Developing nitrogen fertiliser management strategies for canola (Brassica napus L.) under conservation agriculture practices in the Western Cape

[摘要] ENGLISH ABSTRACT: Nitrogen (N) is one of the most important nutrients in canola production systems. Improper N supply due to lack of knowledge regarding canola N management and inappropriate fertiliser guidelines, frequently results in low canola yield and profitability in the Western Cape. Current N guidelines are based on international literature or adapted from wheat guidelines. Conservation agriculture practices have also changed soil N dynamics. More N is mineralised from soil organic matter than conventional systems, which were historically practised. Canola N guidelines should therefore be refined to account for the abovementioned changes. The aim of this study was to determine the effect of different topdress N rates, foliar N application at stem elongation and N source on plant parameters, canola seed yield, oil content and N use efficiency, whilst monitoring the effect of different topdress N rates on the soil mineral N concentration at plant, pre-topdress, post topdress and at harvest. This study was conducted at five canola producing areas in the Western Cape during 2016 and 2017. The trial was laid out as a randomised block design consisting of seven different topdress N rates (0, 25, 50, 75, 105, 135 and 165 kg N ha-1) applied at the rosette stage. For all the above mentioned treatments, 25 kg N ha-1 was applied at planting. A control treatment was included that received no N. A foliar N application that consisted of 20 kg N ha-1 (urea ammonium nitrate) was applied at stem elongation. Five N sources were evaluated, applied as topdressing at rosette stage. Increasing topdress N rate increased (p<0.05) soil mineral N concentration. Plant population at harvest and biomass production did not respond (p<0.05) to topdress N rates, a result not expected but could be ascribed to the relative dry seasons experienced in 2016 and 2017. Canola yield responded (p<0.05) to topdress N rate. Maximum yield response was recorded at lower topdress N rates than expected. The N use efficiency (NUE) decreased (p<0.05) as topdressed N rate was increased, with a drastic reduction in NUE when total N application was increased above 25 kg N ha-1. Foliar N application at stem elongation did not (p>0.05) influence yield or oil content at most sites. Nitrogen source did not influence (p>0.05) plant population, biomass production or yield, except at one site (Langgewens) in the Swartland in 2017 where the urea + inhibitor outperformed LAN. No differences (p>0.05) were recorded in oil content between different N sources in 2016. Generally, in 2017, oil content was lower compared to 2016 and inconsistent results were recorded between N sources. This was possibly due to the dry conditions during 2017, which may have influenced oil production. It is apparent that N fertiliser recommendations have to be adjusted for certain areas. Current N recommendations may result in over-fertilisation and reduced profitability at sites in the southern Cape. Current N recommendation at the Swartland sites has a low NUE and further increase in topdress N rates would likely result in pollution of the environment. Nitrogen source did not affect canola productivity. Selection of N source should be based on cost. In general, CA practices tended to decrease fertiliser N requirement for canola production. Refined N fertiliser guidelines may result in more consistent canola yield and ensure profitability. Guidelines will only be finalised on completion of the research project after at least four years of data capturing.

[发布机构] Stellenbosch University