Effect of water application and plant density on canola (Brassica napus L.) in the Free State

English: Canola serves as a very favorable crop to produce oil world wide. Canola production in South Africa is mainly restricted to the Western Cape Province under winter rainfall conditions. The Protein Research Foundation propagated the production expansion to the central part of South Africa. The semi arid area (Central part of South Africa) is characterized by variable and unreliable summer rainfall. Irrigation is therefore vital for sustainable production of a winter crop like canola. The aim of this study was to establish the crop's plasticity ability, water use, water use efficiency and transpiration coefficient under a range of water application and plant density treatments combinations for the central South Africa. An experiment with a line source sprinkler irrigation system was conducted near Bloemfontein in the Free State Province. Water applications, excluding 57 mm rain were: WI = 118 mm, W2 = 176 mm, W3 = 238 mm, W4 = 274 mm and W5 = 363 mm. These water applications were combined with the following planting densities: PD25 = 25plants m², PD50 = 50 plants m², PD75 = 75 plants m², PD100 = 100 plants m², PD125 = 125 plants m². Seeds (558 - 4653 kg ha-¹) and biomass (1983 - 6733 kg ha-¹) yields induced by the treatments proved that canola has a high plasticity. This is because over the full range of water application treatments optimized yields were realized at only one plant density though different for seed (25 plant m-² ) and biomass (75 plants m-²) yields. Compensation of yields at lower plant densities resulted from branches and hence pods per plant. Total evapotranspiration increased linear (r² = 0.97) from 245 mm with 118 mm water application (W1) to 421 mm with 363 mm water application (W5) but was not influenced by plant density at all. Water use efficiency confirmed the optimum plant density for fodder production is 75 plants m-² and for seed production is 25-² plants m-². The water use efficiency at these two plant densities were 12.9 kg ha-¹ mm-¹ and 9.6 kg ha-¹ mm-¹, respectively. The β coefficient of canola was constant (2.26) for the full to moderate irrigation regimes(W5 - W3), but not for the low irrigation regimes (W2 - Wl). The β coefficient of 2.26 was used to separate the evapotranspiration of the W3 - W5 treatments into evaporation (56%) and transpiration (44%). This method was not suitable to establish the influence of plant density on the two components of evapotranspiration. A transpiration coefficient of 0.0045 was calculated for canola when planted for fodder at an optimum plant density of 75 plants m-² under moderate (W3) to full (W5) irrigation.