Humid Forest Zone Research Articles

Litter Production and Soil Condition Under Agroforestry Trees in Two Agroecological Zones of Southern Nigeria

ABSTRACT Tree species in agroforestry systems contribute to soil improvement through the litter produced on the soil surface and below-ground modification of soil structure by tree roots. Therefore, litter production and soil characteristics under 11 tree species were evaluated in the derived savanna (Ibadan) and humid tropical forest (Onne) zones of southern Nigeria between 1998 and 1999 in 7-year old arboreta. In Ibadan, the soil was a gravelly Alfisol, while in Onne it was an Ultisol. The trees were Alchornea cordifolia, Baphia pubescens, Calliandra calothyrsus, Dactyladenia barteri, Dialium guineense, Grewia pubescens, Inga edulis, Irvingia gabonensis, Nauclea diderrichii, Pterocarpus santalinoides and Treculia africana. Forest and natural regrowth (no-tree) plots were included as control. Litter-fall under the trees ranged from 5 to 14 t ha−1 y−1 compared to 15 t ha−1 y−1 for the forest at Ibadan, while it ranged from 7to16tha−1y−1 compared to 20t ha−1 y−1 for the forest at Onne. Litter production was high under D. barteri and I. edulis and relatively low under A. cordifolia, G. pubescens, I. gabonensis and P. santalinoides. There was a more drastic change in soil chemical properties in the Alfisol than the Ultisol, particularly with available P. Soil bulk density and penetrometer resistance were improved by A. cordifolia, C. calothyrsus, D. guineense, G. pubescens and T. africana. This study showed that ecological differences resulted in differences in annual litter production for only 33% of the species considered. However, the effects of the trees on soil properties were strongly linked to inherent soil properties.

Fodder Yield and Nutritive Value of Browse Species in West African Humid Tropics: Response to Age of Coppice Regrowth

Several indigenous and exotic browse species with potential for development of agroforestry technologies in the humid tropics of west Africa have been identified, but information on their fodder yield and quality, and how this is influenced by age of coppice regrowth is scanty and limited to a few species. The effect of age of coppice of regrowth (8, 12, 16, and 20 weeks) on fodder yield, and concentrations of crude protein (CP), neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (lignin), and acid detergent ash (ADF-Ash) in the fodder of 27 browse species was studied in the humid forest zone of southeastern Nigeria. The fodder yield, and concentrations of NDF, ADF and lignin increased (p<0.05), while CP declined (p<0.05) with increasing age of coppice regrowth for all the browse species. The ADF-Ash concentrations of eight browse species increased linearly (p<0.05), while that of 19 species followed a quadratic (p<0.05) trend in response to increasing age of coppice regrowth. Bauhinia monandra, Calliandra calothyrsus, Dalbergia sissoo, Enterolobium cyclocarpum, Grewia pubescens, Gliricidia sepium, Leucaena leucocephala, Senna spectabilis, and Terminalia superba were identified to have high potential for the development of integrated crop-livestock agroforestry technologies in the west African humid tropics based on fodder yield, concentrations of CP, NDF, ADF and lignin. Coppice regrowth of the promising species could be harvested between 16 and 20 weeks to maximize yield and quality of the fodder. The results showed that fodder yield and chemical composition could be used to identify browse species for the development of agroforestry technologies for smallholder crop-livestock farming systems.

Soil water regime under rotational fallow and alternating hedgerows on an Ultisol in southern Cameroon

