Apply the right herbicide at the right time to slam the door on resistant weeds and kick up yield
As much as we hate to admit it, stress—within limits—often makes us perform better. Soybeans are the same way. Farm Journal studies have found that inducing stress by spraying soybeans at the proper time with certain herbicides can stimulate higher yields.
The burning technique is gaining interest not only because of high soybean prices but because many growers are using additional herbicides to combat resistance to glyphosate. If the herbicide you add prevents or controls resistant weeds and bumps yield a few bushels, that’s a double payoff.
"At today’s soybean prices, a 3-bu. yield response at $11 or $12 per bushel grosses $33 to $36 per acre," says Farm Journal Field Agronomist Ken Ferrie. "That’s a strong incentive to look at the practice, especially if you’re going to be applying a herbicide anyway."
The concept points out an important difference between soybeans and corn. As Ferrie often says, you never want a corn plant to have a bad day because yield will actually decline. In soybeans, a tough day during the vegetative stage can cause plants to come roaring back.
In the Farm Journal Test Plots from 2001 through 2004, Ferrie found an average yield increase of 2.6 bu. per acre from stressing plants with a herbicide. The increase resulted from more nodes per plant, shorter internodes and fewer aborted beans, he says.
Demonstration plots planted in 2011 by Farm Journal Associate Field Agronomist Missy Bauer produced similar results. She sprayed 10 oz. of Cobra herbicide per acre on soybeans at the V3 vegetative stage.
Alternating sprayed strips are visible one day after application in Farm Journal Associate Field Agronomist Missy Bauer’s herbicide-stress demonstration plot. Three of the four varieties in the plot produced more nodes, pods and soybeans as a result of a well-timed chemical application, which spurred the bean's final growth.
In three out of four trials, the stressed soybeans produced more nodes, pods and beans per plant. The average increase was one more node, 1.4 more pods and 3.8 more beans per plant.
Yield increases aren’t automatic, though. In Ferrie’s plots, there was one year when the yield decreased. "That season, because of the weather, the stress delayed maturity and worked against us," he says. "Stressing soybeans in the vegetative stage delays maturity. Stressing them in the reproductive stage speeds up maturity."
In Bauer’s plots, one of the four varieties produced more nodes, but fewer pods and fewer beans. "That might have been because it was the shortest-maturity variety in the plots, only a 0.8," she says. "While it was in the vegetative stage, it might also have been entering flowering, which would also put it in the reproductive stage. We call this the V3/R1+ stage.
"The other varieties had longer reproductive periods, which gave them time to put pods on the additional nodes that resulted from burning," she adds.
Burning soybeans requires thorough management that begins with herbicide selection.
A number of herbicides can be used. "Always remember that weed control is your first objective," Ferrie says. Once you determine which herbicides will control your resistant weeds or prevent resistance from developing, you can investigate what effect those herbicides will have on your soybeans.
After choosing your herbicides, consider the effect of the products on each other. "Tank-mixing some products with glyphosate reduces the efficacy of glyphosate," Ferrie says. "That could force you to use higher rates of glyphosate, or it could lead to weed escapes and cause resistance to develop."
Another problem can arise if you tank-mix a broadleaf herbicide with a postemergence grass control herbicide. "For the post grass products to work, the grass must be actively growing," Ferrie says. "If the broadleaf herbicide burns the grass enough to shut down its metabolism, you won’t get as good a kill. You may want to kill the grass first, and then come back a week later with the broadleaf product."
In areas where rotations are more diverse, such as the South, be aware of cropping restrictions. "You’ll apply the herbicide in midsummer," Ferrie says, "and you don’t want it to disrupt your intended crop rotation. That also applies if you plan to plant a cover crop in the fall."
Timing is everything. To boost yield, the application must be made during the vegetative stage of soybean growth—not the reproductive stage, which will have the opposite effect. That requires keeping a close eye on soybean growth stages, field by field.
"The reproductive stage of a soybean is set by maturity group and day length," Ferrie says. "Height is not an indicator of growth stage—you must look and see if the plants are flowering and setting pods. If a plant is loaded with small pods and you burn it with a herbicide, it may abort the pods and not have time to reset them."
The four-year Farm Journal trials showed that late vegetative through first flowering is the best time to spray diphenyl ether herbicides. Systemic herbicides seemed to work better if they were applied a couple of trifoliates before flowering, so the herbicide could act and the plant rebound by R1.
In three of four trials in a demonstration conducted by Missy Bauer, stressed soybeans produced more nodes, pods and beans per plant. The average increase was one more node, 1.4 more pods and 3.8 more beans per plant.
Consider the growing conditions before you spray. "If a field is clean, there are no glyphosate resistance issues and the plant is struggling under severe stress from drought or spider mites, that’s not the time to burn it," Ferrie says. "You might actually reduce yield instead of increasing it."
Prepare for tough-looking beans, Ferrie warns. "Soybeans look great after a glyphosate application—you rarely see any effect. With this treatment, you’ll see puckering and burning. But the beans will come out of it, and you might gain some yield."
There is some risk. "Because stressing the soybeans delays maturity, it can make them more vulnerable to late-season aphids and rust," Ferrie says.
The key to realizing a yield boost is remembering that weed control takes precedence over burning your soybeans. The growth stage and growing conditions have to be taken into
account before spraying as well.
"If plants are healthy and the herbicide you apply controls weeds and tickles your beans into yielding more, you’ll reap the double payoff," Ferrie says.