varies markedly within and between African smallholder farms, both as a consequence of inherent factors and differential management. Fields closest to homesteads (homefields) typically receive most nutrients and are more fertile than outlying fields (outfields), with implications for crop production and nutrient use efficiencies. Maize yields following application of 100 kg N ha⁻¹ and different rates and sources of P were assessed on homefields and outfields of smallholder farms in Zimbabwe. Soil organic carbon, available P and exchangeable bases were greater on the homefields than outfields. In each of three experimental seasons, maize yields in homefield control plots were greater than in the outfields of farms on a granitic sandy and a red-clay soil. Application of mineral N significantly increased maize yields on homefields in the first season (2.1–3.0 t ha⁻¹ on the clay soil and 1.0–1.5 t ha⁻¹ on the sandy soil) but the effects of N alone were not significant on the outfields due to other yield-limiting factors. Greatest yields of about 6 t ha⁻¹ were achieved on the clayey homefield with 100 kg N ha⁻¹ and 30 kg P ha⁻¹ applied as single super phosphate (SSP). Manure application gave greater yields (3–4 t ha⁻¹) than SSP (2–3 t ha⁻¹) in the sandy homefield and in the clayey outfield. Maize did not respond significantly to N, dolomitic lime, manure and P on the sandy outfield in the first and second seasons. In the third season, manure application (∼17 t manure ha⁻¹ year⁻¹) on the sandy outfield did result in a significant response in grain yields. Apparent P recovery in the first season was 55–65% when P was applied at 10 kg ha⁻¹ on the clayey homefield (SSP), clayey outfield (SSP and manure) and sandy homefield (manure) with apparent P recovery less than 40% when P was applied at 30 kg ha⁻¹. On the sandy outfield, P recovery was initially poor (<20%), but increased in the successive seasons with manure application. In a second experiment, less than 60 kg N ha⁻¹ was required to attain at least 90% of the maximum yields of 2–3 t ha⁻¹ on the sandy homefield and clayey outfield. N use efficiency varied from >50 kg grain kg⁻¹ N on the infields, to less than 5 kg grain kg⁻¹ N on the sandy outfields. Apparent N recovery efficiency by maize was greatest at small N application rates with P applied. We conclude that blanket fertilizer recommendations are of limited relevance for heterogeneous smallholder farms. Targeted application of mineral fertilizers and manure according to soil type and past management of fields is imperative for improving crop yields and nutrient use efficiencies.     

Indigenous legume fallows (indifallows) as an alternative soil fertility resource in smallholder maize cropping systems

Widening the range of organic nutrient resources, especially N sources, is a major challenge for improving crop productivity of smallholder farms in southern Africa. A study was conducted over three seasons to evaluate different species of indigenous legumes for their biomass productivity, N₂-fixation and residual effects on subsequent maize crops on nutrient-depleted fields belonging to smallholder farmers under contrasting rainfall zones in Zimbabwe. Under high rainfall (>800 mm yr⁻¹), 1-year indigenous legume fallows (indifallows), comprising mostly species of the genera Crotalaria, Indigofera and Tephrosia, yielded 8.6 t ha⁻¹ of biomass within 6 months, out-performing sunnhemp (Crotalaria juncea L.) green manure and grass (natural) fallows by 41% and 74%, respectively. A similar trend was observed under medium (650–750 mm yr⁻¹) rainfall in Chinyika, where the indifallow attained a biomass yield of 6.6 t ha⁻¹ compared with 2.2 t ha⁻¹ for natural fallows. Cumulatively, over two growing seasons, the indifallow treatment under high rainfall at Domboshawa produced biomass as high as 28 t ha⁻¹ compared with ∼7 t ha⁻¹ under natural fallow. The mean total N₂ fixed under indifallows ranged from 125 kg ha⁻¹ under soils exhibiting severe nutrient depletion in Chikwaka, to 205 kg ha⁻¹ at Domboshawa. Indifallow biomass accumulated up to 210 kg N ha⁻¹, eleven-fold higher than the N contained in corresponding natural fallow biomass at time of incorporation. Application of P to indifallows significantly increased both biomass productivity and N₂-fixation, translating into positive yield responses by subsequent maize. Differences in maize biomass productivity between indifallow and natural fallow treatments were already apparent at 2 weeks after maize emergence, with the former yielding significantly (P < 0.05) more maize biomass than the latter. The first maize crop following termination of 1-year indifallows yielded grain averaging 2.3 t ha⁻¹, significantly out-yielding 1-year natural fallows by >1 t ha⁻¹. In the second season, maize yields were consistently better under indifallows compared with natural fallows in terms of both grain and total biomass. The first maize crop following 2-year indifallows yielded ∼3 t ha⁻¹ of grain, significantly higher than the second maize crop after 1-year indifallows and natural fallows. The study demonstrated that indigenous legumes can generate N-rich biomass in sufficient quantities to make a significant influence on maize productivity for more than a single season. Maize yield gains under indifallow systems on low fertility sandy soils exceeded the yields attained with either mineral fertilizer alone or traditional green manure crop of sunnhemp.     

