Plant density response and optimum crop densities for canola (Brassica napus L.) in Western Australia

© CSIRO 2016.In 24 experiments conducted across a range of agricultural environments in Western Australia between 2010 and 2014 canola (Brassica napus L.) grain yield response to crop density was adequately described by an asymptotic model (where yield approaches but never quite reaches a ceiling at very high density) in 101 out of 112 individual responses; in the other 11 yield reached a maximum and declined slightly at higher densities. Seed oil was more likely to increase than decrease with increasing density but the effect was always small; less than 1% oil over the range of densities tested. Increasing density also suppressed annual ryegrass (Lolium rigidum (L.) Gaud.) head numbers in six experiments where it was measured, especially at densities below 20 plants/m2. Economic optimum densities ranged from 7 to 180 plants/m2, with a median of 32.2. Mean optima in low and medium rainfall zones (growing season rainfall 1000kg/ha). Predicted optima were more sensitive to seed cost and field establishment (the proportion of viable seeds that become established) than grain price or seed size over the range of values expected in Western Australian agriculture. Field establishment varied from 0.3 to 1 and was higher at low target densities and for hybrid compared with open-pollinated cultivars, with a median of 0.585 at a target density of 40 plants/m2. We identified improving field establishment of canola as an important research priority.