Agronomic evaluation of Leucaena. Part 2. Productivity of the genus for forage production in subtropical Australia and humid-tropical Philippines

Leucaena leucocephala is an important agroforestry species pan-tropically, but relatively little is known of the forage production potential of other species in the genus. The agronomic potential of 116 accessions, represent ing the 28 species and subspecies of the Leucaena genus and several artificial hybrid accessions, was evaluated at Los Baños, Philippines and Brisbane, Australia over a 2.5–year period. Accessions were planted into replicated line plots, with 10 trees/plot spaced 50 cm apart, and with rows spaced 3 m apart. The L. pallida × L. leuco cephala KX2 F ₁ hybrid accessions were highest yielding at both sites, producing dry matter (DM) yields of over 900 g/m row/month at Los Baños and approximately 320 g/m row/month at Brisbane. In the near-optimal growth conditions at Los Baños, L. leucocephala accessions were highly productive, with the best accessions producing total yields of over 500 g/m row/month. The superiority of KX2 hybrids was most pronounced at Brisbane, where high psyllid pressure during summer, and low temperatures during winter severely constrained growth of L. leu cocephala accessions. In the Brisbane environment, psyllid resistant accessions of L. pallida , L. trichandra and L. diversifolia were more productive than L. leucocephala accessions. Leucaena greggii , L. retusa , L. cuspidata , L. confertiflora , L. pulverulenta , L. pueblana and L. involucrata were of inherently low productivity in both the Brisbane and Los Baños environments. Mortality over the experimental period was very low for most species, particularly for L. leucocephala and KX2 accessions. The KX2 F1 hybrid accessions have considerable agro nomic potential as alternatives to L. leucocephala for use in tropical agroforestry.     

Simulation of carbon pool changes in woodlots in eastern Zambia using the CO2FIX model

Agroforestry systems have the potential to contribute significantly to climate change mitigation and adaptation. However, data on tree and soil organic carbon (SOC) pools for most agroforestry systems are lacking because reliable methods for estimating ecosystem carbon (C) pools are scarce. This study quantified the effects of five Leucaena species (L. leucocephala, L. macrophylla, L. diversifolia, L. collinsii and L. pulverulenta) on vegetal and soil C stocks and on mean annual increment (MAI) in aboveground tree C stocks. Specifically, it tested the validity of the CO2FIX model using empirical data from 7?year-old woodlots at Msekera, Zambia, and assessed the impact of converting a degraded agricultural ecosystem to woodlots on C stocks. Measured above- and below-ground tree C stocks and MAI of aboveground biomass differed significantly among the Leucaena species. Measured stem and total aboveground tree C stocks in seven-year old woodlots ranged from 17.1 to 29.2 and from 24.5 to 55.9?Mg?ha^?1, respectively. Measured SOC stocks at 0?C200?cm depth in Leucaena stands ranged from 106.9 (L. diversifolia) to 186.0?Mg?ha^?1 (L. leucocephala). Modeled stem and branch C stocks closely matched measured stocks, but the soil module of CO2FIX did not predict the soil C. The soil C data are inconclusive at this stage. We recommend that a fractionation and a soil aggregate hierarchy study backed by C dating is carried out to explain soil C dynamics in these soils. However, the model can be used only for estimating changes in aboveground tree C stocks in woodlots until soil C module is proven to predict SOC stocks.     

A tree/crop interface design and its use for evaluating the potential of hedgerow intercropping

The paper describes a tree/crop interface (TCI) experiment designed to investigate the effects of row orientation using Leucaena leucocephala Lam. Each TCI plot consisted of a regularly pruned Leucaena hedge in the middle and 12 crop rows on either side. Eight such plots were arranged at 45° around a sole Leucaena plot with rows oriented in four compass directions viz., North-South, East-West, Northeast-Southwest and Northwest-Southeast.Results of four years from 1984 to 1987 did not show any effect of row orientation, and similarly, no effect was seen on crop rows due to their location on the windward or leeward side of the hedge. The TCI effect was positive on the first crop row in the first year because Leucaena grew slowly, but depressed the yield of the first 4 to 6 crop rows(1.8 to 2.7 m from hedge) in subsequent years. The negative effect of Leucaena was noted more on sunflower in a relatively dry year than on sorghum in other years.Results from the TCI plots were used to estimate the yield of five hedgerow intercropping (HI) systems with varying alley widths (2.7 to 9.9 m). Comparison with sole stands of Leucaena and crops indicated that HI was more productive particularly at close alley widths. For example, hedges spaced at 2.7 m and 3.6 m averaged 37% and 25% higher productivity than the respective sole stands; but this advantage may be an overestimation of the real potential.The relevance of TCI experiments for studying agroforestry systems, their merits and limitations, especially of the design employed in this study are discussed.     

