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A farmer is seeding Roundup Ready canola and the field has a lot of winter wheat volunteers and some perennials. Is the farmer better to seed or spray first?
Answer: In this situation, with Roundup Ready canola being able to take a glyphosate application at any time up to the 6-leaf stage, the farmer may want to take advantage of good seeding conditions and seed, then spray as soon as possible after seeding. Reasons:
1. Given the weeds present, seeding today then spraying a couple of days later should still provide good control — as long as the perennials are still within staging size for glyphosate at the in-crop rate. Even if only suppressed with the post-seeding spray, perennials could be knocked back enough that crop competition keeps them down. If the field has a whole spectrum of late-emerging annual and perennials, the farmer will probably want to do a pre-seed burnoff. Also note that if weed control is delayed, these weeds will be taking up moisture and nutrients the crop could use.
2. Spraying first will delay seeding at least three days given that glyphosate needs three days of good (warm, sunny) conditions to get perennial weeds. Read more.
3. A pre-seed burnoff can provide an opportunity to tank mix a different product with glyphosate. However, in this farmer’s situation, selection pressure for glyphosate resistance is low for wheat volunteers and perennials.
The farmer doesn’t have to wait long to spray. Glyphosate can be applied to RR canola at any time, including before or at emergence. However, waiting at least a couple of days after seeding gives weeds a chance to get growing again after the disturbance of seeding. Clean leaf area is important for glyphosate uptake. Dust on leaves from seeding can block uptake. Those extra days also give new disturbed weed seeds a chance to germinate and then be controlled.
Further reading:
Canola Watch podcast with a conversation on tank-mixing glyphosate
Seed first or spray weeds
Use multiple modes in the pre-seed window