Soil moisture depletion during dry seasons by planted hedgerows to lower levels than under natural fallow, would reduce drainage and nutrient losses in the following rainy season when food crops are grown. The volumetric water content of the 0–150 cm soil profile was measured under planted hedgerows (alternating Leucaena leucocephala and Gliricidia sepium) and natural fallow, both either annually cropped to sole maize or in a two-year crop/two-year fallow rotation, in the humid forest zone (annual rainfall 1700 mm) of southern Cameroon during the 1995–1996 and 1996–1997 dry seasons. Hedgerows were cut to 0.05 m height, largely eliminating trees’ water consumption during cropping phases. Differences in total soil water content at 0–150 cm depth, between systems, occurred only in the early phases of the 1996–1997 dry season. In both dry seasons, differences between systems in water content were found in some soil layers, all within 0–60 cm depth, yet, without consistent advantage of any system in exploiting the topsoil water resources. Soil water content was lower under L. leucocephala than G. sepium at 20–40 cm depth only. Below 60 cm depth, no differences in water regimes between systems were found. Under southern Cameroonian conditions it is unlikely that any of the systems has an advantage in accessing or recovering water and thus, if available, nutrients from the sub-soil. None of the systems examined was capable of delaying drainage and thus it appears unlikely that downward displacement of nutrients is delayed after the start of the rains.

Diagnóstico de la capacidad reproductiva de toros en ganaderías de tres municipios del alto Magdalena

El objetivo del estudio fue evaluar la funcionalidad reproductiva de toros del alto magdalena analizando el efecto de la edad y la raza sobre la calidad del semen y otras características reproductivas. La investigación se realizó en los municipios de Paicol, La Plata y Nátaga (Huila - Colombia), cubriendo zonas de bosque seco tropical (bs-T) y bosque húmedo premontano (bh-PM). 60 toros de diferentes razas y edades fueron evaluados bajo condiciones de monta natural y sin reposo sexual previo al examen, usando la metodología recomendada por la American Society of Therigenology (AST). Para evaluar el efecto del tipo racial sobre algunas características reproductivas se utilizó un diseño completamente al azar con tres tratamientos y diferentes número de repeticiones por tratamiento: T1: Bos indicus , T2: Bos indicus x Bos taurus y T3: Bos taurus . El efecto de la edad fue determinado estableciendo un diseño con siete tratamientos y diferente número de repeticiones por tratamientoí: T1: &lt; 24, T2: 24.1 - 30, T3: 30.1 - 36, T4: 36.1 - 42, T5: 42.1 - 48, T6: 48.1 - 54 y T7: &gt; 54 meses de edad. Se aplicó la prueba Diferencia Mínima Significativa para determinar las diferencias estadísticas y se estableció un modelo de predicción para movilidad masal . El comportamiento reproductivo expresado en calidad del material seminal, libido y demás características reproductivas en los toros del alto magdalena, estuvo dentro de los valores normales. El tipo racial no tuvo un efecto significativo sobre las características reproductivas. Con excepción de la circunferencia escrotal y el volumen del eyaculado, la edad tampoco ejerció un efecto sobre las variables analizadas. El modelo de predicción para la movilidad masal (Y), a partir de un valor dado de circunferencia escrotal (X) es: Y = (1.90275) (X).

Integrating local and expert knowledge using participatory mapping and GIS to implement integrated forest management options in Akok, Cameroon

A spatially explicit understanding of local knowledge about land use and tenure is fundamental to developing and assessing sustainable management options for tropical forests in Central Africa. This paper presents a case study in the Humid Forest Zone of Cameroon to demonstrate the effectiveness of integrating participatory rural appraisal techniques with remote sensing and vegetation mapping in a geographic information system (GIS) environment. The full integration of biophysical and socio-cultural patterns within a GIS can identify spatial units important for ecosystem management and relate social institutions to land units across a variety of scales. Key words: landscape, forest-agriculture interface, tropical forest management, land tenure, participatory mapping, local knowledge

Production constraints on cocoa agroforestry systems in West and Central Africa: The need for integrated pest management and multi-institutional approaches

