Keep a crop weed free during a critical period early in its development and you’ll maintain it’s yield potential. Let weeds sneak in, even though the plants are young and small, and you’ll flush away yield before you’re even out of the gate.

Plants don’t like to be crowded. Put an object close to a plant and it will change the way it grows to compensate for the pressure. Research by Clarence Swanton of the University of Guelph in soybeans shows that plants detect changes in the spectrum of light reflected off surrounding surfaces. If they detect competition from other plants for water and nutrients, they’ll change the way they grow. In soybeans, that means they’ll push down shallower roots or reduce size – both things that affect yield.

The same effect is seen across a number of crops, not just soybeans. Weed scientists call it the “critical weed-free period” when plants will change their growth pattern in the face of weed competition. The exact timing of this critical weed-free period changes by crop:

• Spring Cereals: 1 to 3 leaf stage
• Winter Cereals: 500 to 1000 growing degree days with a base of 0°C
• Forages: 4 to 6 weeks after planting
• Canola and Oilseeds: emergence to 6 leaf
• Pulses: 2 to 6 leaf stage

Early Weed Control More Important Than Early Seeding in Spring Wheat

This critical weed-free period might be more important than you think. Research from the University of Saskatchewan shows that early control of winter annuals is more important to yield than early seeding in spring wheat.

Winter annuals are very inefficient water users and rob the crop of a lot of nutrients if left unchecked. Waiting to control weeds until the 6- to 7-leaf stage could cost seven bushels per acre.

Weeds such as stinkweed, narrow-leaved hawk’s-beard, dandelion, volunteer canola, kochia, and cleavers tend to be more competitive with the crop early on as they green up early.

Chose the Right Pre-Emergent Herbicide

Controlling weeds when they are young and actively growing is best. Consider choosing a product or tank-mix that offers extended control. It’s a good strategy to eliminate the weeds that are emerged with a contact herbicide like glyphosate. But consider adding a second herbicide with extended control, perhaps as a tank-mix partner to glyphosate or as an in-crop application, in order to keep the crop clean through the critical weed-free period.

You need to track the herbicide groups you’re using across the crops in your rotation in order to reduce the risk of weed resistance. Use herbicides with the same mode of action year after year and you significantly increase the chance of resistance developing. Choose your products carefully and consider your choices across the whole crop rotation.

Talk to Your Crop Advisor

Your P&H crop advisor can help sort through the options. There’s a lot to consider and fitting a weed management strategy to your operation and crop rotation takes careful planning.

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