Quantifying productivity and water use of sorghum intercrop systems.

Ph. D. University of KwaZulu-Natal, Pietermaritzburg 2015.Rural sub-Saharan Africa (SSA) faces the challenge of achieving food security under water scarcity amplified by climate change and variability. Under these conditions, it is necessary to adopt cropping systems that have a potential to improve productivity. The aim of the study was to assess the feasibility of a sorghum-cowpea-bottle gourd intercrop systems with a view to determine the resource use efficiencies. This was achieved through a series of studies which included conducting critical literature reviews, quantifying water use and water use efficiency of sorghum-cowpea-bottle gourd, and modelling such systems using Agricultural Production Systems Simulator (APSIM). Field trials were conducted at the University of KwaZulu–Natal’s, Ukulinga Research Farm over two seasons (2013/14 and 2014/15) under varying water regimes [full irrigation (FI), deficit irrigation (DI) and rainfed (RF)]. Intercrop combinations considered were sole sorghum, cowpea and bottle gourd as well as intercrops of sorghum–cowpea and sorghum–bottle gourd. Data collected included soil water content, plant height/vine length, leaf number, tillering/branching, leaf area index, relative leaf water content, stomatal conductance and chlorophyll content index as well as biomass accumulation and partitioning. Yield and yield components, water use (WU) and WUE were calculated at harvest. Extinction coefficient, intercepted photosynthetic active radiation (IPAR) and radiation use efficiency (RUE) for biomass and grain were also determined. Land equivalent ratio (LER) was used to evaluate intercrop productivity. Growth, yield and water use (ET) of the sorghum–cowpea intercrop system were simulated using APSIM. The validated model was then used to develop best management practices for intercropping. The review showed that aboveground interactions within intercrop systems have thoroughly been investigated while belowground interactions were mostly limited. The review showcased the potential of bottle gourd as a versatile food crop. The field trials established that sorghum yields were stable across different water regimes. This was mainly achieved through facilitative interaction within the intercrop systems which allowed for greater eco-physiological adaptation resulting in improved water capture and use. Improved water capture and use also increased WUE (50.68%) and RUE (8.96%). The APSIM model was simulated growth, yield and WU of an intercrop system under varying water regimes satisfactorily. The model over–estimated biomass (6.25%), yield (14.93%) and WU (7.29%) and under–estimated WUE (-14.86%). Scenario analyses using APSIM showed that the development of best management practices should be agro–ecology specific to ensure dynamic climate change adaptation strategies and increase resilience. It was concluded that intercropping results in improved productivity, especially under water–limited conditions. As such, it that can be used by farmers located in semi-arid and arid regions as an adaptation strategy for increased productivity. Dynamic agronomic management practices should be adapted to further increase the system’s resilience to predicted climatic uncertainties. Future studies on intercropping should consider root interactions and possibly different plant populations and planting geometry as factors that might influence resource capture and use. Decision support systems should be promoted within farming communities to better manage risks associated with on-farm decision making