To evaluate whether your inoculant is working, cut open several nodules. The insides should be a vibrant pinkish-red color. Non-functioning nodules can have white, green or even black interiors, the latter of which occur when the nodules decay.
These good bugs partner with soybeans to increase nodulation and fix nitrogen.
Soybeans are a workhorse crop for many farmers who count on them to produce solid yields with little or no TLC. With soybean prices in double digits today, however, many farmers are looking to inputs such as inoculants to help boost yields from this affable crop.
Inoculants are made up of tiny bacteria called rhizobia that occur naturally in soils and are also manufactured by various companies.
In both cases, rhizobia partner with soybeans to boost nodulation and fix nitrogen.
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 Extension soybean specialist.
However, he adds: "While plant stand establishment can be improved, we usually do not see a greater yield benefit in a corn-soybean rotation."
Vince Davis agrees. The University of Illinois Extension soybean specialist notes: "A 1 bu. per acre yield increase or less is common."
However, he adds: "With $13 bu. per acre beans and a $2 to $4 per acre cost, it takes very little yield benefit to pay for itself."
For a seed-applied inoculant, that cost range has to do with the seeding rate you use, notes Shawn Conley, University of Wisconsin Extension soybean and wheat specialist.
Most manufacturers recommend that farmers inoculate their soybeans annually.
Ryan Locke, North American marketing manager for soybeans for EMD Crop BioScience, Inc., makes a case for this practice.
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 to 4 bu. per acre on average," Locke says.
He adds that farmers can expect EMD 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, Becker Underwood director of research and development.
Locke estimates that a manufactured inoculant is applied on roughly 40% of all U.S. soybean fields annually. That percentage is double what it was only 10 years ago, he adds.
Not all inoculants are created equal, and their efficacy does vary considerably, according to Conley.
"A paper we published last year indicates the range of success being less than 5% to greater than an 80% break-even, depending upon the product," Conley says.
"Most farmers, and many research stations for that matter, will have difficulty proving an input provided a small fraction of a bushel benefit, so it is an input that is hard to evaluate in many situations," Davis contends.
Charles Wortmann, University of Nebraska Extension agronomist, says the only way for farmers to evaluate inoculants effectively 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 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 growers need to remember that rhizobia are living organisms and that their health is an important factor in efficacy. Farmers need to follow manufacturer recommendations to achieve the best results with their products.
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 timeframe available for the seed-treating process and planting.