Nitrogen requirements of canola (Brassica napus L.) at sustained deficit irrigation levels in central Free State, South Africa

Abstract

In South Africa canola (Brassica napus L.) is cultivated in rotation with wheat under winter rainfall in the Western Cape province. Expansion of the crop to the other eight provinces is propagated to reduce a shortage of locally produced plant oils. At the same time, canola may serve as a rotational crop for wheat in these summer rainfall provinces. In central Free State, agronomic information for canola production is lacking. An experiment with a line source sprinkler irrigation system was therefore conducted to establish the influence of water application levels (175, 233, 295, 351 and 420  mm) in combination with nitrogen (N) application rates (0, 70, 140, 210 and 280  kg ha⁻¹) on the branches, pods and seeds per canola plant. The biomass and seed yields of canola were also measured, allowing calculation of harvest indices. All six variables were strongly influenced by the interaction of water and N applications. An optimal seed yield of 2.907  kg ha⁻¹ was estimated with a 351  mm water application and a 280  kg ha⁻¹ N application. This resulted in a water use efficiency of 8.28  kg seed ha⁻¹ mm⁻¹ water and a nitrogen use efficiency of 10.38  kg seed kg⁻¹ N.