D.E. Johnson’s research while affiliated with University of Greenwich and other places

This book provides a comprehensive overview of the rice sector in Africa and the ongoing rice research and development activities in the region. Specific topics are classified under the following headings: overview of rice in Africa (chapters 1-4), rice genetic diversity and improvement (chapters 5-14), sustainable productivity enhancement (chapters 15-22), rice value chain development (chapters 23-27), working with rice communities (chapters 28-31), and future prospects for rice in Africa (chapters 32-33).

This book provides a comprehensive overview of the rice sector in Africa and the ongoing rice research and development activities in the region. Specific topics are classified under the following headings: overview of rice in Africa (chapters 1-4), rice genetic diversity and improvement (chapters 5-14), sustainable productivity enhancement (chapters 15-22), rice value chain development (chapters 23-27), working with rice communities (chapters 28-31), and future prospects for rice in Africa (chapters 32-33).

Weeds cause considerable yield losses in irrigated rice in West Africa, particularly where sub-optimal crop management or limited resources hamper production. Because of the recent focus of regional breeding programs in West Africa on a wide diversity of interspecific (O.sativaindica×O.glaberrima) and intraspecific (O.sativa×O.sativa) crosses for lowland systems, new genotypes with promising yield potential and putatively improved weed competitiveness are now available. Our study intended to establish a screening methodology for weed competitiveness in irrigated lowlands, to evaluate weed competitive abilities of diverse rice genotypes, and to relate several morphological plant characteristics with competitive abilities. A selection of 25 cultivars and advanced breeding lines was grown under irrigated conditions during the 2000 wet season (WS) and the 2001 dry season (DS) at Ndiaye (Senegal), and their reaction to four different competition levels was assessed. Competition was greatest where the test lines were grown with a mixture of an improved O. sativa indica cultivar and natural weed growth. Average yield losses across rice lines were 36% in the WS and 31% in the DS, but differences between lines were highly significant in both seasons and yield losses ranged from 0 to 84%. Competitive abilities were not related to yields without competition. In the DS, several of the new lines combined high yield with good competitiveness and outperformed the currently widely used check variety. Height, tiller density, specific leaf area (SLA), leaf area index (LAI), and growth duration were mostly negatively related to yield loss. Plant characteristics measured in monoculture had no explanatory value for the competitive ability of tested lines, indicating that screening under conditions of competition with weeds is preferable. Competitive abilities of several lines were stable across sites, years, and seasons; in others, they were strongly influenced by season. The simplified experimental layout can be used to screen advanced breeding lines or established cultivars with moderate resources (e.g., space, seed, labour). Including such a screening in the breeding process would enable more weed competitive cultivars to be identified and would be likely to contribute to increased resource-use efficiency, reduced yield losses, lower costs, and less reliance on herbicides in rice-based lowland systems.

The effects of differing periods of weed management were studied in experiments in five farmers’ direct-seeded, irrigated rice (Oryza sativa L.) fields during the 1999 wet season (1999WS) and seven fields during the 2000 dry season (2000DS) in the Senegal River delta. Ten weed management treatments were used to identify critical periods of competition and to enable the development of more precise management recommendations. Rice yields in 1999WS ranged from 3.2 t ha−1 with no weed control to 6.3 t ha−1 in weed-free plots; a yield loss of 49%. Major weed species encountered in 1999WS were Bolboschoenus maritimus, Oryza longistaminata, Cyperus difformis and Echinochloa colona. In 2000DS, C. difformis dominated the weed flora, and rice yields ranged from 3.7 t ha−1 with no weed control to 7.9 t ha−1 in weed-free plots; a yield loss of 47%. To assess the effects of weed biomass on the rice crop over time a linear mixed model was used. This highlighted the importance of E. colona and C. difformis in the wet and dry seasons, respectively, and of O. longistaminata in both seasons. The presence of E. colona early in the crop seemed related to greater levels of rice biomass in 1999WS, while O. longistaminata always had a negative effect on crop growth even at relatively low levels of infestation. Gompertz and logistic equations were fitted to data representing increasing periods of weed-free growth and weed interference, respectively. Critical periods for weed control, obtaining 95% of a weed-free yield, were estimated as between 29 to 32 days after sowing (DAS) in the WS and 4 to 83 DAS in the DS. The lower temperatures in the early DS resulted in slower growth of rice and weeds, delaying the effects of competition. Weed competition either before or after these critical periods had negligible effects on crop yield.

