Doctoral Degrees (Agronomy)

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Browsing Doctoral Degrees (Agronomy) by Author "Ngezimana, Wonder"

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    The effect of nitrogen and sulphur on the nutrient use efficiency, yield and quality of canola (Brassica napus L.) grown in the Western Cape
    (Stellenbosch : Stellenbosch University, 2012-12) Ngezimana, Wonder; Agenbag, G. A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: There is an increasing demand for canola (Brassica napus L.), an emerging oilseed crop in South Africa. Canola thrives in the Western Cape. However, yet low yields are still obtained within the production areas with poor and or variable responses to nitrogen applications. Crop nutrition and specifically the contribution of sulphur (S) to nitrogen (N) use and selection of nutrient efficient genotypes can be strategies of considerable significance in increasing yields. This study investigated growth, yield and quality responses of canola to different N (0, 30, 60, 90 and 120 kg N ha-1) and S (0, 15 and 30 kg S ha-1) fertilisation rates in field trials at different localities, during the 2009-2011 period. Responses to N and S under optimum growing conditions and responses of different cultivars were investigated in unison in glasshouse trials at the Department of Agronomy of the University of Stellenbosch. Locality and growing season (year) significantly affected nutrient content in plants at flowering (90 days after planting), dry mass production as well as yield and quality of canola in field trials at five different localities during the 2009-2011 period. Growth and yield were also affected by N application rate in both field and glasshouse trials. Sulphur applications did not have an effect on vegetative growth, but rather stimulated flower and pod production in glasshouse trials and resulted in higher grain yields in field trials. Response depends largely on rainfall and S content of the soil. Highest yields were, on average, obtained with application rates of 120 kg N and 30 kg S ha-1, while glasshouse trials showed that even higher rates may be considered under optimum growing conditions. High application rates of N and S also improved water use efficiency from approximately 4-5 kg grain yield to about 8-9 kg grain yield mm-1 of rain during the growing season. Agronomic efficiencies of applied N decreases with increasing N rates and values of about 8 kg grain yield increase per kg of N applied at N rates of 120 kg N ha-1 indicated that high N rates may improve profit margins of canola as long as the cost of N is not more than eight times the producers price of canola. Agronomic efficiencies of N applications are improved if 15 kg S ha-1 is applied complimented with high rainfall, but not with applications of 30 kg S ha-1. Improved agronomic efficiencies of S applications shown at higher N rates, confirmed the dependency of S responses to sufficient availability of nitrogen. Sulphur applications, in contrast to N, resulted in an increase in oil content of the grain in field trials. Yield responses of different cultivars to nitrogen fertilisation under glasshouse conditions differed, with better responses obtained within short and medium season cultivars, than with a late maturing (long season cultivar), in spite of a better vegetative (dry mass) response of the later maturing cultivar. These results may indicate differences in the growth habit of different cultivars, but more research in this regard is needed.