Cocoa-producing countries of West and Central Africa experienced a serious economic crisis in the early 1980s,when the cocoa sector was liberalized and the macroeconomic policies of the sector changed. These institutional changes created new difficulties and challenges for sustainable cocoa farming. Farmers in this region have recently turned to timber and non-timber production to offset the fluctuation of cocoa prices. In a survey of 300 cocoa farmers in the humid forest zone of Southern Cameroon, pest and disease outbreaks were identified as the major limiting factors to sustainable cocoa production. An analysis of pests and diseases affecting the cocoa plantations in the humid forest zone of West and Central Africa revealed strong links to the type of forest cover found on or near the cocoa plantation. An integrated approach to pest management is proposed and the paper concludes with a discussion of current efforts to address constraints posed by pests and diseases on sustainable cocoa farming in the four main cocoa-producing countries of West and Central Africa. Key words: cocoa agroforest, farmer perception, forest landscape, multi-disciplinary approach, multi- institutional approach, Africa

Abundance, dispersion and parasitism of the stem borer Busseola fusca (Lepidoptera: Noctuidae) in maize in the humid forest zone of southern Cameroon

This study was conducted in the humid forest zone of Cameroon, in 2002 and 2003. The main objective was to investigate the effects of intercropping on infestation levels and parasitism of the noctuid maize stem borer Busseola fusca Fuller. Two trials were planted per year, one during the long and one during the short rainy season. Maize monocrops were compared with maize/legume or maize/cassava intercrops in two spatial arrangements: maize on alternate hills or in alternate rows. Spatial analyses showed that the stemborer egg batches were regularly dispersed in the maize monocrop and aggregated in the intercrops, as indicated by b, the index of dispersion of Taylor's power law. Depending on the crop association and planting pattern, intercrops reduced the percentage of plants with stem borer eggs by 47.4-58.4% and egg densities by 41.2-54.5% compared to monocropped maize. Consequently, larval densities were 44.4-61.5% lower in intercrops compared to monocrops. Intercropping maize with non-host plants did not affect larval parasitism. Up to two-fold higher levels of egg parasitism by scelionid Telenomus spp. were recorded in inter- compared to monocrops during the short rainy seasons of 2002 and 2003. No differences were found among the mixed cropping treatments and parasitism was lower during the long compared to the short rainy seasons. It was proposed that differences in levels of parasitism were due to density dependence effects rather than the effect of the presence of non-host plants in the system.

Physico-chemical properties of the African pear and Black olive from different agro-ecological zones of Cameroon

Introduction. Dacryodes edulis and Canarium schweinfurthii are two tropical multi-purpose trees in Cameroon which produce fruits (African pear and Black olive, respec- tively) for human consumption, which are marketed more and more on the national and international markets. This study aimed at characterising these two fruits starting from their physical and chemical properties. Materials and methods. Fruits were collected during the harvest peak in three agro-ecological zones of Cameroon. The physical properties measured were the weight, the length, the fruit circumference and the pulp thickness, while the chemi- cal analyses included the moisture and protein, lipid and ash contents. Results. The fruits from the humid forest were largest (96 g), whereas the smallest came from the grassland (54 g). The highest pulp per fruit content (69% for the African pear and 57% for Black olive) was observed in fruits harvested in the humid forest zone. The strongest correlation (r = 0.25) between the fruit weight and that of pulp was observed for Black olive produced in the humid forest zone. The crude protein rate was highest (8%) in fruits from the humid forest, whereas that of the lipids (42%) prevailed in the semi-forest zone. Conclusion. The humid forest zone offers the best agro-ecological conditions for commercial production of the fruits of D. edulis and C schweinfurthii. These two fruits contain proteins and lipids in significant quantities and can, consequently, be used as alternative sources for these nutritive elements. Cameroon / multiple-use trees / Dacryodes edulis / Canarium schweinfurthii / fruits / proximate composition

Effects of grain legumes and cover crops on maize yield and plant damage by Busseola fusca (Fuller) (Lepidoptera: Noctuidae) in the humid forest of southern Cameroon