Intercropping field crops between rows of Leucaena leucocephala under rainfed conditions in northern India

A field study was conducted for six years (1981–1986) on sandy loam soil on intercropping hedgerows of Leucaena leucocephala (Lam) de Wit. with three field crops viz. maize (Zea mays L), black gram (Vigna mungo L) and cluster bean (Cyamopsis tetragonoloba L Taub.). In treatments 1 and 2 Leucaena hedges were planted as pure crops at close (25 cm × 75 cm) and wide (25 cm × 375 cm) spacings. In treatments 3, 4 and 5 the three field crops were intercropped between the hedgerows of Leucaena at the wide spacing, and in treatments 6, 7 and 8 the field crops were raised as pure crops. Leucaena was topped to 75 cm each time it attained a height of 175 cm. The pure crop of Leucaena at close spacing produced an average, over the six years, of 34 t ha⁻¹a⁻¹ of green fodder and 9.4 t ha⁻¹a⁻¹ of air dry fuelwood. The Leucaena at wide spacing produced 18.9 t ha⁻¹a⁻¹ of green fodder and 6.3 t ha⁻¹a⁻¹ of fuelwood. Intercropping with field crops decreased the yield of green fodder and fuelwood. The yield of all the field crops was less when raised as intercrops than as pure crops. Mean maximum net returns were obtained from intercrops of Leucaena and cluster bean (Rs 3540 ha⁻¹a⁻¹) which were significantly higher than the returns from pure crop of Leucaena at wide spacing but similar to the returns from pure crops of cluster bean. Leucaena with maize (Rs 3273 ha⁻¹a⁻¹) and black gram (Rs 3125 ha⁻¹a⁻¹) gave significantly higher net returns over pure crops of Leucaena at wide spacing, maize and black gram. ICRISAT = International Crops Research Institute for the Semi-Arid Tropics- Hyderabad, India. CIAT = Centro International de Agricultura Tropical - Cali - Columbia     

Agroforestry systems and soil surface management of a tropical alfisol: I: Soil moisture and crop yields

Field experiments were conducted on a tropical Alfisol at Ibadan, Nigeria, to evaluate the effects on soil moisture and crop yields of three agroforestry systems. Effects of agroforestry treatments involving two perennial shrubs (Leucaena leucocephala and Gliricidia sepium), each at 2-m and 4-m row spacings, were compared with no-till and plow-till systems of seedbed preparation. Measurements were made for soil properties, runoff and erosion, nutrient losses in runoff, and crop growth and yield for a uniform maize (Zea mays) and cowpea (Vigna unguiculata) rotation. All of the six plots, each measuring 70 × 10m, were established on a natural slope of about 7%. Alterations in soil properties and effects on crop growth were evaluated for six consecutive years from 1982 through 1987.Seed germination and seedling establishment of Leucaena hedgerows were satisfactory while establishment of Gliricidia from stem cuttings was unsatisfactory. Maize germination and crop stand were normal while that of cowpea were suppressed by both Leucaena and Gliricidia. Maize growth and yield were suppressed only in the vicinity of hedgerows. Maize grain yield in agroforestry systems averaged about 10 percent lower than that of the control. In contrast with maize, agroforestry systems drastically suppressed cowpea grain yield. The average cowpea yield in agroforestry systems was 30 to 50% of the control. Regardless of the mangement system, grain yields declined over time at the rate of 340 and 96 kg ha^–1yr^–1 for maize and cowpea, respectively.Hedgerows of Leucaena and Gliricidia acted as windbreaks. Consequently, soil moisture content in the top 0–5 cm layer in agroforestry systems was generally higher than that in the control during both wet and dry seasons.     