Increased productivity through integrated soil fertility management in cassava–legume intercropping systems in the highlands of Sud-Kivu,DR Congo

Smallholder farmers in sub-Saharan Africa are confronted by low productivity and limited investment capacity in nutrient inputs. Integrated soil fertility management (ISFM) aims at increased productivity through the combined use of improved germplasm, judicious fertilizer application and organic matter management, adapted to the local farming conditions. We hypothesize that the application of these different ISFM components can result in significant increases in productivity and economic benefits of cassava–legume intercropping systems. Participatory demonstration trials were conducted in the highlands of Sud-Kivu, DR Congo with 12 farmer groups during 3 seasons. Treatments included the farmers’ common practice (local common bean and cassava varieties, seed broadcast and manure addition) and sequentially added ISFM components: improved bean and cassava germplasm, modified crop arrangements, compound NPK fertilizer application and alternative legume species (groundnut or soybean). The use of improved germplasm did not result in yield increases without simultaneous implementation of other ISFM components. Modifying the crop arrangement by planting cassava at 2 m between rows and 0.5 m within the row, intercropped with four legume lines, increased bean yields during the first season and permits a second bean intercrop, which can increase total legume production by up to 1 t ha⁻¹ and result in an additional revenue of almost 1000 USD ha⁻¹. Crop arrangement or a second legume intercrop did not affect cassava storage root yields. Fertilizer application increased both legume and cassava yield, and net revenue by 400–700 USD ha⁻¹ with a marginal rate of return of 1.6–2.7. Replacing the common bean intercrop by groundnut increased net revenue by 200–400 USD ha⁻¹ partly because of the higher market value of the grains, but mostly due to a positive effect on cassava storage root yield. Soybean affected cassava yields negatively because of its high biomass production and long maturity period; modifications are needed to integrate a soybean intercrop into the system. The findings demonstrate the large potential of ISFM to increase productivity in cassava–legume systems in the Central-African highlands. Benefits were, however, not observed in all study sites. In poor soils, productivity increases were variable or absent, and soil amendments are required. A better understanding of the conditions under which positive effects occur can enable better targeting and local adaptation of the technologies.     

Critical precipitation period for dryland maize production

Grain yields for dryland maize (Zea mays L.) production in the semi-arid Great Plains of the United States can be unpredictable because of the erratic nature of growing season precipitation. Because of the high input costs for maize production, farmers need to have a tool that will help them assess the risk associated with dryland maize production. The objectives of this work were to determine the critical period for precipitation during the maize growing season and to develop a relationship between critical period precipitation and maize yield to use as a tool to quantify expected yield variability associated with dryland maize production in this region. Maize yield data were collected at Akron, CO from two dryland cropping systems experiments (1984–2009) in which maize was grown in a 3-year winter wheat (Triticum aestivum L.)–maize–fallow rotation. Yields were correlated with weekly precipitation amounts from planting to harvest in search of the period of time in which yield was most influenced by precipitation. Soil water contents at planting were measured either by gravimetric sampling or by neutron attenuation. Yields were found to be most closely correlated with precipitation occurring during the 6-week period between 16 July and 26 August. The data separated into two linear relationships defined by whether the sum of available soil water at planting and May precipitation was less than or greater than 250 mm. These two linear relationships between precipitation during this critical period and yield were used with long-term precipitation records to determine the probability of obtaining a maize yield of at least 2500 kg ha⁻¹ (generally considered to be a break-even yield) at three locations across the central Great Plains precipitation gradient. This analysis quantified the production risk associated with the highly variable corn yields that result from erratic summer precipitation in this region.     

