Early growth of alley shrubs in the Highland region of Rwanda

Early growth of some woody shrubs used in alley cropping were studied in the Buberuka Highland and Central Plateau region of Rwanda. The altitudinal ranges for the two agroecological zones are 1800–2400 m and 1500–1900 m respectively. First, early growth of Sesbania sesban (Sesbania) as affected by intercropped bean, potato, pea and wheat was studied. Second, effect of lime and manure applications on growth of Sesbania, Leucaena sp (Leucaena), Calliandra Calothyrsus (Calliandra) and Markhamia lutea (Markhamia) was investigated. Third, growth of Sesbania and Leucaena at selected locations were examined. Bean grown in association with Sesbania enhanced the growth of the shrub in the Central Plateau region. At both sites, growth of Sesbania was retarded by intercropped pea. Growth of Sesbania increased significantly during the dry season in June and July. Biomass and height after one years' growth of Sesbania were higher in the Buberuka than in the Central Plateau region. Growth of the shrubs is ranked as: sesbania > Leucaena > Calliandra > Markhamia in the Buberuka Highlands. Farmyard manure generally increased the growth rates of the shrubs. Calliandra and Leucaena were most responsive to liming. The response of Leucaena to liming decreased with manure application but no definite trend was observed for Calliandra, Sesbania and Markhamia. Sesbania and Markhamia were least influenced by application of lime. Their respective LRI's were 0.35 and 0.46. The average growth rates for Sesbania and Leucaena at selected locations ranged between 26.4–62.2 cm/month and 13.1–27.4 cm per month respectively.Contribution from the University of Arkansas' USAID supported Farming Systems Research Program (FSRP) in Rwanda (USAID 696-0110), BP 625, Kigali Rwanda     

Site factors associated with the performance of Leucaena leucocephala (Lam.) de Wit and Sesbania sesban (L.) Merill in pure and mixed stands in the northern highlands of Rwanda

Alley cropping with leguminous tree hedgerows planted on contours is an emerging practice in the northern highlands of Rwanda where field slopes range from 4 to 76% and loss of soil fertility due to erosion is the principal impediment to food production. Sesbania sesban and Leucaena Leucocephala, the two woody species recommended for alley cropping in the region, do not perform equally well across sites: Sesbania is faster growing, But is more sensitive to pruning than Leucaena. We tested the hypothesis that the two species could be grown together to make the best use of their relative advantages. Species were planted in pure and 1:1 mixed stands at 26 sites to give a range of altitudes, slopes, azimuths and soil fertility levels representative of the region. During two cropping seasons’ growth spanning 19 months, in pure stands, Sesbania produced more biomass, grew taller and had thicker stems than Leucaena: for the same species, stem diameter and height were not different in pure and mixed stands. Biomass and number of poles from 1:1 mixed stands of Sesbania and Leucaena were equivalent to biomass and pole numbers from Sesbania in pure stands. However, mixed stands produced more biomass and pole numbers than Leucaena in pure stands. Growth of S. sesban was positively correlated with soil pH, potassium and altitude; whereas, growth of L. leucocephala was positively correlated with soil phosphorus, but negatively correlated with field slope. All other soil and site variables tested were nonsignificant. The study suggests that farmers can plant S. sesban and L. leucocephala in 1:1 mixtures in an alley cropping setting and retain levels of biomass production and bean pole numbers equivalent to those from S. sesban alone without compromising future growth and production by Leucaena. As well, the impact of pH, K and P on growth and production of Sesbania and Leucaena as a function of field slope and altitude should be determined before extending their use to other regions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Alley cropping of maize with nine leguminous trees

A maize-leguminous tree alley cropping system was studied on N-deficient soils in Hawaii to determine mulch effects on maize yields. Calliandra calothyrsus, Cajanus cajan, Cassia siamea, Gliciridia sepium, KX1 — Leucaena hybrid (L. pallida X L. diversifolia), L. leucocephala, L. pallida, L. salvadorensis, and Sesbania sesban were evaluated for green manure and yield of intercropped maize. S. sesban, G. sepium, L. pallida, and KX1 produced between 5 and 12 dry t/ha/yr green manure with nitrogen yields between 140 and 275 kg N/ha in 4 prunings. Maize yields responded linearly to nitrogen applied as green manure. Maize yield increased 12 kg for each kg of nitrogen applied. Additions of prunings from hedge rows were able to support maize grain yields at about 1800 kg/ha for two consecutive cropping seasons, while control plot yields averaged less than 600 kg/ha. Maize yields reflected the amount of nitrogen applied as green manure, regardless of tree species from which the nitrogen was derived. In March, maize yields decreased 34% in the row spaced 40 cm from the hedge, relative to the one spaced 110 cm away. In July, increasing the distance away from the hedge to 60 cm and coppicing the hedge earlier in maize growth, significantly improved grain yield. Grain yields decreased only 10% in the row closest to the trees.     

