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 <300 mm) were about 25, 30, and 75 plants/m² respectively for glyphosate-tolerant (Roundup Ready), hybrid triazine-tolerant (TT), and open-pollinated TT cultivars, assuming open-pollinated TT cultivars were grown from farm-saved seed. There was little difference between optimum densities for hybrid and open-pollinated glyphosate-tolerant cultivars, and optima in the high rainfall zone were about 10 plants/m² higher than in low and medium rainfall zones. Yield at optimum density was greater than 90% of maximum yield in 74% of cases. The economic penalty for not achieving the optimum density with hybrids was usually small if the deviation was less than 10 plants/m², and with open-pollinated TT cultivars was small even 50-60 plants/m² below the optimum. The penalty was usually greater for deviations below than above the optimum in medium and high yield potential environments (yield potential >1000 kg/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.