Abiotic constraints override biotic constraints in East African highland banana systems

Banana is the primary food crop in Uganda, but yields are low due to a complex of abiotic and biotic constraints. However, quantitative information on the importance, interactions, and geographic distribution of yields and constraints is scanty. We monitored yields, biotic and abiotic constraints in 159 plots in Central, South and Southwest Uganda in 2006–2007. About half the plots were on-farm demonstrations that received fertilizer (average 71N, 8P, 32 K kg ha⁻¹ year⁻¹) through a development project, the rest were ordinary farmer fields (i.e. controls). Fresh banana yields in controls were significantly (P ≤ 0.05) higher in Southwest (20 t ha⁻¹ year⁻¹) compared with Central (12 t ha⁻¹ year⁻¹) and South (10 t ha⁻¹ year⁻¹). Demonstrations yielded 3–10 t ha⁻¹ year⁻¹ more than controls. Yield losses were calculated using the boundary line approach. In Central, yield losses, expressed as percentage of attainable yield, were mainly attributed to pests (nematodes 10% loss, weevils – 6%) and suboptimal crop management (mulch 25%). In South, poor soil quality (pH – 21%, SOM – 13%, N-total – 13%, and Clay – 11%) and suboptimal crop management (weeds – 20%) were the main constraints. In Southwest, suboptimal crop management (mulch 16%), poor soil quality (K/(Ca + Mg) − 11%) and low rainfall (5%) were the primary constraints. The study revealed that biotic stresses (i.e. pests, weeds) are particularly important in Central, whereas abiotic stresses (i.e. nutrient deficiencies, drought) dominate in South and Southwest. This study concludes that (i) technologies currently available allow farmers to double yields and (ii) past research efforts have mistakenly neglected abiotic constraints.     

Radiation use efficiency,N accumulation and biomass production of high-yielding rice in aerobic culture

The concept of aerobic culture is to save water resource while maintaining high productivity in irrigated rice ecosystem. This study compared nitrogen (N) accumulation and radiation use efficiency (RUE) in the biomass production of rice crops in aerobic and flooded cultures. The total water input was 800–1300 mm and 1500–3500 mm in aerobic culture and flooded culture, respectively, and four high-yielding rice cultivars were grown with a high rate of N application (180 kg N ha⁻¹) at two sites (Tokyo and Osaka) in Japan in 2007 and 2008. The aboveground biomass and N accumulation at maturity were significantly higher in aerobic culture (17.2–18.5 t ha⁻¹ and 194–233  kg N ha⁻¹, respectively) than in flooded culture (14.7–15.8 t ha⁻¹ and 142–173 kg N ha⁻¹) except in Tokyo in 2007, where the surface soil moisture content frequently declined. The crop maintained higher N uptake in aerobic culture than in flooded culture, because in aerobic culture there was a higher N accumulation rate in the reproductive stage. RUE in aerobic culture was comparable to, or higher than, that in flooded culture (1.27–1.50 g MJ⁻¹ vs. 1.20–1.37 g MJ⁻¹), except in Tokyo in 2007 (1.30 g MJ⁻¹ vs. 1.37 g MJ⁻¹). These results suggest that higher biomass production in aerobic culture was attributable to greater N accumulation, leading to higher N concentration (N%) than in flooded culture. Cultivar differences in response to water regimes were thought to reflect differences in mainly (1) early vigor and RUE under temporary declines in soil moisture in aerobic culture and (2) the ability to maintain high N% in flooded culture.     

