Growth and development of Leucaena leucocephala in mixed-silviculture with gmelina intercropped with maize and sorghum in the Nigerian Southern Guinea savanna zone
Department of Crop Production, University of Ilorin, P. M. B. 1515, Ilorin, Nigeria
(Received 10 June, 1995; accepted 14 February, 1996)
Code Number: CS96069 Sizes of Files: Text: 21.5K Graphics: Line drawings (gif) - 22K Tables (jpg) - 424K
The performance of Leuceaena leucocephala in mixed-silviculture with gmelina (Gmelina arborea) and intercropped with maize and sorghum was assessed on a plinthustalf in the Nigerian Southern Guinea Savanna Zone over four cropping seasons. The percentage survival of leucaena ranged from 53.3% when simultaneously cropped with gmelina and sorghum to 93.3% when grown as a sole crop at the end of the first year. During the second and third years, the percentage survival of leucaena simultaneously intercropped with maize was significantly lower than when grown as a sole crop. Sole leucaena achieved a mean annual height growth rate of 2.56 m, but when hedgerow intercropped with maize or sorghum or gmelina in combination with maize or sorghum, annual mean height growth rates were 2.03, 2.02, 2.13 and 2.02 m, respectively. The average bole volume of leucaena when sole cropped was 13.2 m^3 ha^-1 within the first 2 years but increased to 24.3 m^3 ha^-1 at the end of the third year. The effects of gmelina, maize and sorghum on leucaena's dry matter production were similar to that recorded on volume increment.
Key Words: Hedgerow intercropping, leucaena-gmelina hedgerow, planting sequence, pruning, thinning
La performance du Leucaena Leucocephala dans la silviculture mixte avec le gmelina (Gmelina arborea) et croisee avec le Mas et le sorgho etait evaluee en zone guineenne de savane du sud nigerian sur un sol du type plinthustalf et sur quatre saisons de recolte. Le pourcentage de survie de la leucaena s'echelonait de 53,3% quand cela se passe simultanement avec la gmelima et le sorgho a 93,3% quand cela s'eleve comme seule recolte a la fin de la premiere annee. Durant la seconde et la troisieme annee le pourcentage de survie du leucaena qui etait simultanement associe avec le mais etait d'une maniere sensiblement plus bas que quand poussait en monoculture. Leucaena plante seul donnait une croissance annuelle de 2,56 m alors qu'en association avec le mais ou le sorgho, la croissance annuelle etait de 2,03, 2,02; 2,13 et 2,02 m respectivement. Le volume annuel d'un leucaena etait de 13,2 M3 par hectare entre les deux premieres annees; mais elle s'est eleve a 24,3m^3 par hectare a la fin de la 3eme annee quand il est plante seul. Les effets de gmelima, du mas et du sorgho sur la production en matiere seche de leucaena seche sont similaires a ceux enregistres quand son volume augmentait.
Mots Cles: Culture mixte Hedgerow, planter en sequence, elaguer, tanches minces
Leucaena-Leucaena leucoceghala (Lam.) de Wit. is an exotic tree legume in Nigeria whose ability to fix atmospheric nitrogen (N) through root nodulation with the appropriate Rhizobium strain has made it a popular choice as a soil fertility restoring tree species (NRC, 1984; Sanginga et al., 1989a, b). Apart from leucaena's capacity for rapid N fixation, the nutrient contribution from its prunings would be more beneficial to a companion arable crop in alley or hedgerow intercropping (Kang et al., 1984; 1985). However, when any arable crop is intercropped with a tree species, optimal planting densities are adjusted. For instance, Ngambeki (1985) reported leucaena occupying about 20 percent of the surface area when alley cropped with maize. Leucaena population density in agri-silvicultural farming system could be further reduced if there is an interest in its bole formation. Such a reduction in leucaena density could lead to a low yield of prunings. To make up for this loss in prunings, a woody tree species like gmelina - Gmelina arborea (Roxb.) could occur in mixed - silviculture with leucaena in the same hedgerow.
The aim of this research was to investigate the growth and development of leucaena in mixed-silviculture with gmelina in the Nigerian Southern Guinea Savanna Zone with maize or sorghum alley cropped in the inter and intra-spaces along the leucaena-gmelina hedgerow.