Abstract Field trials were run in the humid forest zone of Cameroon to investigate the effects of crop rotation, cover crops and bush fallow on infestations by the noctuid stemborer Busseola fusca (Fuller) and on yield of maize. A continuous maize production system was compared with crop sequence systems, in which maize followed a grain legume (cowpea, soybean), cover crop (mucuna, pigeon pea) or a bush fallow. Compared to the continuous maize cropping system, maize in the crop sequence with legume and fallow systems had significantly higher leaf and stem nitrogen content at 35 and 63 days after planting (DAP), respectively. Differences in K and P at 35 and 63 DAP, and of N in stem and grain at harvest were not significant. Maize in the rotation systems had 1.5–2 times more borers per plant compared to the continuous maize production systems, particularly at the early stages of maize growth. At 63 DAP, larval mortalities were 1.4–1.5 higher in rotation systems than in the continuous maize system. As a result, dead-hearts and tunnel length did not vary significantly among treatments. In the insecticide protected plots, when comparing with the maize–maize cropping system, average grain yields were increased by 15.1–24.6% and 30.3–42.5% in crop sequences of grain legumes with maize and cover crops fallow systems, respectively. However, grain yield losses due to borers were 1.9–3.1 times higher in the continuous maize production system than in the crop sequences of grain legumes with maize and 4.5–11.1 times higher compared to maize after cover crops. It is concluded that an increased nutritional status of the plants lead to an increase in borer attacks at the early stage of plant growth, but also to improved plant vigour, resulting finally in a net benefit for the plant and grain yield.

Simulation of shoot emergence pattern of cogongrass (Imperata cylindrica) in the humid tropics

Cogongrass is a noxious perennial grass that has invaded many countries in the tropical and subtropical regions of the world. Its management has been a significant challenge because of large rhizome and bud reserves in the soil. The emergence pattern of this weed under field conditions has received little attention. Field trials were conducted in 2002 and 2003 in the humid forest zone of southeastern Nigeria to model shoot emergence. The experiment had four treatments: (1) count and tag crop-free cogongrass shoots, (2) count and suppress crop-free cogongrass shoots with paraquat, (3) count and cut crop-free cogongrass shoots, and (4) count and cut cogongrass shoots in cultivated corn. The rationale for these treatments was to determine the effect of different monitoring techniques on shoot emergence of cogongrass. The development of the model was based on hydrothermal time, which was calculated from soil moisture and soil temperature at a 2-cm depth. A Weibull function was fitted to cumulative percent shoot emergence values of Treatment 4 and hydrothermal time. The model closely fit the observed pattern of cogongrass shoot emergence (r2= 0.95,n= 36). It also predicted shoot emergence satisfactorily in six treatments (r2&gt; 0.85, P &lt; 0.001,n= 7 in each treatment) that simulated farmers' practices in southwestern Nigeria. This is the first model developed for cogongrass shoot emergence based on hydrothermal time under field observations. The model should facilitate further analyses of cogongrass emergence patterns and the timing of its management.

Nutrient concentrations and NH4+‐N mineralization under different soil types and fallow forms in southern Cameroon

AbstractTo evaluate the soil‐fertility sustainability of the fallow systems, nutrient concentrations and NH4+‐N mineralization were determined in different soil and fallow types in the humid forest zone of southern Cameroon. Two experiments were conducted, the first comprised planted leguminous tree Calliandra calothyrsus, planted leguminous Pueraria phaseoloides, and regrowth mainly composed of Chromolaena odorata on the Typic Kandiudult. The second experiment made up of a fallow dominated by C. odorata, a fallow with C. odorata removed, and a P. phaseoloides fallow on the Rhodic Kandiudult, Typic Kandiudult, and Typic Kandiudox. In the first experiment, available P, Ca2+, K+ concentrations and effective CEC under C. calothyrsus were, respectively, 40%, 22%, 45%, and 15% lower when compared to P. phaseoloides but no differences were found between soils under P. phaseoloides and C. odorata. Mineralization of NH4+‐N was higher under C. calothyrsus than under C. odorata‐ and P. phaseoloides‐fallow types, indicating the impoverishment of organic material under the former. In the second experiment, the beneficial effect of P. phaseoloides was found in the Rhodic Kandiudult in the 0–10 cm layer throughout its low NH4+ release from mineralization. In the Typic Kandiudult, no differences in NH4+‐N mineralization were found between C. odorata and P. phaseoloides fallows. In the Typic Kandiudox, there was no difference in NH4+ mineralization between the three fallow types. According to the nutrient concentrations and NH4+ mineralization, the fertility sustainability of the different fallow types may be ranked as follow: P. phaseoloides ≥ C. odorata &gt; C. calothyrsus &gt; fallow without C. odorata.