Spectral measurements of the total aerial N and biomass dry weight in maize using a quadrilateral-view optic

Heterogeneous crop stands require locally adapted nitrogen fertilizer application based on rapid and precise measurements of the local crop nitrogen status. In the present study, we validated a promising technique for the latter, namely a tractor-mounted field spectrometer with an oblique quadrilateral-view measuring optic, measuring solar radiation and canopy reflectance in four directions simultaneously. Dry matter yield (kg ha⁻¹), total N content (g N g⁻¹ dry matter) and total aerial N (aboveground N-uptake) (kg N ha⁻¹) in maize were determined in 10 m² calibration areas in 60 plots differing in their N treatment and seeding density three times in each of three years under field conditions. Results show that the sensor used can reliably determine total aerial N ranging from as little as 5 kg N to 150 kg N ha⁻¹ with R²-values ≥0.81 in 2002 and 2004, and with R²-values ranging from ≥0.57 to 0.84 in 2003. Dry matter yields from as low as 0.3–4.2 t ha⁻¹ could be determined with R²-values ranging from 0.67 to 0.91 in 2002 to 2004. The capacity to ascertain DM yield spectrally was drastically reduced in the higher yield range (>6 t ha⁻¹) probably due to decreased sensitivity of the spectral signal. N-contents were generally not well determined. Taken together there is a good potential to determine reliably differences in total aerial N or DM yield from the five leaf stages unfolded to the five node stage where typically nitrogen applications are carried out.     

Dryland maize yields and water use efficiency in response to tillage/crop stubble and nutrient management practices in China

Rainfed crop production in northern China is constrained by low and variable rainfall. This study explored the effects of tillage/crop residue and nutrient management practices on maize (Zea mays L.) yield, water use efficiency (WUE), and N agronomic use efficiency (NAE) at Shouyang Dryland Farming Experimental Station in northern China during 2003–2008. The experiment was set-up using a split-plot design with 3 tillage/crop residue methods as main treatments: conventional, reduced (till with crop residue incorporated in fall but no-till in spring), and no-till (with crop residue mulching in fall). Sub-treatments were 3 NP fertilizer rates: 105–46, 179–78 and 210–92 kg N and P ha⁻¹. Maize grain yields were greatly influenced by the growing season rainfall and soil water contents at sowing. Mean grain yields over the 6-year period in response to tillage/crop residue treatments were 5604, 5347 and 5185 kg ha⁻¹, under reduced, no-till and conventional tillage, respectively. Grain yields under no-till, were generally higher (+19%) in dry years but lower (−7%) in wet years. Mean WUE was 13.7, 13.6 and 12.6 kg ha⁻¹ mm⁻¹ under reduced, no-till, and conventional tillage, respectively. The no-till treatment had 8–12% more water in the soil profiles than the conventional and reduced tillage treatments at sowing and harvest time. Grain yields, WUE and NAE were highest with the lowest NP fertilizer application rates (at 105 kg N and 46 kg P ha⁻¹) under reduced tillage, while yields and WUE tended to be higher with additional NP fertilizer rates under conventional tillage, however, there was no significant yield increase above the optimum fertilizer rate. In conclusion, maize grain yields, WUE and NAE were highest under reduced tillage at modest NP fertilizer application rates of 105 kg N and 46 kg P ha⁻¹. No-till increased soil water storage by 8–12% and improved WUE compared to conventional tillage, thus showing potentials for drought mitigation and economic use of fertilizers in drought-prone rainfed conditions in northern China.     

Nitrogen efficiency in long-term wheat–maize cropping systems under diverse field sites in China

