Bob French1, Mark Seymour2
1 Department of Agriculture and Food, Western Australia, PO Box 432, Merredin, WA, 6415, and Centre for Plant Genetics and Breeding, University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009 firstname.lastname@example.org
2 Department of Agriculture and Food, Western Australia, PMB 50, Melijinup Road, Esperance, WA, 6450
24 canola cultivar × plant density trials were conducted in Western Australia between 2010 and 2014 and 112 individual yield-density response curves fitted. Economic optimum density was chosen as the point on the response curve where marginal return to increasing density equals the marginal cost. This depends on the following parameters: seed cost, seed size, germination percentage, field establishment (the proportion of viable seeds that become established plants), and grain price. Our best estimates of these give optimum densities ranging from 7 to 180 plants/m², with a median of 32.2. Mean optima of Roundup Ready® and hybrid triazine tolerant (TT) cultivars were 25-30 plants/m² in low and medium rainfall areas, and 25-40 plants/m² in high rainfall areas. Mean optimum density for TT open pollinated cultivars was higher, often outside the range of densities achieved in particular experiments, on account of the low cost of farm-retained seed of those cultivars. Among the parameters optimum calculations are based on grain price and field establishment can only be estimated, and the precise shape of the response curve is unknown when a crop is planted. The precise optimum cannot therefore be known. We show for a given response curve optimum density is most sensitive to seed cost and field establishment. Crop gross margin was quite insensitive to density around the optimum: when the deviation from the optimum was less than 10 plants/m² gross margin was generally reduced by less than $10/ha for hybrid cultivars, although more below the optimum than above. Density could often be reduced by up to 60 plants/m² with less than $10/ha change in gross margin for open-pollinated TT cultivars. We conclude that WA growers should aim to establish 25-30 plants/m² of hybrid canola in low and medium rainfall zones and ~35 plants/m² in high rainfall zones, aiming slightly above these targets if there is doubt about conditions for establishment. Growers planting farm-retained open-pollinated seed should aim for 50-60 plants/m² across all rainfall zones.