The adoption of intensive monocrop horticulture in southern Cameroon

AbstractResults from a 1997 survey of 208 households in the humid forest zone of southern Cameroon indicate that African policy makers seeking to intensify agricultural production should focus attention on the horticultural sub‐sector. The survey, which gathered information on horticultural production practices, found that the average expenditure on agro‐chemical inputs by horticultural producers using monocrop production systems was 190US$/ha, which greatly exceeds the FAO reported national average expenditure of 6.50 US$/ha. A logit model of monocrop adoption indicated that the size of land holding per household had a negative effect on adoption, congruent with population‐driven technical change and that increases in unit transportation costs significantly decreased the probability of adoption. These findings suggest that policy makers should target horticultural intensification in areas of higher population density and promote investment in rural roads. The age of the household head had a significant negative and elastic effect on adoption, which in combination with an increase in the cohort of younger farmers in the rural population induced by macro‐economic events contributed to the spread of intensified horticulture. In the study area, roughly two‐thirds of rural households also produce cocoa and the quantity of cocoa produced was positively associated with adoption of intensive horticultural systems suggesting that export crop promotion indirectly facilitated diversification of agriculture. Women's participation in intensive monocrop production was limited and efforts to promote their greater involvement are recommended.

The adoption of intensive monocrop horticulture in southern Cameroon

Results from a 1997 survey of 208 households in the humid forest zone of southern Cameroon indicate that African policy makers seeking to intensify agricultural production should focus attention on the horticultural sub-sector. The survey, which gathered information on horticultural production practices, found that the average expenditure on agro-chemical inputs by horticultural producers using monocrop production systems was 190 US$/ha, which greatly exceeds the FAO reported national average expenditure of 6.50 US$/ha. A logit model of monocrop adoption indicated that the size of land holding per household had a negative effect on adoption, congruent with population-driven technical change and that increases in unit transportation costs significantly decreased the probability of adoption. These findings suggest that policy makers should target horticultural intensification in areas of higher population density and promote investment in rural roads. The age of the household head had a significant negative and elastic effect on adoption, which in combination with an increase in the cohort of younger farmers in the rural population induced by macro-economic events contributed to the spread of intensified horticulture. In the study area, roughly two-thirds of rural households also produce cocoa and the quantity of cocoa produced was positively associated with adoption of intensive horticultural systems suggesting that export crop promotion indirectly facilitated diversification of agriculture. Women’s participation in intensive monocrop production was limited and efforts to promote their greater involvement are recommended.

Growth, cane yield and sugar content of six genotypes of sugarcane in a forest zone of southeastern Nigeria

The growth and yield of six sugarcane genotypes were studied in field plots between 1999 and 2001, at Umudike, in the humid forest zone of southeastern Nigeria. Treatments comprised five improved sugarcane genotypes (NCS 001, NCS 003, NCS 004, NCS 005 and C062175) and one local variety (Umudike Local) arranged in a randomized complete block design (RCBD) with three replications. Cultivar NCS 003 gave significantly the highest stalk lengths and cane yields in both the plant-cane and ratoon crops. The genotype NCS 005, on the other hand, gave the highest cane yield in the plant-cane but not in the ratoon crop, while C062175 gave the least yield in the plant-cane but had one of the highest yields in the ratoon crop. Sugar concentration (Brix %) was highest in Umudike Local in the plant-cane crop. Key words: Sugarcane, genotypes, growth, cane yield, sugar content Agro-Science Vol.2(2) 2001: 15-18