Efficiency of fertilizer N is becoming increasingly important in modern agricultural production owing to increasing food requirement and growing concern about environments. However, there is almost no study regarding its long-term efficiency in wheat and maize cropping systems. Long-term (15 years) experiments involving wheat (Triticum aestivum L.) and maize (Zea mays L.) rotations at five field sites with various soil and climate characteristics in China were used to determine the nitrogen (N) efficiency, including the physiological efficiency, recovery efficiency and N mass balance of soil–plant systems in response to different fertilization treatments. The present study consisted of nine treatments: unfertilized, N, phosphorus, potassium, straw and manure or their combinations. The contribution of N fertilizers to wheat yield was higher than to maize and suggested that wheat could be given priority over maize when determining N application rates. Uptake of 1 kg N produced 35.6 kg of wheat grain and 39.5 kg of maize grain. The deficit of N in soils without applied N ranged from 40 to 103 kg N ha⁻¹ year⁻¹, while N surpluses in soils with applied N fertilizers ranged from 35 to 350 kg N ha⁻¹ year⁻¹. The apparent accumulated N recovery efficiency (NRE_ac) varied widely from 4% to 90%: unbalanced fertilization and other soil limiting factors (such as aluminium toxicity) were associated with low NRE_ac. In the treatments of combination of N, phosphorus and potassium with normal application rates, the average of NRE_ac in four out of five sites reached 80%, which suggested that best management of N fertilizers could recover most of N fertilizers applied to soils. The results will be helpful to understand the long-term fate of N fertilizers and to optimize the N fertilization for agricultural practices and environment protection.     

Direct-seeded rice culture in Sri Lanka: Lessons from farmers

About 95% of the rice grown in Sri Lanka is direct-seeded (wet- and dry-seeding). The average rough rice yield in irrigated predominantly direct-seeded dry zone (DZ) is about 5.0 t ha⁻¹and in the wet zone (WZ) it is about 3.3 t ha⁻¹. However the average realizable yield in DZ and WZ are 8 t ha⁻¹ and 5 t ha⁻¹ respectively. A survey was conducted to understand the cultural practices, farmers’ perceptions and the reasons for the yield gap in direct-seeded rice culture in Sri Lanka. Farmers’ seed rate ranged from 87 to 220 kg ha⁻¹ for intermediate bold-type varieties and from 71 to 176 kg ha⁻¹ for varieties with short round grains. About 90% of the farmers in the DZ and the intermediate zone (IZ) consider both yield potential and duration as criteria in selecting a variety. Among the farmers surveyed, only 21% of the farmers in the DZ, 13% of the farmers in the IZ, and 29% of the farmers in the WZ adhered to the recommended method of basal fertilizer application. Farmers did not adhere to the correct timing of fertilizer application. More than 50% of the cost for rice farming goes to labor, followed by cost of inputs in all climatic zones. Farmers reported that the most important production constraint for direct-seeded rice in the DZ and IZ is the non availability of reliable labor followed by soil problems and weeds. While in the WZ, it is the soil problems specially iron toxicity followed by lower soil fertility. The survey revealed that smaller land holding size, non adherence to the optimum time of farm activity initiation, less efficient use of rain water, higher seed rate and higher cost of production are a few reasons for the existing yield gap. Location-specific technologies for different agro-ecological zones of Sri Lanka should be developed to reduce the cost of production and to increase resource-use efficiency and should be transferred to the farmers to achieve sustainable optimum direct-seeded rice yields.     

Productivity and residual benefits of grain legumes to sorghum under semi-arid conditions in south-western Zimbabwe: Unravelling the effects of water and nitrogen using a simulation model

The APSIM model was used to assess the impact of legumes on sorghum grown in rotation in a nutrient-limited system under dry conditions in south-western Zimbabwe. An experiment was conducted at Lucydale, Matopos Research Station, between 2002 and 2005. The model was used to simulate soil and plant responses in the experiment. Sequences of cowpea (Vigna unguiculata), pigeonpea (Cajanus cajan), groundnut (Arachis hypogaea) and sorghum (Sorghum bicolor) were used in the rotations. Legumes accumulated up to 130 kg of N ha⁻¹ which was potentially available for uptake by sorghum in the following season. The APSIM model predicted total biomass, grain and N yields of the legume phase within the experimental error and performed well in predicting sorghum yield and N supplied in the rotation after cowpea and groundnut. The model generally under-predicted sorghum total biomass and grain yield after pigeonpea. Observed patterns of crop water use, evaporative losses during the dry season and re-charge of soil profile at the start of the rainy season were generally well predicted by the model. An assessment of output on sorghum N and water stresses in the rotation indicated that the legume–cereal rotation is more driven by soil nitrogen availability than water availability even under semi-arid conditions. Further legume–cereal rotation analysis using the model will assist in the understanding of other processes in the rotations in dry environments.     