Alley cropping and analog forests for soil conservation in the dry uplands of Sri Lanka

The use of contour hedgerows of woody legumes to control soil erosion has been promoted amongst smallholders in the Upper Mahaweli catchment of Sri Lanka since 1988. The success of this program depends greatly on the properties of the different production systems occupying the alleys created by these hedgerows such as: profitability, time taken for profit to be realized, labour requirements, seasonal distribution of labour, environmental and economic stability, biophysical sustainability, the contribution of the system to the domestic needs of the farm family, adoptability of the system and successful management of the hedgerows. These properties were used to evaluate seven desktop farm models with different combinations of: seasonal crops, coffee-pepper intercrop, fodder-dairy, woodlot, and analog forest established in the alleys. The analog forest is a perennial polyculture which, like the traditional forest garden, is structurally and functionally analogous to the natural forest. It was found that a model with 40% seasonal crops, 20% perennial crops (coffee-pepper) and 40% analog forest made the best compromise between the properties under consideration. It is recognized that the socio-economic conditions that determine the appropriateness of this model in the Sri Lankan context are not universal and the extension of the analog forest concept into other tropical uplands is discussed.     

Calliandra calothyrsus (Meissn.) in an alley cropping system with sequentially cropped maize and cowpea in southwestern Nigeria

Alley cropping is increasingly becoming accepted as an appropriate technology with the potential to provide stable and sustainable food production in the tropics. However, only a few of the potential trees/shrubs have been tested. The performance of Calliandra calothyrsus (Meissn) as an alley cropping species was evaluated on an Oxic Paleustalf. The treatments were: prunings removal, prunings application; and three N levels, 0,45, and 90 kg N ha^–1, in a factorial arrangement. The cropping sequence was maize (main season) follwed by cowpea (minor season). Nitrogen fertilizer treatments were applied to the maize crop only. Four annual prunings of Calliandra hedgerows produced a total of 6 t ha^–1 of dry matter prunings containing about 200 kg ha^–1 of N. Maize yields were increased by the application of prunings but no benefits were obtained by supplementing the prunings with inorganic N. An average maize grain yield of 3.1 t ha^–1 per year was maintained without any chemical fertilizer input. However, without any prunings, maize yields were substantially increased by the application of inorganic N. Cowpea yield did not respond to application of prunings but plants grown adjacent to the hedgerows had reduced yield probably due to shading. Our results suggest that six rows (0.57 m inter-row spacing) between Calliandra hedgerows spaced at 4 m are optimum for this cowpea variety. The performance of Calliandra was comparable to that of Leucaena which has been widely shown to be effective in alley cropping systems of the region.     

肯尼亚西部印度田菁改良休耕地对玉米生产力和黄独脚金寄生杂草(Sesbania sesban)侵扰的作用

在撒哈拉以南的非洲地区,黄独脚金寄生杂草（Strigahermonthica）侵扰是限制小农产自给性农业生产的主要因素之一。土壤肥力低加之总体环境退化是寄生杂草侵扰产生的重要原因。引入改良的耕作制度来解决寄生杂草侵扰和土壤肥力下降的问题势在必行。本文对肯尼亚西部双峰高原地区内,用豆科植物--印度田菁改良的休耕地对玉米产量和农田寄生杂草侵扰的作用进行了研究。实验处理分阶段进行,处理包括田菁改良6和18个月的休耕地、未经耕作自然植物再生6和18个月的休耕地、连续种植玉米未施肥的耕地和连续种植玉米同时施加氮和磷肥的耕地。结果表明,与未施肥玉米地相比,田菁改良休耕地明显（p〈0.5）增加玉米产量.除草管理降低了第一季度（428000±63000株·hm-2）、第二季度（51000±1500株·hm-2）玉米地寄生杂草植株种群。实验周期内,除草管理降低玉米地土壤中寄生杂草种子种群数。短期田菁改良休耕地对玉米产量的促进作用明显好于未施肥的玉米地,但是短期杂草休耕地对玉米产量无显著影响。种植玉米和除草控制寄生杂草效果要好于休耕。     

