Rotations for weed management

Dec.8, 2011 - Issue 28

For many canola growers, their most challenging weed has become volunteer canola, especially with tighter canola rotations. Growers may need to consider a rotation of herbicide-tolerance systems as a method to help manage the volunteer canola seed bank. And volunteer canola management becomes a goal in non-canola years.

Crop planning and canola rotations also require growers to think about the herbicides used in the 12 months before seeding canola. This may be especially important for growers who tried different products or rates this fall to clean up fields that didn’t get a timely spray in the spring/summer. Many herbicides can cause carryover damage in canola when used in the fall ahead of canola seeding. Click here to see an article that lists products that can be used in the fall ahead of canola.

Benefits of canola for weed control: For growers in less traditional canola growing areas, an extra benefit of canola in the rotation is the opportunity to hit weeds with a different system. Canola can clean up fields for other crops in the rotation that may not have weed control options that are as broad spectrum or cost effective as canola products. With a rotation of wheat and canola and pulses, growers can hit weeds with a wide variety of herbicide groups from year to year. This is good, as long as these products do not become overused as a result.

Risks with tight rotations: The success of herbicide tolerant canola technology to control weeds is one reason why canola rotations have tightened. Economical weed control has increased canola profits. But the reduction in crop diversity can allow some weeds (and insects and diseases) adapted to growing with canola to increase in density and intensity. This places increased pressure on the chemical tools used to manage these pests, thereby increasing the risk of resistance to those products. Some examples of resistance have have already been documented in North America, such as glyphosate resistant kochia in central North Dakota. A predictive model by Hugh Beckie, research scientist with AAFC in Saskatoon, ranks kochia first among weeds at risk of developing glyphosate resistance on the Prairies. Read the article on page 11 of the March 2011 Canola Digest.

When using glyphosate as a pre-seed treatment ahead of non-canola crops, tank mixing with 2,4-D or Heat will help reduce the risk of glyphosate tolerant weeds developing. Do not use these tank mixes ahead of canola. CleanStart with Aim chemistry is a pre-seed option ahead of canola.

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