LLOYDMINSTER, Sask. -ÊIn its early stages, conservation farming relied on herbicides to fight weeds.
But now there are signs the movement is maturing to a new phase of weed management
Studies show cultural control, or integrated weed management, is becoming a reliable and accepted way to fight weeds.
By eliminating tillage, farmers have eliminated a pest control tool, says Agriculture Canada weed scientist Doug Derksen, of Indian Head, Sask.
Cultural weed control uses the natural competitiveness of crops, planted in rotations that suppress weeds, along with careful use of herbicides.
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Research by Derksen and his associates at the Agriculture Canada research centre at Indian Head, for the East Central Soil Conservation project, shows promising results from this type of control.
“The whole issue about rotation and weeds is varied selection pressure,” he told the recent annual meeting of the Saskatchewan Soil Conservation Association.
Varied selection pressure means factors that suppress weeds are constantly changed to reduce the chances of a particular weed dominating the system.
In zero till, two applications of herbicide are used to control weeds: the pre-seeding burn-off and the in-crop control, which in the past as been seen as the “miracle cure,” said Derksen.
The problem with this approach is it compresses weed control in a four- to six-week period.
Problem weeds like dandelions, Canada thistle, night-flowering catchfly and cleavers – which tend to germinate in the summer or fall – are missed with these applications. Winter annuals like stinkweed, shepherd’s purse, flixweed, bluebur and narrow-leaved hawk’s beard also thrive without fall tillage.
Using pre-harvest or fall herbicide applications can help control these weeds. For example, Canada thistle seedlings are not vigorous, Derksen said, and are therefore vulnerable to pre-harvest treatment.
Varying crops in rotations also has the advantage of varying the types of herbicides used. That helps minimize the danger of herbicide-resistant weeds. “The more diverse the rotation, the more diverse the herbicide options,” Derksen said.
For example, quackgrass is more easily controlled in broadleaf crops than in cereals. A rotation that frequently includes broadleaf crops increases the options for controlling quackgrass.
The whole point of cultural control, however, is the judicious use of herbicides by allowing varied selection pressure through these practices:
- Crop sequences. Crops have different competitive abilities. Cereals are generally more competitive than broadleafs, and winter cereals more competitive than spring cereals.
The cereal/oilseed/cereal/pulse rotation shown in the diagram means a farmer can grow wheat, barley, canaryseed, winter cereals, canola, flax, sunflowers, peas or lentils. The sequence not only allows cultural control, but gives farmers some market flexibility.
- Crop life cycle. Summer annual crops select for summer annual weeds, winter cereals for winter annual weeds. Rotating crops with specific life cycles suppresses the build-up of specific weeds.
- Seeding dates. Rotating seeding dates is the least-used weed management tool, and ” … is the thing we need to learn the most about,” said Derksen. Tillage encourages spring weed growth, but weed emergence is slower in zero till and a second flush of growth usually doesn’t occur.
Varied seeding dates allow varied herbicide use. Derksen said for early-seeded crops, the in-crop treatment is most important, whereas the burn-off treatment is most important in late-seeded crops.
- Fallow. Derksen said fallow (including chemfallow) is not weed-neutral and isn’t a complete solution. Work at Indian Head has shown chemfallow favored foxtail barley and dandelions, min-till favored wild tomato and conventional tillage favored wild buckwheat.
