Why canopy closure matters

We know that wider rows make it possible to pull wider drills with the same tractor. Wider rows also make it easier to direct seed into heavy trash. But remember that with every system, there is give and take. Wide row spacing and relatively low seeding rates mean that more of the ground is exposed to full sunlight for longer. This gives weeds more time and more space to emerge and grow. Herbicide tolerant hybrid canola varieties have made it possible to seed wider rows and get away with minimal canopy closure for the first few weeks of the season. But narrower rows mean faster canopy closure and improved crop competition over the weeds. This reduces the need for a second in-crop herbicide application. The fewer times growers use the same herbicide, the more years that herbicide will remain effective on the weeds in their fields. Crops that emerge faster and close the canopy more quickly also tend to mature earlier, reducing the risk of frost damage and quality losses at harvest.

Neil Harker, research scientist with AAFC in Lacombe, Alta., spoke at Farm Tech in Edmonton in January about the benefits of canopy cover as a weed management tool. Simply put, plant leaves absorb red light, which is a trigger for germination of light-sensitive seeds. Without red light hitting them, these seeds don’t germinate. Dandelions are a prime example. Harker cited a Polish study by Tadeusz Gorski showing that with light, 64% of dandelion seeds emerged in the first four weeks and 86% emerged within eight weeks. But if those weeds were under a crop canopy, 0% emerged over an 8-week period. Annual sow thistle was not included in Gorski’s study, but Harker says it is another important light-sensitive weed in Western Canada.

Published on February 7, 2011