Biomass production and nitrogen accumulation and remobilisation by Miscanthus × giganteus as influenced by nitrogen stocks in belowground organs

The nitrogen (N) requirement of dedicated crops for bioenergy production is a particularly significant issue, since N fertilisers are energy-intensive to make and have environmental impacts on the local level (NO₃ leaching) and global level (N₂O gas emissions). Nitrogen nutrition of Miscanthus × giganteus aboveground organs is assumed to be dependent on N stocks in belowground organs, but the precise quantities involved are unknown. A kinetic study was carried out on the effect of harvest date (early harvest in October or late harvest in February) and nitrogen fertilisation (0 or 120 kg N ha⁻¹) on aboveground and belowground biomass production and N accumulation in established crops. Apparent N fluxes within the crop and their variability were also studied.Aboveground biomass varied between 24 and 28 t DM ha⁻¹ in early harvest treatments, and between 19 and 21 t DM ha⁻¹ in late harvest treatments. Nitrogen fertilisation had no effect on crop yield in late harvest treatments, but enhanced crop yield in early harvest treatments due to lower belowground biomass nitrogen content. Spring remobilisation, i.e. nitrogen flux from belowground to aboveground biomass, varied between 36 and 175 kg N ha⁻¹, due to the variability of initial belowground nitrogen stocks in the different treatments. Autumn remobilisation, i.e. nitrogen flux from aboveground to belowground organs, varied between 107 and 145 kg N ha⁻¹ in late harvest treatments, and between 39 and 93 kg N ha⁻¹ in early harvest treatments. Autumn remobilisation for a given harvest date was linked to aboveground nitrogen accumulation in the different treatments. Nitrogen accumulation in aboveground biomass was shown to be dependent firstly on initial belowground biomass nitrogen stocks and secondly on nitrogen uptake by the whole crop.The study demonstrated the key role of belowground nitrogen stocks on aboveground biomass nitrogen requirements. Early harvest depletes belowground nitrogen stocks and thus increases the need for nitrogen fertiliser.     

The effects of cover cropping on yield and weed control of potato in a transitional system

Cover cropping can have various beneficial effects to the cropping system such us the increase of soil nutrient content and weed suppression. In this respect, the species used for covering is of great importance. This paper reports results on the yield and weed control effects in potato crops preceded by different cover crops over a 2-year period (2003 and 2004) in Central Italy (Viterbo). Results were obtained in the frame of a more complex study set up in 2002 where in a 3-year chick-pea/potato/tomato rotation, each crop was preceded by 7 different soil managements: 5 cover crops (rapeseed, Italian ryegrass, hairy vetch, snail medick and subclover) + 1 unfertilised weedy fallow (cover crop absent) + 1 control (weedy fallow fertilised with mineral N at a rate of 170 kg ha⁻¹ for potato). Two different weed control regimes in potato were also applied [weed-free crop (1 inter-row hoeing + 1 hilling up + manual weeding on the row); mechanical control (1 inter-row hoeing + 1 hilling up)]. Cover crops were sown in September and cut and ploughed just before potato planting in March. The potato crops following the cover crops were only fertilised with green manure. Averaged over years, all the cover crops produced more above-ground dry biomass than the weedy fallow (4.79 t ha⁻¹ on average vs 2.36 t ha⁻¹). Hairy vetch and subclover accumulated the highest N in the incorporated biomass (169 and 147 kg ha⁻¹), followed by snail medick (108), rapeseed (99), ryegrass (88) and weedy fallow (47). Rapeseed and ryegrass were the most efficient weed suppressors and had the least proportion of weed biomass (<1%) of the total produced by the cover, while they also reduced weed emergence in the following potato crops (8.8 plants m⁻²vs 25.5 plants m⁻² with all other cover crops). Following subclover and hairy vetch the potato crop yield was similar to that obtained by mineral N-P-K fertilisation (48.5 t ha⁻¹ of fresh marketable tubers). Mechanical weed control compared to weed free crop always reduced potato yield and the reduction, averaged over years, was greater in N-P-K mineral fertilised control (−23.6%) and smaller in ryegrass (−7.9%).     

