Adopting A ‘Two Pass Approach’

By: Jason Jenkins, Mill Creek Communications

Like so many across the Show-Me State this spring, Greg Luce found himself in a planting predicament. After enduring drought in 2018, the Missouri Soybean Association’s director of research was thankful for the more-than-adequate moisture that replenished the soil profile at the Bay Farm Research Facility south of Columbia. However, exceedingly wet weather throughout April and into early May delayed planting.

“I guess you’d say we’ve been chomping at the bit, just like everyone else,” Luce says. “We’re ready to get seed in the ground.”

Among those seeds at Bay Farm are lines of conventional, non-GMO soybean that are part of the North Missouri Soybean Breeding Program, funded by the soy checkoff. While the Bay Farm crew plants plots of experimental beans, they employ the same early-season management strategies applicable to soybean producers growing conventional varieties in commercial fields.

“Conventional beans should be looked upon no differently than the GMO varieties — except for the weed control,” Luce says. “Otherwise, you still need to pick the variety that’s best adapted to your area. Find the maturity you desire along with the important traits such as yield potential and standability, along with disease and pest resistance.”

Greg Luce speaking to farmers during a field day at the Bay Farm Reserach Facility.

Of course, in an age of evermore aggressive, herbicide-resistant weeds, gaining that control is no easy feat. According to Kevin Bradley, University of Missouri Extension weed scientist, the task is even more challenging when growing conventional soybeans.

“We have a lot of farmers now who don’t know how it was before Roundup Ready and other herbicide-tolerant varieties were developed,” he says. “It takes intensive management, incorporating both cultural and mechanical practices, along with herbicides, to achieve good weed control in conventional soybeans.”

Bradley says it’s important for producers to first consider the history of the fields in which they intend to plant non-GMO beans. With an understanding of the types of weeds that were present in a particular field in the past, growers can better prepare for the future.

“I definitely wouldn’t try to put conventional soybeans in one of my fields that I know has the worst weed pressure,” he says, adding that fields with mostly grass weed pressure are better candidates, as grasses are easier to control in conventional soybeans than many broadleaf weeds. “You have to know what options you’re going to have.”

Other non-chemical options for controlling weeds in conventional soybeans include row spacing and seeding rate. More narrow rows planted to optimal populations increases the crop’s ability to outcompete weeds for both nutrients and moisture.

“We can’t be cutting back on seeding rate in conventional beans because that’s competition for the weeds, letting your soybeans do some of the weed control for you,” Bradley explains. “Narrower rows are going to close the canopy that much quicker, and that means less sunlight gets to the soil, which means less weed germination.”

Luce agrees. “An old weed specialist I used to know said the cheapest herbicide you could have is a good canopy on your crop,” he adds.

Chemical weed control can be achieved, but it requires growers to adopt a different philosophy and approach, Luce says. He notes that today, a different weed regime has developed than what existed when many growers last raised conventional soybeans.

“Back in the ’80s, we used to have problems mostly with cocklebur and velvetleaf. Today, it’s small-seeded broadleaf weeds like waterhemp and Palmer amaranth,” he says. “Growers may have to look at some chemistries that they haven’t depended on for quite a few years.”

Both Luce and Bradley recommend that growers adopt a “two-pass approach” to their herbicide program. While the specific products used can vary, the goal is to gain weed control through the application of both pre-emergence and post-emergence soil-residual herbicides with multiple effective modes of action.

Conventional soybean grown in the breeding program at the Bay Farm Research Facility.

During the pre-emergence pass, which can be applied before or just after soybean planting, growers should apply a full rate of an effective soil-residual herbicide. Depending on any herbicide resistance present in a field, these products might include Authority, Valor, Fierce, Sonic, Boundary, Prefix, Zidua Pro or Trivence, to name a few.

“Because our post-emergence herbicide options are more limited in conventional beans, this pre-emergence residual is critical to delaying the emergence of both grasses and small- seeded broadleaf weeds,” Bradley says. “Even then, it’s only going to last so long. In three or four weeks after planting, we almost always have weeds come up.”

At that time, a grower should apply an effective post-emergence herbicide with an overlapping residual and varied mode of action. Options include Group 14 and Group 15 herbicides, though both have their limitations. Group 15 herbicides such as Dual II Magnum, Warrant and Anthem won’t control weeds that have already emerged, and many populations of waterhemp and Palmer amaranth are resistant to Group 14 herbicides such as Cobra, Flexstar and Ultra Blazer.

“We also have to be really aware of the weed size with Group 14 chemistry,” Bradley adds. “If you’re two inches off what it says on the label, you’re probably not going to get control at all.”

When this two-pass approach is successful, the soybean canopy closes and weed control is achieved.

One to two weeks after the post-emergence application, fields should be scouted and any surviving weeds should be rogued before they reach reproductive stage.

“Rogueing out anything that didn’t get controlled is your best bet at a clean field in the future,” Bradley says. “It may not be all that big of a deal from a yield standpoint this year, but from a seed standpoint and what’s going into your soil next year, it’s a huge deal.”

Back at the Bay Farm Research Facility, Luce has adopted the two-pass approach, which he says is just as applicable to GMO beans as it is conventional varieties.

“No matter what herbicide program you’re in, it’s a sound philosophy,” he says. “We simply don’t want weeds to be a factor.”

Want to know more? Download a copy of the Missouri Soybean Seed Guide, which includes guidelines for weed management in conventional soybean and information on conventional soybean varieties developed in Missouri.

EDITOR’S NOTE: This is the third article in a yearlong series examining the production of conventional, non- GMO soybeans in the Show-Me State. Missouri Soybean Farmer will provide insight into both agronomic challenges and marketing opportunities afforded to those who are raising conventional beans in 2019. This issue, we explore early-season crop management.

In the August issue, Missouri Soybean Farmer continues its series on growing conventional soybeans by taking a look at the current state of the 2019 crop — examining the impact that weeds, moisture and disease are having as the growing season progresses.

Read the entire June 2019 issue here.