section 5 : results of agroforestry experiments

The influence of rainfall distribution on the yield of maize under Leucaena leucocephala alley cropping at Mtwapa, Coast Province, Kenya

K. Reshid and A. Getahun

Energy Development International
P.O. Box 62360, Nairobi, Kenya

B. Jama

International Council for Research in Agroforestry (ICRAF)
P.O. Box30677, Nairobi, Kenya

Abstract

The study conducted at Mtwapa, Coast Province, Kenya, during the long rains of 1985 and 1986 on the influence of rainfall distribution pattern on the yield of maize under Leucaena leucocephala alley cropping showed that rainfall distribution pattern had significant effect on the yield of maize even though the total rainfall for both years was the same (1130 mm and 1128 mm for 1985 and 1986, respectively). Maize yields for both years were also consistently higher under high Leucaena densities than under low densities or the sole crop. Yields of maize were very low in 1986 as a result of the heavy rains during the early crop growth stage (vegetative) (91%) and little during the pollination and grain-filling stage (reproductive) (9%) as compared to 1985 where rainfall distribution was relatively uniform whereby 58% of the total rainfall fell during the vegetative growth period and 42% during the reproductive period. As the result yield of maize in 1986 was reduced by about 60% as compared to that of 1985.

Introduction

In the tropics, crop yield fluctuates significantly from year to year, more so because of the variable distribution of rainfall than from the total received. High rainfall at the beginning of growing periods and its absence at later stages of crop growth and development could significantly reduce yields, even when the total rainfall during the growing seasons is normal and adequate for high crop yields. Reduced crop yields under alley cropping in such poorly distributed rainfall could result nutrient leaching and from competition for the same moisture at later crop growth and development stages. This report illustrates the effect of rainfall distribution on the yield of leucaena alley-cropped maize for two consecutive years, the 1985 and 1986 long-rain seasons at Mtwapa, Coast Province, Kenya.

Materials and methods

Leucaena leucocephala K-28 was planted in May 1982 in a parallel systematic design (Bleasdale 1967) comprising 3 between-hedgerow- row spacings (2, 4 and 8 m) and 4 within-row spacings (0.5,1,2 and 3 m). It was cut back to height of 0.5 m on March 1985 after a growth period of three years. The leaves and small twigs were incorporated into the soils during weeding after fresh weights were recorded. Maize (Coast Composite variety) was intercropped at a spacing of 90 x 30 cms on April 1985 and April 1986. Triple superphosphate (TSP) was applied at the rate of 150 kg/ha during planting. Calcium ammonium nitrate (CAN) was applied at the rate of 50 kg/ha twice during growing period.

Due to the vigorous coppicing of the leucaena, two additional prunings were made in April and May. The leaves and twigs less than 5 mm in diameter were incorporated into the soils during weeding. The maize was harvested on 20 August, 125 days after planting.

Similarly, during the 1986 long-rains season, maize was planted as an intercrop with leucaena on 1 April. The leucaena was coppiced at the end of March, April and May. All green leaf manure (GLM) of the three cuttings was weighed and recorded before being incorporated into the soils of their respective plots. TSP and CAN were applied as in the 1985 cropping season. A control plot with five replications was also planted with the same rate of fertilizer application during the cropping seasons of both years. The plots were left fallow between experimental periods. The maize was harvested on 18 August, 138 days from planting in 1986. Data on rainfall distribution of the two years were obtained from the agromet station of the Coast Agricultural Research Centre, Mtwapa, where this study was conducted. A statistical analysis was performed with rainfall distribution in the two years and leucaena spacings as independent variables; and grain yield and green leaf manure as the dependent variables.

Results and discussion

Maize yields under leucaena hedgerows were significantly higher in 1985 than in 1986 even though the total rainfall for the two years was essentially the same, 1130 and 1128 mm, respectively (Table 1 and Figure 1). However, during both years maize yields were significantly higher than those of the sole maize crops.

The reason for the lower yield of maize in 1986 appears to be the poor distribution of rainfall in that year: 91% during the two- month vegetative growth period and 9% during the pollination and grain- filling period. In the previous year, the same amount of rainfall was more evenly distributed: 58% in the first two months, 42% in the second period. We believe that the early heavy rains in 1986 caused excessive leaching of nutrients such as N and K which were then unavailable to the maize crop. Nutrient leaching rates are high in the coastal sandy soils (Anon 1984). A single rain has been reported to have the ability to move nitrates from the soil surface to a depth of 6 cm in 24 hrs (Thompson and Troeh 1979). Vittum, Lathwell and Gibbs (1968) also found that 54 kg/ha of K was leached as a result of adding 10 cm supplementary irrigation per year. Although this experiment was in the temperate zone (New York), they suggested that the observed leaching approximated the loss in tropical humid regions.

