Comparing the precipitation use efficiency of maize-bean intercropping with sole cropping in a semi-arid ecotope

[摘要] English: The study had a major aim of comparing precipitation use by a maize-bean intercrop (1MB) and its component sole crops. In doing so the important variables within the soilcrop system-atmosphere were quantified. The specific ecotope on which the experiment was conducted is Tempe/Valsrivier located at Bloemfontein, South Africa and experiences low and variable rainfall not exceeding 600 mm per annum. The soil was a duplex type with a slowly permeable layer at a depth of about 900-1000 mm depth. The summers are generally very hot with high vapour pressure deficit, and high evaporative demand making it particularly hostile for crop production. The technical problem concerned comparing the cropping systems by quantifying their use of the water resource as well as determining the intervening weather and crop variables influencing water use. The hypothesis was that the intercrop had the potential benefits for water conservation within the ecotope compared to its sole crop components. This property was inferred from past studies which have shown that the intercrop cropping system has a superior water use efficiency. Field experiments were conducted over two summer growing seasons on the ecotope using an additive intercrop of maize and beans to test the hypothesis. Two sowing dates were adopted during each summer of 2000/01 and 2001/02, consequently, four cropping seasons were done. A randomized complete block design was used, with three treatments being intercrop, sole maize and sole bean (1MB,SM and SB) each with three replications. An experiment to determine the transpiration efficiency coefficient was conducted on an adjacent field with a weighing lysimeter and ran parallel to the first planting during both years. Detailed soil water content measurements were made on the ecotope including drained upper limit (DUL- 262 mm). crop determined lower limit (CLL: SM - 114 mm, SB- 103 mm and 1MB- 121 mm) and soil bulk density. Similarly, measurements were made of crop growth and biomass accumulation and weather variables both within the canopy and at an automatic weather station at the experimental site. The measurements made it possible to characterize the precipitation use for the cropping systems within the ecotope. Measurement of soil water content enabled the quantification of the water balance for each season while the component crop transpiration efficiency coefficient made it possible to partition water use between transpiration and soil evaporation. The lysimeter determined transpiration efficiency coefficient for the dry bean was 3.26 ± 0.25 gkPakg-1 which was within range of those found for other legumes. Analysis of the crop extraction limits and soil water balance components revealed that the intercrop had higher plant available water capacity (PAWC) indicating that it extracted more water than the sole crops. It had 7% and 18% more PAWC than SM and SB respectively. Findings from the soil water balance components showed that the 1MB conserved water by losing less through soil evaporation. This attribute was conferred on it by the relatively high leaf area index which reduced the energy flux to the evapotranspiring soil surface. The canopy of the intercrop was more humid decreasing the vapour gradient between the canopy elements and the atmosphere within the canopy. Measured wet and dry bulb air temperatures attest to the presence of relatively higher humidity within the intercrop compared to the other sole maize and bean crops. It is probable that this property made the intercrop conserve more water that was then available for plant use. It can therefore be concluded that the microclimate of the intercrop is favourably modified to conserve water. The estimated soil surface evaporation indicated that the 1MB had the lowest soil surface evaporation (Esg) compared to the other crops. Consequently. the 1MB had the highest transpiration meaning that it was able to produce more biomass than the sole crops as transpiration has a linear relationship to biomass accumulation. An analysis of the total water use did not reveal any significant differences between the cropping systems. with the SM having a slightly higher water use than the 1MB and SB the lowest. These were significant findings as the plant populations were quite different between the cropping systems. The additive intercrop had a plant population of 120.000. sole bean 80. 000 and sole maize crop 40. 000 plants per hectare. Another important product was the quantification of radiation interception and use by the cropping systems within the ecotope. The intercrop intercepted and used more PAR than each of the sole crops. An attempt has been made to mathematically quantify water extraction by the cropping systems using the measured pattern of soil water contents in each cropping systems. This has revealed important possibilities for modeling water extraction by the intercrop. Adopting a particular cropping enterprise such as intercropping involves choice among various alternatives that may be available to the farmer. The choices are both economic and financial and involve foregoing alternative employment of resources. The concept of morecrop per drop should appropriately be more cash per drop of water. Water and therefore any form of precipitation should be allocated to the next best alternative in terms of financial returns. It is the contention that even the small scale farming sector to which this study is aimed has to a large extent been sucked into the economic and financial mainstream in many developing countries. The analysis for PUE was therefore done based on monetary value. It showed that sole beans had the best gross returns per drop of water (37 ± 6 ZAR ha-1 mm-1), the intererop had the second highest value at 32 ± 14 ZAR ha-1 mm-1 and sole maize 14 ± 5 ZAR ha-1 mm-1. The difference between sole bean and the intererop was not statistically significant. The intererop therefore exhibited no statistical difference in total water use despite the relatively higher plant population compared to the other cropping systems within the ecotope. At the same time it had yield advantage overthe component sole crops. It can therefore be concluded that within similar ecotopes, where the preferred choice is one of producing the cereal maize. as it usually is in most small scale farming communities, it would be profitable and nutritionally more advisable to grow the intercrop.

[发布机构] University of the Free State