Canola (Brassica napus L.) water use indicators as affected by sustained deficit irrigation and plant density 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, primarily for seed to make oil. Expansion of the crop to the other 8 provinces is proposed to reduce shortages of locally produced plant oils. At the same time, canola can serve as a rotational crop for wheat in these summer rainfall provinces. In central Free State, information on evapotranspiration and various water use indicators for canola as influenced by sustained deficit irrigation and plant density is lacking. An experiment with a line source sprinkler irrigation system was therefore conducted, comprising of full irrigation as a control with 4 sustained deficit irrigation levels (mean reduction in irrigation depth per event of 67%, 52%, 34% and 19%) and 5 plant densities (25, 50, 75, 100 and 125 plants‧m⁻²). Mean seasonal maximum evapotranspiration amounted to 429 mm across plant densities. Plant density did not significantly influence seasonal evapotranspiration. Reducing the irrigation depth per event by more than 20% decreased seasonal evapotranspiration by a mean 3.5 mm per percentage increase in irrigation depth. A maximum biomass water productivity of 22 kg‧ha⁻¹‧mm⁻¹ was measured with full irrigation and a plant density of 75 plants‧m⁻². Seed water productivity amounted to a high of 11 kg‧ha⁻¹‧mm⁻¹ with full irrigation and a plant density of 25 plants‧m⁻². A percentage reduction in irrigation depth and increase in plant density above 25 plants‧m⁻² will reduce seed water productivity by 0.071 and 0.033 kg‧ha⁻¹‧mm⁻¹, respectively. Sustained deficit irrigation increased water use efficiency by a mean 0.5% per percentage reduction in irrigation depth per event.