Alley cropping rice (Oryza sativa var.Barafita) with cassia (Cassia siamea): soil fertility and crop production

Rice grain weight and quality (weight of larger size fractions), soil pH, and N, P, K and organic matter content were studied where rice was alley cropped with cassia on a semiarid site in The Gambia. The four treatments, control (no cassia prunings or inorganic fertilizer added), only prunings added, prunings plus half the recommended fertilizer rate and prunings plus full recommended rate of fertilizer, were applied in a Latin square design with 10×8 m plots, each sub-divided into two 4-m wide alleys. Fertilizer was applied twice; full rates were 93.7 kg/ha NPK (8:24:24) plus 32.4 kg/ha urea followed two weeks later by 100 kg/ha urea applied as side dressing. Soil samples collected before and after cropping at 0–10 cm and 10–15 cm depths and cassia pruning samples were analyzed for pH, N, P, K and organic matter content.Our results do not show significant benefit of cassia prunings applied as mulch to grain weight or quality in alley cropping rice with cassia. The addition of inorganic fertilizer plus cassia prunings did not increase rice grain and straw weights (p=0.3447 and p=0.0691, respectively) compared to the control and prunings only treatments. In all treatments, the outer rows, those within 80 cm of hedgerows, produced significantly less grain (p=0.0002) and straw than inner rows. Neither the larger grade A nor the smaller grade B grain weights were significantly different (p=0.6017 and p=0.0629, respectively) between treatments. Weight of grain, straw, and larger grade A and smaller grade B quality grain did not differ significantly for inner and outer rows (p=0.6329, p=0.7148, p=0.7171 and p=1.000, respectively).     

The role of agroforestry in the farming systems in Rwanda with special reference to the Bugesera-Gisaka-Migongo (BGM) region

The Rwandan farmers, faced with a perpetual land shortage, have evolved certain intensive systems of organic agriculture. These systems, particularly the homestead (compound) farming, involve the combination of food, fodder and tree crops. to a certain extent these systems can satisfy the multiple needs of the subsistence farmers living under several risks and constraints. However, they cannot cope with the expanding food demand of the rapidly increasing population. Some multipurpose, low-input technologies and agroforestry approaches have been designed to improve the productivity of these traditional systems; these include inter/mixed cropping systems and rotations, alley cropping with leguminous trees and shrubs, use of planted fallow, planting tree legumes on anti-erosive lines, mixed farming,community forestry and woodlots, and tree planting on farm/field boundaries. The essential aspects of these technologies are briefly discussed.ISAR-IITA FSR PROJECT, B.P. 629, Kigali RwandaISAR-Swiss Intercooperation, c/o Forestry Department, B.P. 617, Butare, Rwanda     

Alley cropping maize (Zea mays var.Jeka) with cassia (Cassia siamea) in The Gambia: crop production and soil fertility

The relative contributions of cassia prunings and inorganic fertilizer to yield of maize (number of ears and weight of grain, ears, cobs and stover), maize grain quality (weight of 3 largest size fractions) and soil pH, N, P, K and organic matter contents were investigated at a semi-arid site in The Gambia. Four treatments: control (no prunings or fertilizer applied), only prunings applied, prunings plus half the recommended rate of fertilizer, and the full recommended fertilizer rate plus prunings, were replicated in a Latin square design. At the full fertilizer rate, 125 kg/ha of NPK (82424) plus 43.5 kg/ha urea was broadcast at the time of sowing followed by 100 kg/ha of urea applied as side dressing two weeks after crop germination. Soil N, P, K organic matter and pH were not significantly different either before cropping or after crop harvest in both 0–10 and 10–15 cm soil depth.Crop yield, as measured by number and dry weight of ears, stover, grain and cob weights, was significantly different among treatments (p=0.0001, p=0.0001, p=0.0001, p=0.0001 and p=0.0001, respectively). The application of prunings plus full recommended fertilizer produced the highest yields. The weights of the three largest grain size fractions, grades A, B, and C, differed significantly between treatments (p=0.0026, p=0.0001 and p=0.0001, respectively). Yield of grade A grain increased by 202% over control with application of prunings plus full rate fertilizer and declined by 31% relative to the control with application of only prunings. More grade A, B and C grain was produced with application of full rate fertilizer plus prunings than in control treatments. However, application of half rate fertilizer plus prunings produced a higher proportion of grade A grain.     

