Soybeans are a workhorse crop that are expected to produce solid yields with little or no TLC. With soybean prices in double digits today, however, farmers are looking for inputs, such as inoculants, to help boost yields from this easygoing crop.
Inoculants are made up of tiny bacteria called rhizobia that occur naturally in soils and are manufactured, as well, by various companies. In both cases, rhizobia partner with soybeans to boost nodulation and fix nitrogen.The better your soybeans handle these two tasks, the better they will tend to perform in the field.
A manufactured inoculant is applied on approximately 40% of all U.S. soybean fields annually,
estimates Ryan Locke, North American marketing manager for soybeans for EMD Crop BioScience Inc. That percentage is double what it was only 10 years ago, he adds.
When to inoculate. The challenge for most soybean producers is determining when it makes sense to use manufactured inoculants, which are based on the Bradyrhizobium japonicum strain of rhizobia.
"If I’m planting soybeans after a field has been in CRP [Conservation Reserve Program] or in a field that’s not had soybeans for three years, then I’m going to make the investment in a manufactured inoculant," says Bill Wiebold, University of Missouri soybean Extension specialist.
Wiebold’s colleagues in other Midwestern states, including Illinois, Indiana, Nebraska and Wisconsin, offer similar recommendations that range between three and five years.
The timing recommendations for manufactured inoculants vary based on how well the naturally occurring rhizobia work in various soils.
Flooded soybean fields are also candidates for an inoculant, depending on the length of time a field has been under water, according to Wiebold.
"Bacteria are aerobic, and when in water for prolonged periods of time, they drown," he says.
Some of the Extension specialists say a couple of the newer manufactured inoculant products, including Optimize 400 and Vault HP, provide additional benefits.
"These components can help with early plant growth, especially in cool and wet soils associated with early planting," says Shaun Casteel, Purdue University soybean Extension specialist. "However, while plant stand establishment can be improved, we usually do not see much of a yield benefit in a corn–soybean rotation."
University of Illinois soybean Extension specialist Vince Davis notes that a yield increase of 1 bu. per acre or less is common. "With $13 per bushel beans and a $2 to $4 per acre cost, it takes very little yield benefit to pay for itself," he says.
For a seed-applied inoculant, the cost range is influenced by seeding, notes Shawn Conley, University of Wisconsin soybean and wheat Extension specialist.
New bacteria. EMD’s Locke says that, even in continuous soybeans, the impact of seasonal stresses on existing rhizobia can reduce their populations. For that reason, soybeans benefit from an annual infusion of new bacteria that can help support seedling development and emergence in early spring.
"We see a gain of 2 bu. to 4 bu. per acre on average," Locke says.
He adds that farmers can expect EMD’s Optimize 400 to provide a return on investment 70% of the time.
Becker Underwood reports that its Vault HP contains a patented growth enhancer that supports increased nitrogen-fixing nodules. "This can lead to more robust root structure, better
nitrogen fixation and more yield potential," says Eda Reinot, the company’s director of research and development.
Not all inoculants are created equal, and their efficacy does vary considerably, according to Wisconsin’s Conley.
"The range of success can be less than 5% to greater than an 80% break-even, depending upon the product," Conley says.
It’s hard to isolate a yield boost in soybeans to a single input because there are so many variables, Davis contends.
Effective evaluation. University of Nebraska Extension agronomist Charles Wortmann says the only way for farmers to effectively evaluate is to conduct strip trials to compare soybean yield results with and without inoculant treatments.
"There should be at least six replications, giving 12 strips, with yield determined for each strip separately," he explains. "All other management should be consistent across strips.
"We like to see such trials conducted in three different fields or in one field over three years before we have much confidence in the results," he adds. "Such trials are easily conducted when farmers are equipped with guidance equipment and yield monitors."
Locke encourages farmers to pull root samples from treated fields to evaluate efficacy. "Cut a nodule open. If it’s white or slightly pink, that product isn’t performing like it should," he says. "You want to see a strong, vibrant, pink color. If the nodules are all white, even if you see 100 of them, there’s no nitrogen fixation going on."
Davis adds that farmers need to remember that rhizobia are living organisms and that their health is an important factor in efficacy. To achieve the best results with manufactured inoculant products, follow the manufacturer’s recommendations.
Becker Underwood claims an on-seed survival rating of 125 days for the rhizobial inoculant component in its Vault HP. According to Renoit, this rating helps extend the time frame available for the seed-treating process and planting.