Effect of winter cover crop species and planting methods on maize yield and N availability under irrigated Mediterranean conditions

Under semiarid Mediterranean conditions irrigated maize has been associated to diffuse nitrate pollution of surface and groundwater. Cover crops grown during winter combined with reduced N fertilization to maize could reduce N leaching risks while maintaining maize productivity. A field experiment was conducted testing two different cover crop planting methods (direct seeding versus seeding after conventional tillage operations) and four different cover crops species (barley, oilseed rape, winter rape, and common vetch), and a control (bare soil). The experiment started in November 2006 after a maize crop fertilized with 300 kg N ha⁻¹ and included two complete cover crop-maize rotations. Maize was fertilized with 300 kg N ha⁻¹ at the control treatment, and this amount was reduced to 250 kg N ha⁻¹ in maize after a cover crop. Direct seeding of the cover crops allowed earlier planting dates than seeding after conventional tillage, producing greater cover crop biomass and N uptake of all species in the first year. In the following year, direct seeding did not increase cover crop biomass due to a poorer plant establishment. Barley produced more biomass than the other species but its N concentration was much lower than in the other cover crops, resulting in higher C:N ratio (>26). Cover crops reduced the N leaching risks as soil N content in spring and at maize harvest was reduced compared to the control treatment. Maize yield was reduced by 4 Mg ha⁻¹ after barley in 2007 and by 1 Mg ha⁻¹ after barley and oilseed rape in 2008. The maize yield reduction was due to an N deficiency caused by insufficient N mineralization from the cover crops due to a high C:N ratio (barley) or low biomass N content (oilseed rape) and/or lack of synchronization with maize N uptake. Indirect chlorophyll measurements in maize leaves were useful to detect N deficiency in maize after cover crops. The use of vetch, winter rape and oilseed rape cover crops combined with a reduced N fertilization to maize was efficient for reducing N leaching risks while maintaining maize productivity. However, the reduction of maize yield after barley makes difficult its use as cover crop.     

Effects of transition season management on soil N dynamics and system N balances in rice–wheat rotations of Nepal

In the low-input rice–wheat production systems of Nepal, the N nutrition of both crops is largely based on the supply from soil pools. Declining yield trends call for management interventions aiming at the avoidance of native soil N losses. A field study was conducted at two sites in the lowland and the upper mid-hills of Nepal with contrasting temperature regimes and durations of the dry-to-wet season transition period between the harvest of wheat and the transplanting of lowland rice. Technical options included the return of the straw of the preceding wheat crop, the cultivation of short-cycled crops during the transition season, and combinations of both. Dynamics of soil N_min, nitrate leaching, nitrous oxide emissions, and crop N uptake were studied throughout the year between 2004 and 2005 and partial N balances of the cropping systems were established. In the traditional system (bare fallow between wheat and rice) a large accumulation of soil nitrate N and its subsequent disappearance upon soil saturation occurred during the transition season. This nitrate loss was associated with nitrate leaching (6.3 and 12.8 kg ha⁻¹ at the low and high altitude sites, respectively) and peaks of nitrous oxide emissions (120 and 480 mg m⁻² h⁻¹ at the low and high altitude sites, respectively). Incorporation of wheat straw at 3 Mg ha⁻¹ and/or cultivation of a nitrate catch crop during the transition season significantly reduced the build up of soil nitrate and subsequent N losses at the low altitude site. At the high altitude site, cumulative grain yields increased from 2.35 Mg ha⁻¹ with bare fallow during the transition season to 3.44 Mg ha⁻¹ when wheat straw was incorporated. At the low altitude site, the cumulative yield significantly increased from 2.85 Mg ha⁻¹ (bare fallow) to between 3.63 and 6.63 Mg ha⁻¹, depending on the transition season option applied. Irrespective of the site and the land use option applied during the transition season, systems N balances remained largely negative, ranging from −37 to −84 kg N ha⁻¹. We conclude that despite reduced N losses and increased grain yields the proposed options need to be complemented with additional N inputs to sustain long-term productivity.     