Increased demand for land is forcing many farmers to intensify theirupland rice-based systems with a gradual shift from bush fallow rotation towards sedentary agricultural production. However, farmers are not adapting appropriate management practices to the intensified land use, which may threaten sustainability. Diagnostic field trials were conducted at four sites in during 1994 and 1997, comparing 228 farmers' fields with diverse cropping histories. The objective was to improve future research priority setting and to guide site and systems-specific targeting of available technical options. Soil samples (0–20 ) were analyzed for pH, organic matter quantity andquality, and the potential soil N supplying capacity. Weed species and dry biomass and rice grain yield were determined under farmers' management and in three super-imposed researcher-managed subplots (hand weeding at 28, 56, and84 days, 30 mineral fertilizer Napplication, and a combination of both). Yield gaps were attributed to weeds and nitrogen, based on yield response to researchers' management in intensified systems. Increased cropping intensity and reduced fallow duration were associated with yield reduction, which was largest at the sites in the derived savanna (1.48 vs. 1.15 )and the bimodal forest zones (1.55 vs. 1.02). Intensification-induced yield loss was about 25% and appeared to be related mainly to increased weed infestation (72% more weed biomass) and declining soil quality (about 20% less soil organic C content and N supply). Weeds were the dominant factor responsible for rice yield loss in the forest area (explaining 65% of the yield gap) and appeared to play a lesser role in the savanna. The reduction in soil organic carbon and N supplying capacity was strongest in the derived savanna zone where N supply explained 35% of the yield gap. In the two savanna environments, a large share of the intensifiaction-related yield reduction could not be ascribed to either weeds or N (probably related tochanges in soil physical parameters or pests). Long-term upland rice productivity can thus not be sustained at current intensification practices.Improved management strategies should aim primarily at reducing weed pressure and improving soil organic matter content and N supply. Technical options arelikely to vary with agro-ecological zones.

Much of the rapidly-growing demand for rice in West Africa will be met from production in inland valley swamps which are abundant and relatively robust with regard to cropping intensification. Rice yields in the traditional, predominating non-bunded systems are low (about 1.5 Mg ha−1). Informal development of traditional rainfed lowlands is currently occurring across the region, mainly with low-cost interventions such as the building of field bunds. Relating lowland rice yield to quantitative information on resource base quality and agronomic practices will improve future research priorities and guide site and system-specific targeting of available technical options. Diagnostic on-farm trials were conducted in 62 traditional and 40 improved (bunded) rainfed lowland fields in three agro-ecological zones of Côte d'Ivoire (Guinea savanna, bimodal forest, and monomodal forest zones) to quantify effects of improved water control on productivity. Weed biomass and rice grain yield were determined in farmers' fields and in super-imposed researcher-managed subplots (clean weeding, zero N control treatment, and mineral fertilizer N application). Grain yield variability was attributed to agronomic practices using multiple regression analysis. Bunding significantly increased rice grain yield across sites by about 40% and controlled weeds, with approximately 25% less weed biomass in bunded than in open plots. Mineral fertilizer N application significantly increased rice yields (18% on average across sites) only in bunded fields, where N use efficiency was 12 compared to 4 kg of rice grain per kg of applied N in open fields. Across environments, about 60% of the observed variability in rice grain yields was explained by water control and the timing of agronomic management interventions (weeding, N application). The increased development of lowland rice areas through the construction of field bunds has the potential to significantly increase rice production in West Africa, while also possibly reducing labor requirements for hand weeding and allowing for a more efficient use of mineral N fertilizers. The benefits from construction of bunds are likely to be largest in the relatively well-drained inland valleys of the savanna and bimodal rainfall forest, compared to the high-rainfall monomodal forest zones.

Experiments in over 250 farmers’ lowland rice fields of West Africa determined yield gaps and responsible management factors. Simulated potential yields increase from 6.8 Mg ha−1 in the forest zone to nearly 10 Mg ha−1 in the Sahel. Actual farmers’ yields rarely exceed 1.8 Mg ha−1 in rainfed and 3.5 Mg ha in irrigated fields. Between 30 and 60% of the observed yield gap was related to N deficiency. Particularly water management, early weed control and timly N application increased N use efficiency with the largest impact in irrigated fields of the savanna zone.

