Rate and timing of nitrogen fertilisation for canola production in the Western Cape of South Africa

[摘要] ENGLISH ABSTRACT: Canola (Brassica napus) is increasing in popularity as a cereal crop in the Western Cape. Nitrogen (N) is generally the most limiting nutrient to canola production. Nitrogen fertiliser guidelines for canola are currently adopted from international literature or adopted from guidelines for wheat, and should be refined for the local environmental conditions. The aim of this study is to determine the optimal rate of N fertilisation, and the distribution thereof in the Western Cape. Field experiments were conducted in 2015 at Langgewens and Altona (Swartland) and Roodebloem (southern Cape). The trial was laid out as a factorial arranged in a randomised block design, with six N rates (0, 20, 60, 90, 120 and 150 kg N ha-1), of which each rate was applied in either one, two or three increments after planting, replicated in four blocks. Twenty kg N ha-1 was applied at planting. The rest was divided in equal increments either 30 days after planting (DAP), 30 and 60 DAP or 30, 60 and 90 DAP. Soil mineral N, leaf area index (LAI) and biomass was determined at 30, 60 and 90 DAP. Grain yield and thousand kernel mass (TKM) was recorded. Using these values obtained, agronomic N use efficiency (ANUE), water use efficiency (WUE) and profitability was determined by means of a sensitivity analysis. Treatments had no effect (P>0.05) on total soil mineral N content at any locality at any physiological stage with the exception of 90 DAP at Langgewens. Neither LAI nor biomass was affected by treatments (P>0.05) at any locality, at any physiological stage. Yield at Roodebloem was affected (P<0.05) by N fertilisation and treatments which had the highest yield were those who received 20 kg ha-1 at planting, and 70 or 100 kg ha-1 at 30 DAP respectively. These treatments did not differ (P>0.05) from treatments which received more than 60 kg N ha-1, regardless of the distribution. At Langgewens and Altona, N fertilisation had no effect (P>0.05) on yield, while TKM was not affected by treatments at any locality. No differences (P>0.05) were observed for ANUE at Altona and Langgewens, while treatments had a significant effect on ANUE at Roodebloem. The treatment that received 20 kg N ha-1 at planting and 100 kg N ha-1 at 30 days after planting and no N later, had the highest WUE, but did not differ (P>0.05) from a number of treatments that received more than 90 kg N ha-1 at various time intervals. The WUE at Altona and Langgewens was not affected by treatments (P>0.05). At both Altona and Langgewens the highest gross income was obtained by treatment that received no N at all, while at Roodebloem the highest gross income was obtained by applying 90 kg N ha-1 for the entire duration of the growing season. Preliminary results indicate optimum N fertiliser rate of 90 to 120 kg N ha-1, applied as 20 kg N ha-1 at planting and the remainder at 30 DAP at Roodebloem. No significant response to N applications was recorded in the Swartland. These results could be ascribed to drought conditions during 2015, which may have prohibited efficient uptake of N in the Swartland localities of Altona and Langgewens during critical periods.The differences between treatments at Roodebloem in the southern Cape might be due to a combination of sufficient rainfall over the growing season, lower temperatures and sufficient N being available during early, rapid vegetative growth. This in turn enabled a higher LAI which allowed for higher biomass accumulation, and consequently higher translocation to seeds. It is recommend that this study be repeated before results could be used to develop fertiliser guidelines for canola production in South Africa.

[发布机构] Stellenbosch University