Effect of pruning intensities of three woody leguminous species grown in alley cropping with maize and cowpea on an alfisol

Field trials were carried out on an Oxic Paleustalf in the humid zone of southwestern Nigeria withLeucaena leucocephala (Lam.) de Wit,Gliricidia sepium (Jacq.) Steud. andSesbania grandiflora (L.) Pers. alley cropped with maize and cowpea. The three leguminous woody species were grown in hedgerows spaced at 2 m. Trials were carried out one year after establishment of the hedgerows using a split-plot design with four replications. TheLeucaena trial had twenty pruning combinations consisting of five pruning heights (25, 50, 75, 100 and 150 cm) and four pruning frequencies (monthly, bi-, tri- and six-monthly). TheGliricidia andSesbania hedgerows were subjected to nine pruning intensities consisting of three pruning heights (25, 50 and 100 cm) and three pruning intensities (monthly, tri- and six-monthly). For the three woody species, biomass, dry wood and nitrogen yield from the hedgerow prunings increased with decreasing pruning frequency and increasing pruning height. Biomass, dry wood and nitrogen yields were in the following orderLeucaena >Gliricidia >Sesbania. The various pruning intensities had no effect on survival ofLeucaena plants. Pruning frequency had a larger effect than pruning height on survival ofGliricidia andSesbania plants. With monthly pruning, about 25 percent of theGliricidia and all of theSesbania plants died within six months of repeated pruning. Even with lower pruning frequencySesbania plants showed lower survival rates thanGliricidia orLeucaena. The various pruning intensities of all the hedgerow species had more pronounced effects on the grain yield of the alley cropped cowpea than on maize grain yield. Higher maize and cowpea yields were obtained with increasing pruning frequency and decreasing pruning height. IITA Journal paper number 335     

Effect of pruning intensities of three woody leguminous species grown in alley cropping with maize and cowpea on an alfisol

Field trials were carried out on an Oxic Paleustalf in the humid zone of southwestern Nigeria withLeucaena leucocephala (Lam.) de Wit,Gliricidia sepium (Jacq.) Steud. andSesbania grandiflora (L.) Pers. alley cropped with maize and cowpea. The three leguminous woody species were grown in hedgerows spaced at 2 m. Trials were carried out one year after establishment of the hedgerows using a split-plot design with four replications. TheLeucaena trial had twenty pruning combinations consisting of five pruning heights (25, 50, 75, 100 and 150 cm) and four pruning frequencies (monthly, bi-, tri- and six-monthly). TheGliricidia andSesbania hedgerows were subjected to nine pruning intensities consisting of three pruning heights (25, 50 and 100 cm) and three pruning intensities (monthly, tri- and six-monthly).For the three woody species, biomass, dry wood and nitrogen yield from the hedgerow prunings increased with decreasing pruning frequency and increasing pruning height. Biomass, dry wood and nitrogen yields were in the following orderLeucaena >Gliricidia >Sesbania.The various pruning intensities had no effect on survival ofLeucaena plants. Pruning frequency had a larger effect than pruning height on survival ofGliricidia andSesbania plants. With monthly pruning, about 25 percent of theGliricidia and all of theSesbania plants died within six months of repeated pruning. Even with lower pruning frequencySesbania plants showed lower survival rates thanGliricidia orLeucaena.The various pruning intensities of all the hedgerow species had more pronounced effects on the grain yield of the alley cropped cowpea than on maize grain yield. Higher maize and cowpea yields were obtained with increasing pruning frequency and decreasing pruning height.IITA Journal paper number 335     

Alley cropping in the moist savanna of West-Africa: II. Impact on soil productivity in a North-to-South transect in Togo

