Effets des precedentes plantes de couverture sur la production de l'igname en zone de savane au Benin et au Togo

L'igname (Dioscorea spp.), est généralement cultivée sur défriche de jachère longue pour bénéficier du niveau élevé de fertilité de sol mais l'augmentation de la pression démographique entraîne une la réduction générale de la durée des jachères. Dans le but de mettre au point des systèmes de production durables à base d'igname des essais ont été menés respectivement à Gobé au centre du Bénin et à Laouno et Tchébébé au centre du Togo en vue d'évaluer l'effet des jachères des légumineuses Aeschynomene histrix, Mucuna pruriens et Pueraria phaseoloides sur l'amélioration du rendement de l'igname. La quantité de biomasse aérienne sèche la plus élevée a été produite généralement par le Mucuna qui a réalisé l'accumulation biologique d'azote la plus élevée de l'ordre de 91 à 120kg/ha. Sur tout les essais, le précédent jachère de Mucuna a augmenté de 3,2t/ha (50%), en moyenne le rendement de tubercules frais d'igname Des accroissements hautement significatifs ont été enregistrés à Laouno en 2002 sur des parcellcs de deux ans de jachère (2,7t/ha), et à Tchébébé après un an de jachère (4,0t/ha). Dans ces cas le précédent jachère de Mucuna a été économiquement rentable avec des taux marginaux de rentabilité supérieur à 100%. La pratique de la jachère plantée de Mucuna pourrait se substituer à la pratique de la jachère de longue durée en vue d'intensifier l'exploitation des terres pour la production de l'igname. (Résumé d'auteur

Experiences with Mucuna in West Africa

In IDL-1387

The Photoelectron-spectrum of Hexacarbonyl-mu-di-tert-butylacetylene-di-iron - A Joint Experimental and Theoretical-study

The He I and He II photoelectron spectra of [Fe2(CO)6(ButC2But)] are reported and assigned on the basis of ab initio self-consistent field (SCF) and configuration interaction (Cl) calculations carried out on the model system [Fe2(CO)6(HC2H)] assuming a perfect Cs symmetry. The calculations at the SCF level lead to the possibility of a triplet ground state for the model system. However, But substitution and observed deviations from Cs symmetry, along with the pair correlation energy not accounted for in the present work, are expected to reverse this trend. The ionization energies have therefore been computed assuming a closed-shell ground state for the neutral molecule. The highest occupied molecular orbital can be described in terms of a backdonation interaction from both metal atoms to a carbonyl ligand displaying some semibridging character. The Cl calculations, carried out on the ionized states using the molecular orbital set of the neutral molecule were expected to account for both the correlation effects (through two-electron excitations) and the relaxation effects (through one-electron excitations). These relaxation effects were, however, not completely accounted for, especially for localized d orbitals. The origin of the five bands reported between 7 and 12 eV can, however, be assigned with good confidence from the joint interpretation of the experimental and theoretical results

Response of selected crop associations to groundwater table depth in an inland valley

Available online 1 July 2003The success of inland valley crop production in humid and subhumid areas of subSaharan Africa depends on the availability of appropriate crop varieties with some tolerance to excess moisture. Performance of upland crop associations [cassava (Manihot esculenta) + sweet potato (Ipomoea batatas) and cassava + soybean (Glycine max)] was observed in a trial conducted along a small valley toposequence. Groundwater table depth (GWT) was a significant covariate for cassava yield and regression analysis showed that cassava clones of similar yield potential responded similarly and positively to increasing GWT depth (r2=0.77) regardless of associated crop. Sweet potato tuber production was related to GWT depth for variety TIS 9465 (r2=0.47) but not for TIS 8441. Sweet potato was more tolerant of shallow GWT than cassava. Combined cassava and sweet potato tuber production was also positively correlated to GWT depth (r2=0.83). The range of GWT depth to be considered by plant breeders screening cassava and sweet potato varieties for excess moisture in inland valleys should include depths 0.2 to 0.4 m, because those depths were shown to be stressful to these crops

Utilization of rock phosphate by crops on a representative toposequence in the northern Guinea savanna zone of Nigeria: response by maize to previous herbaceous legume cropping and rock phospate treatments

Mucuna pruriens and Lablab purpureus can immediately benefit from rock phosphate (RP), but as both herbaceous legumes have only limited economic value, these benefits are only meaningful if yields of a subsequent maize crop are improved. The impact of RP application to Mucuna and Lablab on grain yield, total N, and total P uptake of a subsequent maize crop was evaluated for a set of non-acidic soils on a representative toposequence (‘plateau', ‘slope', and ‘valley' field) in the Northern Guinea savanna (NGS) zone. Urea-N (45 kg N ha−1) was applied to all treatments on the ‘slope' and ‘valley' fields, while on the ‘plateau' field, plots were split into sub-plots with and without fertilizer N addition. Application of RP to Mucuna and Lablab led to site- and species-specific increases in grain yield, total N, and total P uptake of a subsequent maize crop. On the ‘plateau' and ‘valley' fields, maize grain yields, and total aboveground N and P uptake were significantly higher (1741 kg grains ha−1 on the ‘plateau' field and 910 kg grains ha−1 on the ‘valley' field) in the treatments following legumes treated with RP, compared to the treatments following legumes without RP addition. These increases were highest for the Mucuna treatment in both fields. On the ‘slope' field with high initial Olsen-P content application of RP to preceding legumes did not influence maize yield or N and P uptake. Improvements of maize yields on the ‘plateau' field were the results of an improvement in the soil P status after growing legumes supplied with RP, as maize did not respond to the addition of urea in the absence of RP addition to the preceding legumes. Recovery of RP-P by the maize varied between 3.1 and 5.8%. On all fields, legumes also improved the soil N status, giving average apparent legume N recoveries of 17% for Mucuna and 32% for Lablab. The results clearly show that soil fertility management in the NGS needs to take into consideration both N and P. The studied legume–maize rotations supplied with RP during the legume phase and minimal amounts of inorganic N during the maize phase are good examples of promising soil fertility management technologies alleviating N and P deficiencies. Recommendations need to be diversified following the position of the field on the toposequence and previous P application history

