Weed’Em or Weep: Strategies for Soybean Growers to Stay Ahead of Weeds

Written By Geoff Geddes

The early bird gets the weed

hand on lowly weeds, Bryan Dion has a wealth of experience. An agronomist and co-owner of Jonair (1988) Limited near Portage la Prairie, Dion and his company specialize in aerial and ground application of pesticides as well as providing agronomic services for their clientele.

“It’s really important to keep soybeans clean so they can compete better,” said Dion. “Studies show that early weed removal is very beneficial for soybeans, and it’s something that growers and agronomists need to work on more to make sure they are seeding in a clean field.”

Two of the biggest weed threats to soybeans are volunteer canola and wild buckwheat. In both cases, Dion finds that the sooner you address them, the better off you’ll be.

“You need to get on them early. If you do a pre-seed burn off and follow that with herbicide application on the first trifoliate and a second application on the third or fourth trifoliate, you should be okay. If you miss that burn off though, you’ll need to use a higher rate of glyphosate with your add-on.”

Dion stresses that as weeds like wild buckwheat get bigger, the efficacy of spraying glyphosates is diminished. So just like dealing with the annoying house guest, timing is critical.

Another proponent of early action on weeds in soybean fields is Paula Halabicki with BASF, the largest chemical producer in the world. In her role as Technical Service Specialist for Manitoba, Halabicki has seen firsthand the havoc that weeds can wreak on soybeans and the bottom line.

“Until the plant gets to a larger stage where it can compete with weeds, any weeds thatemerge are actually robbing you of yield, so controlling weeds early on is vital,” said Halabicki. “If you don’t, you can lose 50 to 60 percent of yield potential. By keeping as many weeds away from that plant as possible and keeping competition down, you will maximize yield potential at the end of the day.”

Hit them with your best shot

Whether it’s an unwelcome visitor or an unwieldy weed, you may have to approach them from different angles before they get the message.

“At the first trifoliate stage, you can use Viper ADV, Solo ADV, Odyssey, or Pinnacle as add-ons to your glyphosate, or use Flexstar GT, which contains both glyphosate and Reflex,” said Dion. “Once you get to the third or fourth trifoliate, spray again with straight glyphosate as soybeans aren’t very competitive.”

“Growers are hesitant to spray if they think the fields are clean,” Dion said, but he stressed that if you don’t spray your second in-crop pass at the fourth trifoliate, you’re liable to regret it as there are always weeds that go through. 

“There are always people calling me in August in a panic wanting to spray again as they didn’t do their second in-crop pass,thinking that their fields were clean.”

Whatever your approach, the experts agree that prevention is key. Because once that boorish guest settles into a comfy chair or an unprotected field, you’re in for a long night.Not surprisingly, one of the biggest challenges in giving intruders the boot is resistance.

“Canada is number three in the world in terms of resistant weed species with 61 unique varieties,” said Halabicki. “We recommend adding a second product to the tank to control weeds, such as a pre-seed option prior to planting or before the plant emerges from the ground.”

For example, she suggests tank mixing Heat LQ with glyphosate to address broadleaf weeds.

“When you combine those two and target the same weed species, you’re attacking it from two different angles, which can be highly effective in delaying resistance. Just make sure that when you are tank mixing and have a particular species in mind, that both products in the tank will control that weed. If the weed has a natural genetic mutation making it resistant to one of the herbicides, the other one will control it and prevent it from reproducing.”

In the case of wild buckwheat, which is not well controlled by glyphosate, adding Heat LQ or Viper ADV to the tank of glyphosate can do the trick. And with the proper timing, these same combinations can work against volunteer Roundup Ready canola.

“It is easy to lose one bushel per acre or more of canola grain out the back of a combine or through shattering losses, which goes back into the field and becomes a weed the next year. Volunteer canola will often emerge before your soybeans, and you need to get control of it so it doesn’t compete with your soybean crop.”

As Halabicki points out, this can often be accomplished by applying Heat LQ pre-emerge or Viper ADV in-crop.

The rotation situation

For many people, variety is the spice of life; for soybean growers, it can be the key to their livelihood.

“Rotating your crops year to year is important for both resistance management and disease control,” said Halabicki. “In the same way, you want to rotate your herbicides. If you alternate two herbicides, you can delay resistance longer than you would by just using one, and if you tank mix the two, it’s even more effective in postponing resistance.”

As part of this process, keep in mind that different herbicides pose different risks when it comes to resistance, which is where the herbicide resistance risk triangle comes in.

“Every herbicide has an active ingredient that’s classified into a certain Group, and each Group targets one part of the plant. As an illustration, Viper ADV is a mix of Group 6 and Group 2, with Group 6 targeting broadleaf and Group 2 aimed at both grasses and broadleaves.”

While Groups 1 and 2 pose a high risk of developing resistance, Groups 14 and 6 are a low to moderate risk, so you would have to apply the latter many more times before resistance would appear.

For that reason, Dion warns against using a Group 2 herbicide more than once in a growing season.

Weed warnings

Both Dion and Halabicki see common mistakes when it comes to weed management.

For Dion, it’s not spraying a pre-seed burn off and seeding into dirty fields. Dion notes that in the spring, you can get 10 days of rain where you can’t spray and soybeans are growing through, competing with weeds, so he urges growers to employ the burn off more than they have been.

In Halabicki’s experience, people don’t always clean their machinery between fields, moving resistance problems from one field to the next. She recommends always using clean, certified seeds. In addition, as glyphosate is a very important tool for growers, Halabicki advises adding a second mode of action to glyphosate when possible.

Whatever your approach, the experts agree that prevention is key. Because once that boorish guest settles into a comfy chair or an unprotected field, you’re in for a long night.