Soybean stages for weed control

It’s important to get an early jump on the weeds before they rob soybeans of their potential yield

A soybean plant in the unifoliate stage.

Early weed control is essential to protect Roundup Ready soybean yields, says Dennis Lange, a farm production manager with Manitoba Agriculture, Food and Rural Development (MAFRD) in Altona.

Glyphosate can be safely applied when soybeans are in the first trifoliate stage right up to flowering.

Most farmers split their applications, first hitting weeds when they are small and then again later before soybean flowering.

Roundup Ready soybeans are registered for two applications at the 360 grams acid equivalent per acre rate or one application at 720 grams acid equivalent per acre.

“If you’re on top of your weeds twice is usually enough to clean up the field before flowering,” Lange said in an interview June 18.

If Roundup Ready canola volunteers are flowering in a soybean crop, yield potential has already been compromised, Lange said. “The horse is out of the barn as they say.”

Farmers should scout their soybean fields before and after spraying for weeds, Lange said. It’s important to know what weeds are there and the stages of the weeds and the crop.

Not all glyphosate is registered for use on Roundup Ready soybeans, so select one that is. Also be aware of what crops are bordering the Roundup Ready soybean field and avoid drift.

“Keep in mind surrounding crops — the dry bean versus the soybean,” Lange said. “It sometimes is tough enough to tell them apart from a photograph on the screen never mind when you’ve got a 100-foot boom and you’re trying to figure out if it’s soybeans or dry beans next door.”


Dry beans are not glyphosate tolerant and neither is Liberty Link canola.

There are several in-crop herbicide options for Roundup Ready soybeans. Details are on pages 178 to 179 and page 184 of MAFRD’s Crop Protection Guide. Here’s a summary:

  • Basagran Forte — Group 6. It cannot be mixed with glyphosate. No crop staging restrictions.
  • Odyssey — Group 2. Be aware of recropping restrictions. Apply at the one to three trifoliate stage.
  • Reflex — There are label restrictions, including use only in the Red River Valley. Apply at the one to two trifoliate stage but not before the first trifoliate.
  • Pursuit — Group 2. Be aware of recropping restrictions. Apply up to and including the third trifoliate stage.
  • Flexstar GT — Apply as a pre-seed burn-off or pre-emergent or early post-emergent at the one to two trifoliate stage of glyphosate tolerant soybeans only and only in the Red River Valley.
  • Viper ADV — Group 2 and 6. Do not mix with glyphosate. Apply during the three to six true leaf stage.

Glyphosate works best when temperatures are warmer, Lange said. Avoid spraying at temperatures under 10 C.

Iron chlorosis, which is characterized by interveinal yellowing of soybean leaves, is due to a combination of high soluble salts, high carbonate levels, high soil pH and cool, wet weather, Lange said. Applying Pursuit can add to the yellowing.

Chlorosis tolerance varies between soybean varieties. Seed Manitoba rates each variety for tolerance.

“When you get into some warmer weather typically the new growth looks fine,” Lange said. “You’ve just got to walk away and come back a week later.”

Farmers should note fields and varieties that are prone to iron chlorosis and consider growing a more tolerant variety on that land in future years, he said.

About the author


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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