EVALUATION OF CASSAVA/SOYBEAN INTERCROPPING SYSTEM AS INFLUENCED BY CASSAVA GENOTYPES

The effects of three cassava genotypes (NR 8212, TMS 91934 and TMS 30572) grown sole or intercropped with soybean were investigated in two field experiments in 2000/2001 and 2001/2002 cropping seasons at Umudike in the lowland humid forest zone of south-eastern Nigeria. The plant height, canopy diameter, number of leaves per plant and leaf area index (LAI) of soybean and cassava were significantly (P

Preliminary reconstruction of the desert and sandy land distributions in China since the last interglacial period

The desert and sandy land are the products of arid climate. The spatial distribution of modern deserts and sandy land in China and their relation to climate show following characteristics: arid and hyper-arid desert zones, at isohyet of less than 200 mm, are dominated by mobile dunes; semi-arid steppe and arid desert steppe with the precipitation between 200–400 mm, are dominated by semi-fixed and fixed sand dunes; the precipitation of sub-humid forest grassland and humid forest zones with scattered fixed sand land is higher than 400 mm. With this as reference, in combination with considerable amount of paleoclimatic data in desert regions and adjacent regions, the distributions of desert and sandy land in China during the last interglacial period, the last glacial maximum (LGM), and the Holocene megathermal, were preliminarily reconstructed. The results compared with that of today show that the distribution of desert and sandy land in China was greatly dwindled during last interglacial period, and the mobile dune area was about two-thirds of that of today’s, but greatly expanded during LGM. However, the dwindling area of desert and sandy land in the Holocene megathermal was smaller than that in the last interglacial period. The forcing mechanism was mainly related to the changes of East Asian winter and summer monsoon, south-northward swing of the westerlies and the variations of the Qinghai-Tibet Plateau monsoon intensity caused by global climate changes during the cold and warm intervals since the last interglacial period.

Long-Term Phosphorus Fertilizer Effects on Phosphorus Uptake, Efficiency, and Recovery by Upland Rice on an Ultisol

Phosphorus (P) deficiency is a major constraint to upland rice production on highly weathered, low activity clay soils in the humid zone of West Africa. There is a paucity of information on the long-term fertilizer P effects on rice on these soils. A field experiment was conducted for six years (1993–1998) to determine the response of four upland rice cultivars to fertilizer P applied at 0, 45, 90, 135, and 180 kg P ha−1 only once in 1993, and to residual P in 1994, 1995, 1996, and 1998. The experimental site was located on an Ultisol, low in available P, in the humid forest zone of Côte d'Ivoire, West Africa. This paper discusses long-term P effects on P uptake and efficiency of upland rice cultivars. The cultivars evaluated differed in cumulative agronomic and physiological P efficiencies, and the efficiencies were higher at lower P rates for the P-efficient cultivars. The P uptake response and P harvest index were affected by P rate and its residual effect, which decreased with time after P was applied. The mean cumulative recovery of applied P in five crops of four rice cultivars varied from 5.5 to 9.4%. The results suggest that the variation in P efficiency of the cultivars is due to differences in the efficiency of utilization of P for grain production.

Long-Term Cropping Effect on Phosphorus Fractions in an Ultisol of the Humid Forest Zone in West Africa

