The effect of nitrogen and sulphur on the nutrient use efficiency, yield and quality of canola (Brassica napus L.) grown in the Western Cape

Thesis (PhD(Agric))--Stellenbosch University, 2012.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.AFRIKAANSE OPSOMMING: Canola (Brassica napus L.), ‘n relatief nuwe oilsaadgewas wat goed aangepas is, word in ‘n toenemende mate in die produksiegebiede van die Weskaap verbou. Lae opbrengste en wisselvallige reaksies teenoor stikstofbemesting word egter verkry ten spyte van die gewas se hoë stikstofbehoefte en dit mag moontlik aan swaweltekorte toegeskryf word. In hierdie ondersoek is die groei-, opbrengs- en kwaliteitsreaksie van canola teenoor verskillende N (0, 30, 60, 90 en 120 kg N ha-1) en S (0, 15 en 30 kg S ha-1) bemestingspeile in droëland proewe op verskillende lokaliteite bestudeer gedurende die 2009-2011 groeiseisoene. Reaksies teenoor N en S onder optimale groeitoestande en vir verskillende cultivars is in glashuisproewe van die Departement Agronomie van die Universiteit van Stellenbosch, uitgevoer. Die chemiese samestelling van die plante tydens blomstadium (90 dae na plant), asook droëmateriaal produksie, graanopbrengs en kwaliteit het betekenisvol verskil tussen die lokaliteite, maar lokaliteitsverskille is ook deur die seisoene beïnvloed. Die ontwikkeling, groei en graanopbrengs van die canola is ook beïnvloed deur die stikstofbemestingspeile in beide die veld en glashuisproewe. Swawelbemesting het nie die vegetatiewe groei van canola beïnvloed nie, maar het blom en peulproduksie in glashuisproewe en graanopbrengste in veldproewe verhoog. Die reaksie van canola teenoor die swawelbemesting is grootliks bepaal deur die swawelinhoud van die grond asook klimaatsfaktore soos reënval. In die algemeen is die hoogste canola opbrengste in veldproewe met toedienings van 120 kg N en 30 kg S ha-1 verkry, maar glashuisproewe het getoon dat hoër toedieningspeile nodig mag wees onder optimale groeitoestande soos in besproeiingsgebiede. Hoë toedieningspeile van N en S het veroorsaak dat die waterverbruiksdoeltreffendheid toegeneem het van 4-5 kg graanopbrengs per mm reën tot sowat 8-9 kg graan opbrengs per mm reën. Agronomiese doeltreffendheid van toegediende stikstofbemesting het afgeneem met toenemende N peile, maar waardes van ongeveer 8 kg opbrengsverhoging per kilogram N toegedien met stikstofpeile van 120 kg ha-1, toon dat hoë N toedieningspeile mag steeds winsgrense verhoog mits die prys van een kilogram N nie meer is as agt maal die produsente prys van canola is nie. Agronomiese doeltreffendheid van stikstofbemesting is verhoog deur ook 15 kg S per hektaar toe te dien, maar nie deur die toediening van 30 kg S ha-1 nie. Die agronomiese doeltreffendheid van S toedienings het slegs by die gelyktydige toediening van hoë stikstoftoedienings toegeneem, wat die wisselwerking tussen N en S ten opsigte van graanopbrengs bevestig. In teenstelling met stikstof het swawel toedienings die olie-inhoud van canola in die veldproewe verhoog. In glashuisproewe is gevind dat kort en medium groeiseisoen cultivars, ten spyte van ‘n groter vegetatiewe reaksie van die lang groeiseisoen cultivars, groter opbrengsreaksies teenoor stikstof- en swawelbemesting toon. Meer navorsing word egter in hierdie verband benodig