The influence of cassia siamea hedgerows on plant water use of cowpeas (vigna unguiculata (l) walp) in the semi arid area of kibwezi in Kenya

Abstract

The aim of this work was to quantify the impact of a modified agroforestry microclimate on water use of a cowpea crop as compared to a sole stand in the Arid and semi-arid lands (ASALs). Agroforestry systems have been developed in the ASALs to help the local farmers alleviate the myriad problems associated with water management in crop growth. However, most of the past studies using the same system had emphasized on soil fertility and its physical improvement. Equally, tree pruning associated with the system are simply too important as a source of fodder to be returned to the soil. Under such circumstances the only benefit from agroforestry is expected to be derived from a greater utilization of physical resources such as micro meteorological variables and hence the importance of their study. The agroforestry system adopted in this case was alley cropping using the Cassia siamea tree. The experimental site was located at Kibwezi Dryland Field Station of the University of Nairobi in Makueni District. The study was conducted during the short and long rainy seasons of 1998 and 1999 respectively. However the amount received was inadequate as compared to the seasonal mean. Parameters assessed include, solar radiation, relative humidity, air temperature, wind speed and wind direction, pan evaporation, sunshine duration, rainfall, soil temperature, soil moisture, above-ground dry matter production, actual and potential evapotranspiration, and water use efficiency (WUE). Determinations of these variables were made both at the site and at a nearby agrometeorological station which was about 70 m to the south of the site. Two alleys measuring 5 m by 1m and 70 m long were used. They were subdivided into three blocks with each of them having two treatments measuring 20 m by 5 m. The hedgerows were lopped to size of 1 m in one treatment (PS) and 2 m on the other (PT) in the same block. These were replicated thrice. The control plot (CP) with the same dimensions was established at the periphery of the agroforestry system (cf. Fig. 3). It was replicated twice. All the treatments were planted with the same density of cowpea cultivar CY.TT945-2264-4. Results indicated that in the entire microclimatic variables measured, only wind speed was significantly less by approximately 50°'0 in the alleys Soil moisture varied very little between I m and 2.5 m, and between PS and PT in the alley cropping. In the open crop, measured soil moisture was more by 11%. fIhe mean actual evapotranspiration at the sole stand was reduced by about 32 and 27% as compared to agroforestry system in the first and second season respectively. The values of crop water use as compared to potential evapotranspiration and rainfall revealed that, in the first season, only the initial stage of the crop that received enough rainfall. Subsequent growth stages had deficits. A similar scenario was observed during the second season. Cowpea dry biomass was the only yield obtained. No grain yield was obtained as the crop failed to reach maturity due to lack of sufficient rainfall. The study indicated that, there was a severe reduction in dry matter production in the alleys as compared to sole stand. During the first and second season, mean biomass production in the agroforestry system was reduced by 47 and 56% respectively as compared to the sole stand. Water use efficiency (\VUE) was generally very low and particularly in the alleys where it ranged from 0.15 to 0.5 g/mm from the initial to the vegetative stage. In ttie'sole stand the range was from 0.2 to 2.2 g/mrn. On the average, the monocultural stand outperformed the alley cropping by about 69%. Since there was insignificant modification in all parameters measured apart from wind speed, the poor crop performance in the alleys could not be ascribed to microclimatic modification. Therefore, as observed by other researchers, during the seasons of below average rainfall as was in this case, competition for moisture between the trees and the crops severely reduced crop yields. Thus, it was conclusive that, intercropping of cowpea with Cassia Siamea does not benefit the cowpea crop and in fact is serious to the crop when moisture is limiting.