The moist savanna of West-Africa is characterized by a wide range of climates and soil types. The impact of the biophysical environment on hedgerow N uptake, wood production and maize grain yield was assessed for three years in three alley cropping trials with a selected number of hedgerow species in Glidji (Southern Togo), Amoutchou (Central Togo), and Sarakawa (Northern Togo). Senna siamea hedgerows accumulated significantly more N in the first pruning in Glidji (129−138 kg N ha−1) and Sarakawa (102−185 kg N ha−1) than in Amoutchou (17–26 kg N ha−1). This difference in N uptake was attributed to the infertile subsoil in Amoutchou, which was sandy up to 1 m and had a shallow groundwater-table. The amount of N accumulated in the Gliricidia sepium biomass varied between 38 kg N ha−1 in Glidji and 142 kg N ha−1 in Amoutchou. Averaged over all species and sites, 9 to 29% and 9 to 39% of the annual N accumulation in the hedgerow biomass is incorporated in the second, respectively third pruning. The Gliricidia trees produced between 12 and 26 ton fresh matter ha−1 of wood and the Senna trees between 4 and 38 ton fresh matter ha−1. Maize grain yield in Glidji was not affected by treatments (3196 kg ha−1, on average). In Amoutchou, the highest grain production was observed in the Gliricidia treatment (2774 kg ha−1 vs 1007 kg ha−1 in the control), while in Sarakawa, the Gliricidia (3786 kg ha−1) and Senna (3842 kg ha−1) plots produced a greater grain yield than the control plots (2123 kg ha−1). Maize yield increase in the alley cropping systems relative to the control plots was related to the soil total N content. Top and sub-soil characteristics were shown to be an important modifier of the functioning of alley cropping systems and should be taken into account when deciding on whether to use alley cropping and when selecting the hedgerow species. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Alley cropping of sorghum with Leucaena during the post-rainy season on Vertisols in semi-arid India

A field experiment was conducted for two years (1989– 1991) on a Vertisol in Bijapur, India in a split-plot design, replicated four times, to evaluate the potential of alley cropping post-rainy season sorghum between Leucaena hedgerows. Leucaena produced on average 2.74 t per ha of prunings and 1.57 t per ha of wood annually. Alley cropping decreased sorghum yields by 28 to 45% when all Leucaena prunings were removed from the system and by 21 to 24%, when on average 1.92 t per ha prunings were applied to the soil annually. The reduction of sorghum yield increased as higher rates of N were applied to sorghum. Although alley cropping increased organic carbon by 21% and available N by 19% at the time of crop sowing, it did not result in increased crop yields because of competition for water between hedgerows and crops. Calculation of land equivalent ratios based on total Leucaena biomass indicated that alley cropping was more productive than sole cropping of sorghum only in one year, and that, too, when no N was applied to sorghum. Therefore, alley cropping of Vertisols with post-rainy season sorghum is not likely to have any advantage in the short term. This revised version was published online in June 2006 with corrections to the Cover Date.     

Incidence of Mesoplatys ochroptera Stål (Coleoptera: Chrysomelidae) on Sesbania sesban in pure and mixed species fallows in eastern Zambia

The sesbania beetle, Mesoplatys ochroptera Stål (Chrysomelidae: Coleoptera), is a serious defoliator of Sesbania sesban in eastern and southern Africa. Developing integrated pest management practices against the beetle is an important aspect of the adoption of S. sesban as an improved fallow species. Field studies were conducted in eastern Zambia with the objective of determining the incidence of M. ochroptera on S. sesban in pure and mixed species fallows. M. ochroptera did not successfully feed or breed on species other than S. sesban either in the pure or mixed species fallows. The density of adult and immature stages of M. ochroptera was higher in mixtures of S. sesban with Mucuna pruriens, Macrotyloma axillare, Macroptilium atropurpureum or Crotalaria grahamiana compared to a pure S. sesban fallow. Although mixed fallows of S. sesban with M. axillare and M. pruriens may produce fodder, increase the amount and quality of organic inputs and improve nutrient cycling in the soil, they appear to be incompatible with management of M. ochroptera. Mixing S. sesban with G. sepium neither increased larval and adult populations of M. ochroptera feeding on S. sesban nor damage to it. While this mixture appeared to favour development of pupae to the adult stage, it increased mortality of emerging adults. Therefore, it is concluded that mixing S. sesban with G. sepium is more robust in nutrient cycling, improving resource utilisation and management of M. ochroptera on S. sesban.This revised version was published online in November 2005 with corrections to the Cover Date.     

