Doctoral Degrees (Agronomy)

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Browsing Doctoral Degrees (Agronomy) by Subject "Agricultural conservation"

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    New perspectives on plant population and row spacing of rainfed maize
    (Stellenbosch : Stellenbosch University, 2020-03) Haarhoff, Stephanus Johannes; Swanepoel, Pieter Andreas; Kotze, Theunis Nicolaas; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: Recent maize grain yield increases are attributed to genetic advances and changes in soil and crop management practices, including no-tillage (NT) and additional conservation agriculture (CA) practices. Management practices such as plant population and row spacing should be adapted for NT and other CA practices to optimise maize grain yield and promote sustainable production. However, there is a lack of information reporting on the influence of environmental and management factors and its relationship with plant density and maize grain yield. This study was initiated to generate novel perspectives on the complex concept of interplant competition of rainfed maize under various soil and crop management practices and climate conditions. The study entailed five research themes. The first research theme consisted of a critical review of the current soil and crop management practices followed in rainfed maize production regions of South Africa. Sustainable and alternative agronomic management approaches were highlighted. Alternative agronomic management practices, such as NT, crop intensification and diversification, crop residue retention, and livestock integration may provide pathways to increase the sustainability of these rainfed maize systems. Improved soil water content may support higher plant populations. The second research theme entailed consolidation of global published data from rainfed maize plant population field trials to investigate the effects on yield and to determine the influence of rainfall, soil tillage and nitrogen on the relationship between plant population and yield. Data was extracted from 64 peer-reviewed articles. Maize grain yield responded positively to increased plant population in high rainfall environments, while yields in rainfall limited environments were highly variable. The optimal plant population under NT was lower than under conventional tillage. However, at a given plant population, maize grain yield under NT outperformed the yield obtained under conventional tillage. As a third research theme, the effects of plant population and row spacing on soil water, soil temperature and maize grain yield under CA in a sub-tropical environment, were evaluated over three seasons. Although maize grain yield was not affected by plant population in the season with the highest early-season rainfall, maize grain yield increased with increasing plant population in the average rainfall and drier seasons. The fourth and fifth research themes involved a two-year trial in a semi-arid environment. In this trial, the effects of plant population and row spacing on the aboveground growth, water use efficiency and root morphology were evaluated under NT. A row spacing of 0.76 m was advantageous in the drier season. Plant populations of 20 000 to 50 000 plants ha-1 out-yielded plant populations more than 25 000 plants ha-1 at 0.52 m row spacing. Rainfall affected maize root growth while plant population had a small effect on maize root morphology. Optimising maize grain yield using plant population and row spacing requires a flexible systems-based (i.e., CA) approach. Conservation agriculture should incorporate management practices (such as plant population and row spacing) tailored for specific context.