On the other hand, the small amount of rain that fell during the next two months of reproductive period (103 mm) was likely to inhibit pollination and grain filling due to moisture stress. Water stress caused either by low soil potential or high transpiration demand has consequences that involve physiological functions ranging from primary biochemical processes to effects on whole-plant physiology (Crafts 1968; Kozlowski 1968a, 1968b, 1972,1976,1978).

Table 1 Maize yield (kg/ha) of 1985 and 1986 under leucaena alley cropping.

Figure 1 Rainfall distribution patterns of 1985 and 1986 long rains seasons.

Though the main effect of water stress on photosynthesis is attributed to the opening mechanism of the stomatal aperture, there is some evidence that an inhibition of photosynthesis also occurs as a result of water stress in the cells (Salisbury and Ross 1969). This moisture stress could have been aggravated by competition from the leucaena hedgerows which coppice vigorously after every pruning.

Maize yields from narrow leucaena hedgerows has been observed to be significantly lower than under wider hedgerows, all other factors being equal (Kedir and Bashir 1986). This showed that under moisture stress, leucaena could also compete with the food crop. Under the conditions of the present experiment, however, higher yields of maize (4000 and 2598 kg/ha for 1985 and 1986 respectively) were attained by intercropping with high density leucaena hedgerows as compared with yields obtained with the lower-density hedgerows (2500 kg/ha and 475 kg/ha in 1985 and 1986, respectively) and the sole maize crop during both years (Table 1). The reduction in yield of maize in 1986 was about 35% in the 2 x 0.5 m plots when compared to the maize yield reduction of the sole maize crop that had a reduction in yield of 82% from 1985 to 1986.

The positive correlation between maize yields and amount of GLM incorporated into the soil during both years (Figure 2) showed that the higher yield of maize in the high density leucaena hedgerows (e.g., 2 x 0.5 m) was also associated with the high GLM produced (Table 2). Conversely, maize yields were consistently negatively correlated with increase in hedgerow spacing (Figure 3) which produced decreases in GLM yields.

Figure 2 : Correlation between maize yield and green manure incorporated into the soils during the long rains seasons of 1986 and 1987

In the higher density leucaena hedgerows (2 x 0.5 m), the effect of high evaporation rates and wind velocity that are characteristic of the coastal area might have been reduced as a result of the high density hedgerows serving as a windbreak. Also partial shading of the soil could have created a favourable microclimate of stable humidity for the development of the intercrop under high leucaena density plantings.

As is the practice in alley cropping systems (Kang, Wilson and Lawson 1984), the leucaena hedgerows were left to grow during the dry season and develop a dense canopy. This controls the growth of weeds and protects the soil from the intense heat of the sun. The high yields of GLM that were obtained from the three cuttings of the high- density leucaena hedgerows, and the extensive root system of leucaena, could have added more nutrients and organic matter to the soil. This may have substantially increased the moisture retentive capacity of the sandy soils. Thus, nutrients and moisture would be more readily available to the crop than in lesser leucaena hedgerow densities and sole maize crop (Table 2).

Agriculture in the coastal strip of Kenya is currently characterized by predominantly tree-based systems in which trees such as coco palm, cashew nuts and mangoes are intercropped with cassava and/or maize, with or without associated livestock grazing. Our results indicate that under the conditions of uneven rainfall distribution, high evaporation rates, high wind velocities and weeds that occur in the coastal strip of Kenya, alley cropping may be a useful alternative farming system. This should ameliorate the high leaching rates of the soils and reduce the need for chemical fertilizers.

Table 2 Leucaena green manure yield (t/ha) of 3 prunings for 1985 and 1986 long rains seasons.

Figure 3 Correlation between maize yield and spacing during the 1985 and 1986 long rains seasons.

References

Anon. 1984. Coast Agricultural Research Station (CARS) Annual Report. Bleasedale, J.K.A. 1967. Systematic designs for spacing experiments. Exp. Agric. 3:73-85.

Crafts, A.S. 1968. Water deficit and physiological process. In T.T. Kozlowski (ed.), Water deficit and plant growth Vol. 2. New York: Academic Press.

Kang, B.T., G.F. Wilson and T.L. Lawson. 1984. Alley cropping: a stable alternative to shifting cultivation. IITA. Balding and Mansell Ltd.

Kedir, R. and J. Bashir. 1986. The effect of planting distance of maize from leucaena hedgerows on the growth of height, tesseling and yield of maize at Mtwapa, Coast Province, Kenya. Unpublished report of the Kenya Renewable Energy Development Project (MOERD).

Kozlowski, T.T. 1986a, 1968b, 1972,1976,1978. Water deficit and plant growth, Vols. 1-5. New York: Academic Press.

Salisbury, F.B. and C. Ross. 1969. Plant physiology. Belmont, California: Wadsworth Publishing Co.

Thompson, L.M. and F.R. Troeh (eds.). 1979. Soil and soil fertility. New Delhi:

Vittum, M.T., DJ. Lathwell and G.H. Gibbs. 1968. Cumulative effect of irrigation and fertilization on soil fertility. Agron. J. 60: 503-565.