Gains in yield and profitability of irrigated rice (Oryza sativa L.) in the Senegal river region resulting from improved fertilizer and weed management practices were studied in collaboration with farmers’ organizations, national research institutes and farmers. The profitability of different fertilizer and weed management treatments was compared to farmers’ practice using partial budgeting techniques and the net benefit of all treatments was estimated. Improved fertilizer management increased grain yield by 0.9 t ha−1, while the recommended weed management resulted in a yield increase of 1.0 t ha−1. The effect of recommended management practices was additive and gave a mean yield increase of 1.8 t ha−1 compared to farmers’ practices. Principal weed species in rice were Echinochloa colona (L.) Link, Diplachne fusca (L.) P. Beauv. ex Stapf, Sphenoclea zeylanica Gaertner, Bolboschoenus maritimus L., Schoenoplectus senegalensis (Steudel) Raynal, Cyperus iria L. and Cyperus difformis L. Indications of propanil-resistant E. colona were found. Weed infestation was severe and weed biomass comprised between 24 and 49% of the combined biomass of weeds and rice at panicle initiation. Average losses in rice yield caused by weeds were 0.56 t ha−1 per 10% of relative weed biomass. The value/cost ratios were between 2.1 and 4.6 for the improved treatments, and improved soil fertility and weed management resulted in an increase in net revenues of 40–85% compared to farmers’ practice.

Traditional upland rice-based cropping systems in West Africa rely on periods of fallow to restore soil fertility and prevent the build-up of insect pests and weeds. Demographic growth and increased demand for land is forcing many farmers to intensify their rice production systems. Declining fallow length and increasing number of crops before leaving the land to extended fallow result in a significant yield reduction. Promising cropping system alternatives include the use of site specific, weed-suppressing, multi-purpose cover legumes as short duration fallows. Constraints to rice production related to intensification were determined in 209 farmers' fields in four agro-ecological zones during 1994 and 1995. Nitrogen accumulation and weed suppression were evaluated in 54 legume accessions, grown for six months during the dry season, under a range of hydrological and soil conditions in 1994/95. Their effect on the yield of upland rice was determined in 1995. To increase benefits from improved fallow technology, the timing of legume establishment in relation to rice and the effect on crop and weed growth of removing, burning, mulching, or incorporating allow residues prior to the rice crop were determined. Intensified land use resulted in a significant plot-level yield reduction that was highest in the derived savanna and the bimodal forest zones where it was associated with a doubling of the weed biomass in rice and a significant reduction in soil N supply. Legume fallows appear to offer the potential to sustain rice yields under intensified cropping. Legume biomass was in most instances significantly greater than in the weedy fallow control and several legume species suppressed weed growth. Nitrogen accumulation by legumes varied between 1-200 kg N ha(-1) with 30-90% Ndfa. Rice grain yield following legume fallows increased by an average of 0.2 mg ha(-1) or 29% above the weedy fallow control. Relay establishment substantially increased legume biomass. However, seeding of the legume at 28 days or earlier significantly reduced grain yield due to interspecific competition. Incorporating or mulching of fallow residues provided no significant yield advantage as compared to burning. Absolute effects varied as a function of site, legume species, and management practice.

Traditional upland rice-based cropping systems in West Africa rely on periods of fallow to restore soil fertility and prevent the build-up of insect pests and weeds. Demographic growth and increased demand for land is forcing many farmers to intensify their rice production systems. Declining fallow length and increasing number of crops before leaving the land to extended fallow result in a significant yield reduction. Promising cropping system alternatives include the use of site specific, weed-suppressing, multi-purpose cover legumes as short duration fallows. Constraints to rice production related to intensification were determined in 209 farmers' fields in four agro-ecological zones during 1994 and 1995. Nitrogen accumulation and weed suppression were evaluated in 54 legume accessions, grown for six months during the dry season, under a range of hydrological and soil conditions in 1994/95. Their effect on the yield of upland rice was determined in 1995. To increase benefits from improved fallow technology, the timing of legume establishment in relation to rice and the effect on crop and weed growth of removing, burning, mulching, or incorporating fallow residues prior to the rice crop were determined. Intensified land use resulted in a significant plot-level yield reduction that was highest in the derived savanna and the bimodal forest zones where it was associated with a doubling of the weed biomass in rice and a significant reduction in soil N supply. Legume fallows appear to offer the potential to sustain rice yields under intensified cropping. Legume biomass was in most instances significantly greater than in the weedy fallow control and several legume species suppressed weed growth. Nitrogen accumulation by legumes varied between 1–200 kg N ha-1 with 30–90% Ndfa. Rice grain yield following legume fallows increased by an average of 0.2 mg ha-1 or 29% above the weedy fallow control. Relay establishment substantially increased legume biomass. However, seeding of the legume at 28 days or earlier significantly reduced grain yield due to interspecific competition. Incorporating or mulching of fallow residues provided no significant yield advantage as compared to burning. Absolute effects varied as a function of site, legume species, and management practice.