Nitrogen fertiliser demand of canola is low when produced in conservation agriculture systems in South Africa

Abstract

Current fertiliser guidelines for nitrogen (N) application for canola under conservation agriculture (CA) in South Africa, may result in over-fertilisation. This study aimed to evaluate the effects of N rates and foliar applied N on biomass, seed and oil yield of canola grown under CA. Trials took place from 2016 to 2019 at Darling, Porterville, Moorreesburg, Riviersonderend and Riversdale in the Western Cape, South Africa, representing diverse soil and climatic conditions. The study evaluated the application of 0, 25, 50, 75, 105, 135, 160 and 190 kg N ha⁻¹ N rates using limestone ammonium nitrate followed by a urea ammonium nitrate foliar application of 20 kg N ha⁻¹ vs a control (no foliar). The responses of biomass, seed and oil yield to top-dressed fertiliser N rate was site specific and varied with in-season rainfall. Foliar N application had no effect (p > 0.05) on biomass (all sites), seed yield (Darling, Porterville and Riversdale) and oil yield (Darling). At Moorreesburg (2016 and 2017) and Riviersonderend (2016, 2017 and 2019) foliar N increased seed yield. The lack of response by biomass, seed and oil yield to increased N application rates under different soil and rainfall conditions suggests lower fertiliser N demand under CA conditions.

Keywords:

• Brassica napus L.
• canola biomass production
• canola oil yield
• canola seed yield
• foliar N application
• nitrogen fertilisation of canola

Acknowledgements

The SASOL Agricultural Trust, the Protein Research Foundation of South Africa and Southern Oil Ltd. are acknowledged for financial assistance. The Western Cape Department of Agriculture are thanked for their facilities and labour. We also acknowledge the producers, N Loubser, WG Treurnicht and J Joubert who made land available on their farms for the research sites. Dr Marde Booyse is thanked for undertaking the statistical procedures and assistance with interpretation.