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Pre-harvest glyphosate on soybeans?

Yes, no or maybe — it really all depends on the weeds

Whether to apply glyphosate or a true desiccant before harvesting soybeans depends on the weeds in the crop, Manitoba Agriculture weed specialist Jeanette Gaultier, said Sept. 20 during the Crop Talk Westman webinar.

“If your issues are winter annuals and perennials I would definitely go in with glyphosate because obviously it’s a systemic — it’s moving down to the roots and doing a good job on those,” Gaultier said.

Applying glyphosate post-harvest can often be more effective, but there’s the risk wet weather could delay the operation or a killing frost stop plants from growing, preventing the herbicide from working.

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“So my recommendation is just get it done when you can,” Gaultier said. “We are talking increments of effectiveness. Do it when you have a chance… that way you don’t have to worry about it anymore.”

Apply glyphosate when soybean seed is 30 per cent moisture or less. Don’t forget, glyphosate won’t kill Roundup Ready soybeans because they tolerate glyphosate. The goal is controlling perennial weeds.

Gaultier said it takes seven to 14 days, depending on the weather, for the glyphosate to kill perennial weeds. Glyphosate works faster under warmer temperatures and when weeds are actively growing.

If weeds are mostly green, annuals, including kochia, consider applying a true desiccant such as diquat, Heat or CleanStart, Gaultier said. The goal is burning off weeds to get the soybeans through the combine sooner. True desiccants, which are contact weed killers, do that faster than glyphosate, she said.

Sometimes it makes sense to apply a combination of glyphosate and a desiccant. However, don’t use diquat in the mix because it burns plants down so quickly weeds will not have time to fully absorb the glyphosate, Gaultier said.

Desiccants can be applied at 80 to 90 per cent leaf crop with yellow to brown pods.

Diquat can dry down weeds in as little as one day, but it and the other desiccants usually burn down weeds in seven to 10 days, depending on the weather, Gaultier said.

Apply CleanStart when soybean seed moisture is 30 per cent or less, Manitoba Agriculture’s 2017 Guide to Field Crop Protection, says.

After applying Heat or CleanStart do not harvest soybeans for at least three days.

Always check with buyers about the pre-harvest aids they accept and follow herbicide label instructions.

About the author

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Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.

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