Mineral fertilizer response and nutrient use efficiencies of East African highland banana (Musa spp., AAA-EAHB,cv. Kisansa)

Poor yields of East African highland bananas (Musa spp., AAA-EAHB) on smallholder farms have often been attributed to problems of poor soil fertility. We measured the effects of mineral fertilizers on crop performance at two sites over two to three crop cycles; Kawanda in central Uganda and Ntungamo in southwest Uganda. Fertilizers were applied at rates of 0N–50P–600K, 150N–50P–600K, 400N–0P–600K, 400N–50P–0K, 400N–50P–250K and 400N–50P–600K kg ha⁻¹ yr⁻¹. In addition 60Mg–6Zn–0.5Mo–1B kg ha⁻¹ yr⁻¹ was applied to all treatments, with the exception of the control plots which received no fertilizer. Fresh bunch mass and yield increased with successive cycles. Yield increases above the control ranged from 3.1 to 6.2 kg bunch⁻¹ (average bunch weight for all treatments 11.5 kg bunch⁻¹) and 2.2–11.2 Mg ha⁻¹ yr⁻¹ (average yield for all treatments 15.8 Mg ha⁻¹ yr⁻¹) at Kawanda, compared with 12.4–16.0 kg bunch⁻¹ (average bunch weight for all treatments 14.7 kg bunch⁻¹) and 7.0–29.5 Mg ha⁻¹ yr⁻¹ (average yield for all treatments 17.9 Mg ha⁻¹ yr⁻¹) at Ntungamo. The limiting nutrients at both sites were in the order K > P > N. Potassium, N and P foliar nutrient mass fractions were below previously established Diagnosis and Recommendation Integrated System (DRIS) norms, with the smallest K mass fractions observed in the best yielding plots at Ntungamo. Total nutrient uptakes (K > N > P) were higher at Ntungamo as compared with Kawanda, probably due to better soil moisture availability and root exploration of the soil. Average N, P and K conversion efficiencies for two crop cycles at both sites amounted to 49.2 kg finger DM kg⁻¹ N, 587 kg finger DM kg⁻¹ P and 10.8 kg finger DM kg⁻¹ K. Calibration results of the model QUEFTS using data from Ntungamo were reasonable (R² = 0.57, RMSE = 648 kg ha⁻¹). Using the measured soil chemical properties and yield data from an experiment at Mbarara in southwest Uganda, the calibrated QUEFTS model predicted yields well (R² = 0.68, RMSE = 562 kg ha⁻¹). We conclude that banana yields can be increased by use of mineral fertilizers, but fertilizer recovery efficiencies need to improve substantially before promoting wide-scale adoption.     

Mixing groundnut residues and rice straw to improve rice yield and N use efficiency

Groundnut as a pre-rice crop is usually harvested 1–2 months before rice transplanting. During this lag phase much of N in groundnut residues could be lost due to rapid N mineralization. Mixing of abundantly available rice straw with groundnut residues may be a means for reducing N and improve subsequent crop yields. The objectives of this experiment were to investigate the effect of mixing groundnut residues and rice straw in different proportions on (a) growth and yield of succeeding rice, (b) groundnut residue N use efficiency and (c) N lost (¹⁵N balance) from the plant–soil system and fate of residue N in soil fractions. The experiment consisted of six treatments: (i) control (no residues), (ii) NPK (at recommended rate, 38 kg N ha⁻¹), (iii) groundnut residues 5 Mg ha⁻¹ (120 kg N ha⁻¹), (iv) rice straw 5 Mg ha⁻¹ (25 kg N ha⁻¹), (v) 1:0.5 mixed (groundnut residues 5 Mg: rice straw 2.5 Mg ha⁻¹), and (vi) 1:1 mixed (groundnut residues 5 Mg: rice straw 5 Mg ha⁻¹). After rice transplanting, samples of the lowland rice cultivar KDML 105 were periodically collected to determine growth and nutrient uptake. At final harvest, dry weight, nutrient contents and ¹⁵N recovery of labeled groundnut residues were evaluated.