MATERIALS AND METHODS
This study was carried out at Ilorin (located at 80 3' N and 40 33' E) in the Southern Guinea Savanna Zone of Nigeria. The topography of the area is generally undulating with an average altitude of about 200 m above sea level. The soil is in the order Alfisol. The zone has monthly mean temperatures ranging between 20.70 C and 32.80 C, while rainfall follows a bimodal distribution with the first peak in June, followed by a short break in August and another peak in September. Annual rainfall ranges from 770 mm to 1,300 mm with a 10-year mean of 1050 mm. The duration of rainfall spans 6 to 8 months.
The experimental plot was cleared and ploughed by tractor in May of the first year, but was neither harrowed nor ridged throughout the duration of this experiment. Also inorganic fertilizers were not applied to the plot.
The area was divided into 6 main blocks, each 16 x 36 m, separated from one another by a 5 m footpath. Two blocks comprised one replication. A block was further sub-divided into 6 (16 x 6 m) compartments. The 6 subplots in each main block were planted with the factorial combinations of gmelina, maize and sorghum. The 4 subplots in each main block with arable crops (i.e., maize or sorghum) were further split on the basis of the planting sequence (simultaneous and sequential). Simultaneous intercropping consisted of maize and sorghum sown in the alleys and intra-spaces of the leucaena - gmelina hedgerows from the first cropping season. Sequential intercropping consisted of fallowing the land between tree crops for the first 2 seasons before sowing the cereal crops during the third and fourth cropping seasons. One crop of each of the 2 cereals was grown per annum. The period from June to December comprised one cropping season for the cereals, while June of one year to the following June was the growth year for the trees. The experimental layout was a randomised complete block design (RCBD) with split - plot and split - split - plot arrangements (Fig.1).
Records kept were: yearly percentage tree survival, bi-annual height, and crown and stem diameter using relascope/girth tape. Plant parts were partitioned into bole, branch and leaf. Each partition was sub-sampled and oven dried at 700 C for 48 hours. Ratios of dry to fresh weight were used to estimate total oven dry weight of each component (bole, branch and leaf) using the relationship:
Dry wt. = X(Z/Y), where X = fresh weight of the component Y = fresh weight of the sample Z = oven dried weight of the sample
Volume of the bole of leucaena was estimated by the use of a modified Smalian's formula (FAO, 1980).
Statistical analysis on some variables was carried out as split-split-plots (gmelina being the main plot, arable and planting sequence as factors), ignoring compartments without arable crops, 10 (see a-j in Fig.1) treatment combinations were employed for the analysis. Subsequent to an analysis of variance, means were tested by the LSD test on the condition that the corresponding F-ratio for treatment was significant.
Fifteen surface soil samples were taken within each block at 0-15 and 15-30 cm and bulked for analysis. Particle size analysis was done by the hydrometer method (Bouyoucos, 1951). Soil pH was measured in 1:2 soil-water suspension using pH meter (McLean, 1965), while nitrogen was determined by the regular macro-Kjeldahl method of Jackson (1962) and available P by the Bray P-1 method (Bray and Kurtz, 1945). Organic matter was determined by wet dichromate oxidation method as modified by Piper (1944). Exchangeable cations were extracted with neutral normal NH4OAC. Potassium, Ca and Na in the extract were determined by flame photometry while Mg was determined by atomic absorption spectrophotometry. Available Mn and Zn were determined by extracting 2.5 g of soil with 25 ml of modified NaHCO3 (Hunter, 1972). Concentrations of these micronutrients in the filtrate were read off the atomic absorption spectrophotometer.
The result of both the physical and chemical analysis of the soil of the experimental plot is presented in Table 1. The values of most soil variables determined increased with depth except Na and silt.
The effects of gmelina, cereal and planting sequence on leucaena height, diameter at breast height (DBH) and crown diameter are presented in Table 3. Gmelina and cereal crops did not lead to significant differences in leucaena height, DBH and crown diameter. Simultaneous cropping led to significantly lower height, DBH and crown diameter when compared with sequential cropping. The effect on leucaena due to maize or sorghum or gmelina in combination with either cereal led to the lowest height growth of leucaena at 6, 12 and 18 months after seeding. Interaction effect of gmelina by arable crop were not significant, but gmelina by arable crop by time were significant during the first 18 months. At 30 and 36 months, there were no significant differences in height (Fig.2).