Forage potential of eight woody species: intake and growth rates of local young goats in the highland region of Rwanda

Eight woody fodder species adapted to the highlands of Rwanda were evaluated in terms of dry matter intake in one experiment. Animals were offered a daily diet comprising 4.0 of fresh matter of Setaria splendida grass supplemented with or without one of the eight fodder species tested. In all cases, total daily feed intake was increased by the addition of woody fodder. Daily intake of the woody fodder was high for Acacia koaia, Mimosa scabrella and Acacia koa at 43.7, 42.6 and 41.9 g/kg BW^0.75, respectively. The dry matter intake of the other five species (Alnus acuminata, Chamaecytisus palmensis, Hagenia abyssinica, Acacia mearnsii and Acacia melanoxylon) ranged from 18.9 to 30.1 g/kg BW^0.75 per day. In another experiment, a basic daily diet of 4.0 kg of fodder (in fresh weight) was given to each animal. This ratio comprised S. splendida supplemented with M. scabrella and incorporated at 0 (control), 45% and 66% (fresh weight basis) of the total daily diet. Improved weight gain was obtained when setaria was supplemented with M. scabrella with daily weight gain of 31, 47 and 51 g/animal for 0, 45 and 66% M. scabrella, respectively.     

Effect of spacing on growth and biomass production of Gliricidia sepium (jacq) walp in an alley cropping system in Sierra Leone

This paper describes a study into the influences of spacing on the early performance and biomass production of Gliricidia sepium in an alley cropping system in southern Sierra Leone. Four between-row spacings of 2, 4, 6 and 8 m were combined with three within-row spacings (0.25, 0.50 and 1.00m) in a split plot experimental design.Survival, tree height and leaf nitrogen content were not affected by between- or within-row spacings. For the other parameters measured, namely root-collar diameter, branch production, total biomass and nitrogen yields per hectare, it was found that for equivalent tree densities, the lower the rectangularity of planting, the better the performance of the individual trees, and consequently the greater the yields per hectare.Total biomass production per unit area was, expectedly, greatest where the spacings between hedgerows were closest, while production per plant decreased with closer within-row spacings. The total fresh and dry weights of leaves and stems, as well as leaf nitrogen yields per unit area were strongly influenced by between-row spacing and less so by within-row spacing.     

The effects of alley cropping withLeucaena leucocephala and of different management practices on the productivity of maize and soil chemical properties in lowland coastal Kenya

The effects of leucaena hedgerows, mulching with leucaena foliage (0,50 and 100% of harvested foliage), cowpea intercropping and adition of dairy cattle slurry (55 t ha^–1 per maize crop) on the yield of maize from a sandy soil were assessed. The four-year results from five maize crops are reported.Except in the first year, yields of maize grain and stover were significantly reduced by 30% in the presence of leucaena hedgerows. Use of leucaena mulch eliminated this effect; application of all the harvested leucaena mulch (100%) increased the total maize grain yield of the five crops by 44% over sole maize. Hedgerow and mulching management required an additional 36 mandays labour ha^–1 which was more than compensated by the increased maize yields. Furthermore leucaena hedgerows substantially depressed the growth of weeds between cropping seasons.Intercropping with cowpea significantly depressed yields of maize grain and stover when both crops were sown together, but not in later seasons when cowpea was sown four weeks after the maize. Application of slurry increased the total yields of maize grain and stover by 35 and 37%, respectively. The grain yield of maize in leucaena hedgerow treatments fertilized with slurry did not respond to application of more than 50% of leucaena foliage, which suggested that half of the foliage could be spared for feeding to livestock. The cumulative yield of maize grain from the highest yielding organic system was 85% of the yield from the fertilizer treatment.The study, which is continuing, demonstrates that large increases in agricultural productivity are possible through the intercropping of maize with woody forage and grain legumes and the integration of dairy cattle production into the system. It thus shows the importance of exploiting crop/livestock interactions.     