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.     

Agroforestry systems and soil surface management of a tropical alfisol:

Field runoff plots, 70 x 10 m each, were established on a tropical Alifisol in southwestern Nigeria to monitor water runoff, soil erosion and nutrient loss in water runoff. The non-agroforestry control treatment (A) was established at two levels: plow-till and no-till systems of seedbed preparation. There were two agroforestry systems based on contour hedgerows of (B) Leucaena leucocephala and (C) Gliricidia sepium established at 4-m and 2-m spacings. Field plots were established in 1982 and hydrological measurements were made for uniform maize-cowpea rotation for 12 consecutive growing seasons from 1982 through 1987. Once established hedgerows of Leucaena at 2-m spacing were extremely effective in reducing water runoff and controlling erosion. Runoff, erosion and nutrient losses were generally more from maize grown in the first season than from cowpea grown in the second. Mean seasonal erosion from maize was 4.3, 0.10, 0.57, 0.10, 0.64 and 0.60 t/ha for plow-till, no-till, Leucaena-4m, Leucaena-2m, Gliricidia-2m treatments, respectively. Mean runoff in the first season from treatments listed in the order above was 17.0, 1.3, 4.9, 3.3, 4.3, and 2.4 percent of the rainfall received. There were high losses of Ca and K in water runoff from the plow-till treatment. In contrast to runoff and erosion, losses of bases in water runoff from agroforestry treatments were relatively high, high concentration of bases in runoff was probably due to nutrient recycling by the deep-rooted perennials.     

Forage dry matter yields and psyllid resistance of thirty-one leucaena selections in Hawaii

Forage yields ofL. leucocephala (Lam). de Wit have been reduced as the result of psyllid damage sinceHeteropsylla cubana Crawford invaded the Hawaiian Islands in 1984. The forage productivity and psyllid resistance of 31Leucaena species and interspecific hybrids were assessed from 5 harvests in Hawaii during 1991 and 1992. The trial consisted of an augmented randomized complete block with 22Leucaena selections in all 4 replicates, 2 selections in 3 replicates and 7 selections in 1 or 2 replicates.Forage (leaves and stems < 6 mm diam.) dry matter (DM) biomass yield over a 13-month period ranged from 1.4 to 34 Mg ha^–1 from total DM ranging from 1.9 to 63.7 Mg ha^–1. Percent forage fractions ranged from 49 to 78% (forage DM/total DM). The 10 selections in this trial of eitherL. pallida Britton & Rose, and its hybrids withL. leucocephala consistently produced both the highest forage and total DM yields averaging 22 and 40 Mg ha^–1, respectively. This represented a three-fold increase in forage production when compared toL. leucocephala K636 (a standard around the world).The excellent performance of theL. pallida lines was attributed to high psyllid resistance and seedling vigor. AllL. pallida selections with the exception of K953 exhibited good psyllid resistance.Leucaena diversifolia Benth. K749,L. pallida K376, andL. esculenta (Moc. & Sesse) Benth. K950 had the highest psyllid resistance (p<0.05). Psyllid damage was negatively correlated to forage DM yield at both harvest 2 and 5 (r=–0.55,p<0.01,n=94). Forage DM was positively correlated to seedling vigor for the first harvest (r=0.83,p<0.001,n=74) and combined harvests (r=0.88,p<0.001,n=74).     

Alley cropping on an Ultisol in subhumid Benin. Part 1: Long-term effect on maize, cassava and tree productivity

In southern Benin, West Africa, two alley cropping systems were studied from 1986 to 1992. Yield development was followed in a maize and cassava crop rotation vs. intercropping system, with alleys of Leucaena leucocephala (Lam.) de Wit and Cajanus cajan (L.) Millsp. vs. a no-tree control, with and without NPK fertiliser. Without alleys, NPK fertilisation maintained high yield levels of 2–3 t maize dry grain plus 4–6 t ha^–1 cassava root DM in intercropping, 3–4 t ha^–1 maize and 6–10 t ha^–1 cassava in solercropping. Without NPK, final yields seemed to stabilise at about 1 t maize plus 2 t cassava in intercropping and twice as much in each solecrop. Alley cropping induced significant yield increases by about 50% with both tree species in unfertilised, intercropped maize, and with Cajanus in fertilised, solecropped cassava. In monetary terms, the NPK-fertiliser response of stabilised yields was significant for all treatments except the solecropped Leucaena alleys. It is concluded that on Ultisols with low nutrient status in the upper rooting zone, alley cropping with low-competitive tree species may improve food crop yields but the greatest monetary output is achieved by intercropping with mineral fertiliser independent of the presence or absence of an agroforestry component.     