The nature of phosphorus (P) pools in soils after application of P fertilizers should be considered when designing management practices to raise P fertility. Five rates of P: 0(P0), 45(P45), 90(P90), 135(P135) and 180(P180) kg ha−1 in the form of triple superphosphate (TSP) were applied once in 1993 on a Typic Paleudult at Man in Côte d'Iviore and the soil was cropped successively for five years (i.e., up to 1998) with four upland rice cultivators. At the end of the cropping season in 1998, soil samples were collected from the various P treated plots and the following P fractions were analyzed: resin-P, NaHCO3-Pi, NaOH-Pi, HCl-P residual P and organic P. The P fractions were compared with those of stored soil samples collected in the field before the initiation of the experiment in 1993. Five years of cropping the soil reduced labile Pi (resin-P+NaHCO3-Pi) from 10.8 mg P kg−1 in the uncultivated soil to 7.62 mg P kg−1 in the control (P0) and to 9.71 mg kg−1 in the P45 treatment. Labile Pi in P90 was the same (10.7 mg P kg−1) as in the uncultivated soil but in the P135 and P180 soils, labile Pi increased by 1.3 and 2.7 mg P kg−1 above that of the uncultivated one. The NaOH-Pi and residual P fractions of lower availability increased with increasing rates of P application. The results suggested that on the Ultisol studied, P applied once to a field, cropped successively over a period of time was not an adequate management option. Frequent P application at low rates at each cropping season is suggested and merits investigation on the Ultisol studied.

Factors Affecting Infestations of the Stalk BorerBusseola fusca(Lepitoptera: Noctuidae) on Maize in the Forest Zone of Cameroon with Special Reference to Scelionid Egg Parasitoids

The effect of vegetation, intercropping, and Telenomus spp. egg parasitism during the vegetative stage of maize on Busseola fusca (Fuller) (Lepitoptera: Noctuidae) infestations and maize yields at harvest were studied during two consecutive cropping seasons, in the humid forest zone of Cameroon. Six locations grouped into three blocks, representing gradients in human population densities of 15–88 inhabitants/km2 were chosen. The fields were located at 20–90 min walking distance from the next road. Differences between blocks in the measured variables were mostly not significant indicating that the population density gradient was not strong enough to affect length of fallow period and thereby vegetation, the population dynamics of pests and natural enemies, and maize yields in forest fields. Numbers of egg batches per plant but also egg parasitism by and sex ratio of Telenomus spp. were significantly higher during the second than the first season (66.9 versus 19.9% and 0.67 versus 0.21, respectively). As a result, the number of borers per plant was only 0.22 versus 0.73 during the first season. Similarly, total mortality of immature B. fusca was 60.5% during the first and 93.2% during the second season. The seasonal differences in unexplained mortality was probably because rainfall caused drowning of migrating first instars. High mean parasitoid mortality at the beginning of the first season was probably caused by high superparasitism promoted by low host density conditions. Across seasons, abundance of grasses in the surroundings of a field was negatively related to egg batch density. Likewise, numbers of B. fusca decreased with increasing density of the nonhost cassava in the field, indicating increasing mortality of migrating larvae. Egg numbers per plant was negatively and egg parasitism or parasitoid sex ratio was positively related to yield.

Phosphorus input-output on an ultisol cropped to upland rice in West Africa

A long-term (1993-98) experiment was conducted in Man, Cote d'Ivoire, West Africa, to study the effect of four proved upland rice cultivars to direct phosphorus (P) in 1993 and to residual P in 1994, 1995, 1996 and 1998, applied at five rates only once in 1993. The soil at the site in the humid forest zone is an Ultisol. The four rice cultivars were WARDA-bred cultivars WAB56-125, WAB56-104 and WAB556-50 and check IDSA6, and the five rates of P fertilizers, applied as triple superphosphate were (0, 45, 90, 135 and 180 kg P/ha). All plots received a uniform application of N (applied as urea at 100 kg/ha) and K (applied as KCl at 100 kg/ha). At harvest, rice grain and straw yields were recorded and total P uptake in rice biomass was computed by analysing the P content of the grain and straw samples. The P input-output value (amount of P applied and amount of P taken up in the grain plus straw in five crops) was negative in the no-P treatment but positive in treatments where P fertilizer was added. The results showed that in five crops, only a small fraction of the applied P fertilizer was removed by the rice crop, which varied from 5.5-9.4%, and this value decreased with the increase in the rate of P applied. In 1998, however, after 5 years of successive cropping, available P in the soil was found to be similar and not affected by the rate of P fertilizer applied in 1993