Effects of gmelina, cereal crops, their different combinations and planting sequence on leucaena volume increment at 24, 30 and 36 months after sowing are shown in Table 4. Gmelina and cereal crops did not lead to any significant effects but planting sequence did. Sequential planting also led to significantly greater bole volume than simultaneous. The greatest mean volume of 13.2 m^3 ha^-1 was obtained within the first 2 years in leucaena sole cropping. This increased to 24.3 m^3 ha^-1 at the end of the third year. The lowest volume increment was recorded when leucaena was simultaneously intercropped with gmelina plus maize, although this was not significantly different from either sorghum or maize sown without gmelina. The average upper and lower mean annual bole volume increment of leucaena were 8.09 and 1.2 m^3 ha^-1, respectively.
The number of luecaena seedlings/saplings that survived was not significantly affected by the presence or absence of gmelina or by the two cereal crops. The adverse effect of simultaneous intercropping of sorghum or maize on leucaena during the first year was probably due to the shading effect of the arable crops on the vulnerable seedlings coupled with low soil nutrient status. By the end of the second year, the effect of planting sequence was no longer significant since the seedlings were able to establish.
Observation of the height growth of leucaena on a bi-annual basis indicated that gmelina and arable crops did not produce a significant effect, but planting sequence did: sequential planting was superior to simultaneous. However, this was only during the first 18 months. At 30 months of growth, leucaena had recovered from the earlier negative effect due to simultaneous intercropping with either sorghum or maize or gmelina in combination with maize or sorghum. The greatest detrimental effect on leucaena height growth was from simultaneous intercropping involving sorghum. The maximum height growth of 7.8 m over 3 years seems poor compared with 5.0 m recorded for Leucaena leucocephala in one year in Kevela, South India; 5.5 m in 9 months in Reduit, Mauritius; 8.0 m in 18 months in East Java, Indonesia (NRC, 1984). Comparing 2.56 m mean annual height growth of leucaena in this study with the 2.74 m recorded for gmelina (Fagbemi, 1994) on the same plinthustalf, one could conclude that leucaena is less adapted to the Nigerian Southern Guinea Savanna Zone than gmelina. Since the interaction effect of gmelina by arable by time did not go beyond the first 18 months, it would appear that the legume could be established under any of the cropping systems.
Gmelina and cereal crops did not significantly affect leucaena volume increment but planting sequence did. Whereas leucaena that was simultaneously intercropped recorded 2.6, 4.2 and 5.6 m^3 ha^-1 of bole volume, sequential intercropping resulted in 11.4, 15.2 and 21.0 m^3 ha^-1 during 24, 30 and 36 months of growth, respectively. The implication of this finding is that if the objectives of tree management include provision of wood from the bole, then sequential intercropping will be preferred to simultaneous planting. The average mean annual bole volume increment of leucaena ranged from 1.2 m^3 ha^-1 when simultaneously intercropped to 8.1 m^3 ha^-1 when allowed to establish for 2 years before intercropping with any of the cereal crops.
Soil nutrient status might have affected early establishment of leucaena when it was intercropped simultaneously with maize or sorghum in the Nigerian Southern Guinea Savanna Zone. The use of fertilizer to enhance the productivity of arable crops is a common practice in the savanna zone of Nigeria. Leucaena could benefit from such an inorganic input to facilitate early establishment before establishing a symbiotic relationship with the soil native rhizobia.
The effects of gmelina, cereal crops and planting sequence on biomass partitioning in leucaena was similar to that of bole volume. A total of 5.5 and 6.4 t ha^-1 were obtained from the prunings (leaf + branch) of 2-year old leucaena intercropped with sorghum and maize, respectively. Yield of leucaena prunings in the order of 7 t ha^-1 has been reported (Kang et al., 1985). The low biomass production from leucaena simultaneously intercropped with either cereal led to a reduction in the quantity of prunings used to mulch the soil. However, the inclusion of gmelina as a component of the leucaena hedgerow could raise the total quantity of prunings produced (Maclean et al., 1992).
This study demonstrated that leucaena could perform better when sequentially intercropped than in simultaneous intercropping involving maize or sorghum. However, the enhanced cereal yield from simultaneous intercropping may also make the planting sequence a viable option.
One problem found associated with leucaena was the high density of free volunteer leucaena seedlings arising from seedset after the first year of establishment. These seedlings were found to be difficult to uproot during manual weeding due to their deep penetrating taproots. To make this species acceptable to peasant farmers, a use must be found for the seeds so as to encourage their harvest from the field. The results of this investigation have, however, shown that leucaena could be integrated into the agroecosystems of the Southern Guinea Savanna Zone of Nigeria
The contribution of Prof. J. A. Ogunwale in reviewing the manuscript is greatly appreciated.
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Copyright 1996 The African Crop Science Society
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