Psyllid resistance in Leucaena. Part 1. Genetic resistance in subtropical Australia and humid-tropical Philippines

One hundred and sixteen accessions of Leucaena were assessed for resistance to the leucaena psyllid that has devastated stands of L. leucocephala pan-tropically since the mid-1980s. Damage to young leaf was assessed using a ratings scale, developed at the University of Hawaii, in replicated field trials at Brisbane, Australia and Los Baños, Philippines. The study identified a continuum of resistance among the accessions, from highly resistant to highly susceptible. Resistance levels were consistent between the two sites with few exceptions. L. collinsii subsp. collinsii, L. confertiflora, L. esculenta, L. pueblana, L. retusa, L. greggii and L. matudae were highly resistant at both sites, while L. leucocephala, L. lempirana, L. involucrata and L. multicapitula were highly susceptible at both sites. Other species were intermediate and were allocated to either resistant or susceptible categories. There was considerable variation in psyllid resistance within some species especially, L. trichandra, L. diversifolia and L. collinsii. Regression analysis was used to determine the influence of environmental variables on psyllid populations. Psyllid population outbreaks were restricted to a temperature range of 10?°C minimum temperature and 33?°C maximum temperature, but additional, unidentified factors influenced populations within this range. Leaf chemical characteristics were examined for their potential to confer psyllid resistance, but none were strongly correlated with accession damage ratings.     

Agronomic evaluation of Leucaena. Part 1. Adaptation to environmental challenges in multi-environment trials

A multi-environment trial was conducted to evaluate the performance of high yielding accessions from most taxa of Leucaena, for broad and specific adaptation to growth-limiting environments, and to develop an understanding of environmental parameters limiting growth of this important multipurpose tropical tree-legume genus. Eighteen sites were selected to include environments known to constrain the growth of L. leucocephala. Accessions were planted in replicated line plots, with 10 trees/plot, spaced 50 cm apart, and with rows spaced 3 m apart. Environmental parameters measured were maximum and minimum temperatures, rainfall and psyllid pressure. Soil fertility and acidity indices were derived from soil analyses. A KX2 F1 hybrid accession proved broadly adapted with high yield rankings in all environments, and with yield advantage greatest at low temperature and high psyllid pressure sites. Specific adaptation to low temperature environments was identified in L. trichandra OFI53/88, and to a lesser extent in L. pallida CQ3439. No accessions were specifically adapted to acid-infertile soils. Highest dry matter production was achieved in fertile, hot, humid, low psyllid environments. Very low growth rates were achieved in acid-infertile and low temperature environments. Yield was strongly reduced by mean monthly minimum temperatures of 22 °C, mean monthly maximum temperatures of 29 °C, psyllid pressure of >2.5 (1–9 scale of increasing pressure based on damage to young leaves), acidity index of 6 (1–7 scale of decreasing acidity based on soil and Al saturation) and fertility index of 9 (1–12 scale of increasing fertility based on critical soil nutrient levels). This revised version was published online in June 2006 with corrections to the Cover Date.     