Effect on maize growth of the interaction between increased nitrogen availability and competition with trees in alley cropping

Maize growing next toErythrina hedgerows had 44% lower biomass (p<0.01) and 35% lower N content (p<0.1) than maize growing in the middle of the alleys. Maize growing next toGliricidia hedgerows had the same biomass but 56% higher N content (p<0.1) than maize growing in the middle of the alleys. However these differences did not develop until 2 months after sowing of the maize.Spatial variability in soil nitrogen mineralization and mulch nitrogen release did not explain any of the differences in growth or N uptake of the maize with respect to distance from the trees. It is hypothesized that the slower growth of the maize next to theErythrina trees after 2 months is due to increasing light and/or nutrient competition from the trees as the trees recover from pollarding. The apparent lack of competition fromGlirigidia may be due to different rates of regrowth or different shoot and root architecture.A theoretical model is described demonstrating that if a crop is to take advantage of the higher nutrient availability under alley cropping it must complete the major part of its growth before the trees recover significantly from pollarding, and start competing strongly with the crop.     

Comparative performance of seventeen upperstorey tree species associated with crops in the highlands of Uganda

Trials were established at three sites in Uganda to test the suitability of multipurpose trees (MPTs) as upperstorey in crop lands to provide poles, small timber and fuelwood. The three sites were Kachwekano District Farm Institute (1°16 S, 29°57 E, 2000 m.a.s.l.) in Kabale District, Kabanyolo University Farm (0°28 N, 32°27 E, 1250 m.a.s.l.) in Mpigi District and Bushenyi District Farm Institute (0°34 S, 30°13 E, 1610 m.a.s.l.) in Bushenyi District. The MPTs were planted in single rows at intra spacing of 2 m and each plot contained seven or nine trees. On both sides of the tree row, crops were raised. Data on crop yields were collected every season, while data on the growth of the trees were collected four times each year.In terms of tree growth,Grevillea robusta, Casuarina cunninghamiana andAlnus acuminata performed well with height growth of 1.8–2.4 m per year at Kachwekano, while at BushenyiGrevillea robusta, Casuarina junghuhniana, Cupressus lusitanica andCedrela serrulata averaged 1.6–2.0 m height per year. At Kabanyolo,Melia azedarach, Cassia siamea, Jacaranda mimosifolia, Grevillea robusta andMaesopsis eminii had height increments ranging from 1.8–2.7 m annually. Crop yields were affected by the presence of the MPTs, withMaesopsis eminii being the most competitive (averaging 60% reduction, over five seasons). The crop rows nearest to the tree line were the most affected. OnlyAlnus acuminata seems to have had a positive effect on crop yields. The installation of a root mesh to reduce tree root competition for nutrients and water in four species increased yields in plots with MPTs by 5% (Melia azedarach) to 152% (Maesopsis eminii), but the control plot still had significantly higher bean yields, suggesting that shading could also be important. In the case of maize, suppression seems to be due mainly to root competition because after its elimination yields obtained thereafter did not differ significantly from those of the control except for the Maesopsis plots. The negative influence of the MPTs could, therefore, be minimized by periodically pruning the tree crowns and roots.     

Response of <Emphasis Type="Italic">Sesbania sesban</Emphasis> (L.) Merr. to rhizobial inoculation in an N-deficient soil containing low numbers of effective indigenous rhizobia

Sesbania sesban (L.) Merr. (sesbania) is a fast growing N₂-fixing widely used as an improved fallow species by smallholder farmers in eastern and southern Africa to restore fertility of their N-deficient soils. In order to establish the need for inoculation, the population of sesbania rhizobia in soil collected from a site where the species is intended for introduction was assessed using the most probable number (MPN) plant infection assay. The MPN of sesbania rhizobia was low (21, 6–81 fiducial limits at P=0.05 g⁻¹soil) but with N₂-fixation potential comparable to sesbania inoculant strain KFR 651. Evaluation of an indigenous sesbania rhizobial isolate GSS 1 from the MPN assay in potted field soil showed that it was more effective than strain KFR 651 in terms of plant growth and shoot dry matter (DM) accumulation at 9 and 12 weeks after planting, respectively. Total shoot N content was also higher for plants inoculated with isolate GSS 1 than inoculant strain KFR 651 and uninoculated control treatments 12 weeks after planting. These results demonstrate that it is better to inoculate with effective indigenous than exogenous rhizobia where the need for inoculation has been established.