Canola was investigated as part of the Optimising Irrigated Grains research programme over three years. Growing canola under irrigation with the aim of producing 5t/ha has illustrated significant penalties in yields and margins from growing crops that are too thin. With higher yield potential under irrigation small differences in plant population have a “magnifying” effect in terms of yield. With plant populations below the optimum there are significant yield penalties, whilst in the same varieties’ populations that might be regarded as above the optimum have been either equal or higher yielding than the optimum. As a result, dropping to populations between 10-20 plants/m2 can produce a significant drop in productivity compared to plant populations that are above 40 plants/m2 when canola has been grown under irrigation.