Incidence and Severity of Bean Rust (Uromyces appendiculatus) in Alleys between Leucaena Hedgerows in Kenya

The effects of inter-row spacing of Leucaena leucocephala (Lam.) de Wit in an alley cropping system on the incidence and severity of rust (Uromyces appendiculatus (Pers.) Unger) on intercropped beans (Phaseolus vulgaris L.) and their yield were examined over 2 years (1993 and 1994) at Chepkoilel Campus in Kenya. Each experiment consisted of three randomized blocks with treatments of three alley widths (2 m, 4 m and 8 m) and a treeless control with two intra-row spacings of Leucaena (0.5 m and 1.0 m). Hedgerows were coppiced at 1.0 m height and pruned subsequently at 2–3 months intervals. No fertilizer was applied but Leucaena loppings were incorporated as green leaf manure. Rust on beans was assessed at three growth stages in each season, using the Centro International de Agricultura Tropical (CIAT) scales. Microclimate was monitored in treatments, in 1994. Bean growth and yield were also measured. Rust increased with increasing alley width and was less severe in bean rows adjacent to hedgerows. Bean yield was highest in the treeless control plots and declined with decreasing alley width. Beans in 2 m alleys had significantly lower (p < 0.05) yields than 8 m alleys and treeless control plots. Bean growth was greatly modified in 2 m alleys and close to hedgerows. Light availability and diurnal temperature increased with alley width but relative humidity and leaf wetness duration decreased. Proximity to hedgerows also had marked effect on microclimate. The changes in yield, rust incidence and severity were examined in relation to microclimate, inoculum survival and dispersal.     

Psyllid resistance in Leucaena. Part 2. Quantification of production lossesfrom psyllid damage

The psyllid insect (Heteropsylla cubana) is known to reduce biomass production in Leucaena species, but little information is available on the level of reduction, or whether a commonly used plant damage rating scale can be related to production losses. Biomass production losses due to predation by the psyllid were determined for 12accessions of Leucaena in a randomised split-plot field experiment conducted at Brisbane, Australia. Regrowthfrom well-established Leucaena trees, cut back to bare stems 50 cm high, was measured over a 9–week period from plants subjected to psyllid damage and from plants with psyllids controlled by spraying with chlorpyrifos (1 mg ai/l water). Psyllid damage was scored using a ratings scale developed by the University of Hawaii. Lossof potential DM production due to psyllids ranged from 10% for L. trichandra OFI53/88 to 76% for L. collinsii ssp. zacapana OFI56/88. Production losses were significantly correlated with psyllid damage ratings for 10 of the 12 accessions, and also for a combined data set comprising these 10 accessions. From correlations for the combined data set, a 50% loss of potential DM production occurred at a psyllid damage rating of approximately3.2. Results indicated that a “0” rating should be added to the scale to indicate the absence of psyllids. This revised version was published online in June 2006 with corrections to the Cover Date.     

Disease incidence and severity on beans in alleys between Leucaena hedgerows in Kenya

The effects of inter-row spacing of Leucaena leucocephala in an alley cropping system on the incidence and severity of diseases on intercropped beans (Phaseolus vulgaris L.) and their yield were examined over two years (1993 and 1994) at Chepkoilel Campus in Kenya. Each experiment consisted of three randomized blocks with treatments of three alley widths (2 m,4 m and 8 m) and a treeless control with two intra-row spacings of Leucaena (0.5 m and 1.0 m). Hedgerows were coppiced at 1.0 m height and pruned subsequently at two-to-three months intervals. No fertilizer was applied but Leucaena loppings were incorporated as green leaf manure. Incidence of angular leaf spot (Phaeoisariopsis griseola) and anthracnose (Colletotrichum lindemuthianum) on beans were assessed at three growth stages in each season, using the Centro International de Agricultura Tropical (CIAT) scales. Microclimate was monitored in treatments, in 1994. Angular leaf spot and anthracnose decreased with increasing alley width and were more severe in bean rows adjacent to hedgerows. Light availability and diurnal temperaturereaching the bean canopy increased with alley width but relative humidity and leaf wetness duration decreased. Proximity to hedgerows also had markedly reduced light levels, lower temperatures and higher relative humidity. Higher incidence and severity of angular leaf spot and anthracnose on beans in alleys than on beans in treeless plots were examined in relation to microclimate, inoculum survival and dispersal. The pattern of the diseases was best explained by microclimate changes induced by Leucaena hedgerows, especially effects of humidity. This revised version was published online in June 2006 with corrections to the Cover Date.     