Adoption of improved fallows in West Africa: lessons from Mucuna and Stylo case studies

Traditional shifting cultivation systems can no longer be sustained in West Africa because of rapid increases in human and livestock populations. Short-duration, improved fallows are among the alternative land-management strategies that have evolved. This paper reviews how velvetbean or mucuna (Mucuna pruriens) and stylo (Stylosanthes hamata and Stylosanthes guianensis) management systems were developed and disseminated in West Africa. Mucuna was first adopted by farmers in southwestern Benin between 1988 and 1992, and the number of testers of the innovation rose to 10,000 farmers throughout Benin by 1996. Suppression of spear grass (Imperata cylindrica) was perceived as the main benefit of mucuna fallows. The stylo technology was introduced in the late 1970s, and it was primarily targeted to livestock production in the subhumid monomodal rainfall zone. The uptake of stylo has been relatively slow and modest in West Africa in contrast to the faster rate of adoption of mucuna in southwestern Benin. Some of the contributory factors to the slower adoption of stylo than mucuna include rainfall regime, lack of motivation of livestock keepers, insecure land tenure, limited capability and facilities of extension staff, poor communication among scientists, and unsatisfactory establishment of the crop. Recommendations to increase the adoption of improved fallows include the use of a participatory approach in problem identification, expansion of the genetic base of cover crops for use in fallows, optimization of the multiple benefits of cover crops, management of the improved system, promotional strategies, and appropriate policies

Utilization of rock phosphate by crops on a representative toposequence in the northern Guinea savanna zone of Nigeria: response by Mucuna pruriens, Lablab purpureus, and maize

The availability of P from rock phosphate (RP) is often too low to demonstrate an immediate impact on cereal production. Legumes may improve the immediate availability of P from RP and eventually benefit subsequent maize crops. The ability of Mucuna pruriens (L.) var utilis (Wright) Burck and Lablab purpureus L. to use P from RP and the changes in selected plant and symbiotic properties and in the soil available P and particulate organic matter (POM) pool as affected by the addition of RP were measured for a set of soils on a representative toposequence (‘plateau', ‘slope' and ‘valley' field) in the Northern Guinea savanna zone of Nigeria. At 18 weeks after planting (WAP), Mucuna accumulated significantly more N and P in the total biomass in the plots treated with RP compared to the plots without RP addition on all fields. Nitrogen accumulation of Mucuna reached 175, 177 and 164 kg N ha−1 in the treatments with RP on the ‘plateau', ‘slope' and ‘valley' fields, respectively. Phosphorus accumulation of Mucuna was highest at 18 WAP in all sites and reached 10, 14 and 10 kg P ha−1 in the treatments with RP on the ‘plateau', ‘slope' and ‘valley' fields, respectively. Lablab accumulated significantly more N and P at 18 WAP only on the ‘plateau' field, but some of the potential differences in N or P accumulation may have been masked by various pests especially affecting Lablab. A highly significant negative correlation was observed between the aboveground biomass at 16 WAP and the nematode population. The addition of RP significantly increased arbuscular mycorrhizal fungi (AMF) infection of the Mucuna (from 24 to 33%) and Lablab roots (from 15 to 28%) to a similar extent in all fields. This increased AMF infection was most likely caused by specific processes in the rhizosphere of the legumes as AMF infection of the maize roots (8%) was not affected by RP addition. Increases in nodule numbers and fresh weight were site- and species-specific and highest for the ‘plateau' and ‘slope' fields. The number of nodules increased on average from 8 to 19 (3 plants)−1 and from 7 to 30 (3 plants)−1 for Mucuna and Lablab, respectively, after RP addition. Although nearly all the aboveground legume biomass had disappeared from the soil surface at 51 WAP, both the Olsen-P status and POM N concentration were increased by the presence of legumes. Mucuna significantly enhanced the Olsen-P content of the soil after RP addition compared to the Lablab or maize treatments on the ‘plateau' and ‘valley' fields. Due to the relatively high initial Olsen-P content of the ‘slope' field (14 mg kg−1), differences between treatments were not significant. The N concentration of the POM pool was significantly higher under legumes than under maize on the ‘slope' and ‘valley' fields, and indicates incorporation of part of the legume biomass in the POM pool. The addition of RP to herbaceous legumes was observed to lead to site- and species-specific changes in the tripartite legume–rhizobium–mycorrhizal fungus, driven by processes taking place in the rhizosphere of the legumes, and in the soil available P pool. A cereal following these herbaceous legumes could benefit from this improvement in soil fertility status