The potential of alley cropping im improvement of cultivation systems in the high rainfall areas of Zambia II. Maize production

Theee trials to evaluat the potential of alley cropping in maize production on the low fertility, acidic soils in Northern Zambia are described. Leucaena leucocephala, Gliricidia sepium, Sesbania sesban, Albizia falcataria, Fleminga congesta, and Cassia spectabilis, were grown in alley crops with hybrid maize and soybean. All trials received recommended rates of P and K fertiliser; N fertiliser was applied at three rates as a subplot treatment. One trial received lime before establishment.Only in the limed trial was there a significant improvement in maize yields through alley cropping; when no N fertiliser was applied, incorporation of Leucaena leucocephala prunings resulted in an increase of up to 95% in yields, with a smaller improvement being produced by Flemingia congesta. There was a significant correlation between the quantity of prunings biomass applied and the proportional increase in maize yields over the control treatment. It is suggested that the lack of effect of most of the tree species on crop yields was due to low biomass production.An economic analysis showed that alley cropping with limed Leucaena was only profitable when fertiliser costs were high in relation to maize prices. However, lime is both expensive and difficult to obtain and transport for most small scale farmers in the region, and is therefore not a practical recommendation. It is suggested that future alley cropping research should focus on screening a wider range of tree species, including other species of Leucaena, for acid tolerance and higher biomass production.     

Productivity of annual cropping and agroforestry systems on a shallow Alfisol in semi-arid India

An experiment was conducted at ICRISAT Center, Patancheru, India from June 1984 to April 1988 on a shallow Alfisol to determine whether the productivity of annual crop systems can be improved by adding perennial species such as Leucaena leucocephala managed as hedgerows. Except in the first year, crop yields were suppressed by Leucaena due to competition for moisture. The severity of competition was high in years of low rainfall and on long-duration crops such as castor and pigeonpea. Based on total biomass, sole Leucaena was most productive; even on the basis of land productivity requiring both Leucaena fodder and annual crops, alley cropping had little or no advantage over block planting of both components. Application of hedge prunings as green manure or mulch on top of 60 kg N and 30 kg P ₂ O ₅ha⁻¹ to annual crops did not show any benefit during the experimental period, characterized by below average rainfall. Indications are that (i) alley cropping was beneficial in terms of soil and water conservation with less runoff and soil loss with 3 m alleys than with 5.4 m alleys, and (ii) root pruning or deep ploughing might be effective in reducing moisture competition.     

Alchornea cordifolia,a promising indigenous browse species adapted to acid soils in southeastern Nigeria for integrated crop-livestock agroforestry production systems

Dry matter (DM) production, crude protein, phosphorus fibre contents and goat preference for eight indigenous browse species,Alchornea cordifolia, Diallum guineense, Ficus capensis, Baphia nitida, Manniophytum fulvum, Homalium aylmeri, Glyphaea brevis andRauwolfia vomitoria, and for two exotics,Leucaena leucocephala andGliricidia sepium, in cultivated plots were compared on acid soil in southeastern Nigeria. Total DM production was higher (P<0.05) forAlchornea cordifolia than for the other browse species.Glyphaea brevis andL. leucocephala were the most preferred species, whileA. cordifolia, G. sepium andR. vomitoria were the least. Mean crude protein content of browse species in this study was higher, whileP and neutral detergent fibre were lower than reported for other browse species in Nigeria. It is argued that the ultimate goal of a crop-livestock agroforestry system such as alley farming, could be better achieved through the complementary use of browse species.     

Productivity and nutrient cycling in young Acacia senegal farming systems on Vertisol in the Blue Nile region, Sudan

Acacia senegal, the gum arabic-producing tree, is the most important component of traditional dryland agroforestry systems in the␣Sudan. The spatial arrangement of trees and the type of agricultural crop used influence the interaction between trees and crops. Tree and crop growth, gum and crop yields and nutrient cycling were investigated over a period of 4 years. Trees were grown at 5 × 5 m and 10 × 10 m spacing alone or in mixtures with sorghum or sesame. No statistically significant differences in sorghum or sesame yields between the intercropping and control treatments were observed (mean values were 1.54 and 1.54 t ha⁻¹ for sorghum grain and 0.36 and 0.42 t ha⁻¹ for sesame seed in the mixed and mono-crop plots, respectively). At an early stage of agroforestry system management, A. senegal had no detrimental effect on crop yield; however, the pattern of resource capture by trees and crops may change as the system matures. A significant positive relationship existed between the second gum picking and the total gum yield. The second gum picking seems to be a decisive factor in gum production and could be used as an indicator for the prediction of the total gum yield. Soil organic carbon, N, P and K contents were not increased by agroforestry as compared to the initial levels. Soil OC was not increased by agroforestry as compared to sole cropping. There was no evidence that P increased in the topsoil as the agroforestry plantations aged. At a stocking density of 400 trees ha⁻¹ (5 × 5 m spacing), A. senegal accumulated in its biomass a total of 18.0, 1.21, 7.8 and 972 kg ha⁻¹ of N, P, K and OC, respectively. Agroforestry contributed ca. 217 and 1500 kg ha⁻¹ of K and OC, respectively, to the top 25-cm of soil during the first four years of intercropping.     

Beneficial Effects of Intercropping on the Growth and Nitrogen Status of Young Wild Cherry and Hybrid Walnut Trees

Growing concern for economic and environmental issues emphasizes the potential value of intercropping systems in temperate regions. However, the selection of relevant tree species to be associated with crops has been little documented. The growth and the nitrogen nutrition of two economically valuable species, wild cherry (Prunus avium L.) and hybrid walnut (Juglans nigra L.×Juglans regia L.), were compared over six years after plantation. These two species were associated with non-irrigated cereal crops in the agroforestry treatment or grown separately (weeded control and fallow). Intercropping increased diameter growth as soon as year 2 in the two species. Leaf biomass assessment using allometric models showed an earlier and greater leaf biomass increase in hybrid walnut than in wild cherry tree. After six years, the relative growth increase of the agroforestry trees with respect to the control trees varied with the parameter considered (diameter at breast height from +26 to +65%, leaf biomass from +54 to +142%) and with the tree species (higher relative growth for hybrid walnut trees). The beneficial effect on tree growth can be accounted for in terms of enhanced nitrogen nutrition. The tree–crop association in intercropping systems, which improves tree growth, might thus allow the planting of more demanding trees on soils of lower fertility.     

Forage yield and compositional analysis of Leucaena species and hybrids adapted to cool sites

Four species of Leucaena (L. leucocephala, L. diversifolia 2n and 4n, andL. pallida) and three interspecific hybrids KX1 (L. diversifolia × L. pallida), KX2 (L. leucocephala × L. pallida), and KX3 (L. leucocephala × L. pallida) were evaluated for forage yield on a cool upland site at the Mealani Research Station on the island of Hawaii (900 m elevation). Two-month-old seedlings were planted at a density equivalent to 40,000 trees/ha and coppiced (harvested) every six months for a two-year period. Leaf material collected from the plots was oven-dried, finely ground, and evaluated for forage components and digestibility.Two Holstein steers fitted with cannulae in the rumen and proximal duodenum were used to conduct a replicated trial to examine rumen and post-rumen dry matter and crude protein digestibility of leaf material. Nylon bags containing leaf material from K636 (L. leucocephala), KX2 (L. leucocephala × L. pallida), or K376 (L. pallida) were suspended in the rumen for periods of 0, 4, 8, 12, 24, 36, and 48 hours. Weight loss was measured and nitrogen content of each sample was determined using a macro-Kjeldahl procedure. Subsequently, rumen-incubated material was placed in nylon bags, inserted into the duodenal cannulae of the two steers and collected in the feces for periods of 24 h after insertion.Results indicate that varieties and hybrids of Leucaena are adapted to cool sites and produce good yields of high quality forage. No significant difference was found in either ruminal or intestinal dry matter or crude protein disappearance between K636 and KX2 whereas K376 was significantly lower for both values. Nearly 40% of dietary crude protein in the K636 and KX2 leaf material escaped ruminal digestion whereas nearly 70% was non-digested for K376. For K636 and KX2, intestinal availability of that protein which escaped ruminal digestion was nearly 50% whereas only 20% of ruminal escape protein of K376 was available in the intestines. Because KX2 produced higher forage yields, plantings on cool sites would favor the use of KX2 over K636 if animal acceptability and rumen retention of dry matter are not problems.