To optimize yields, some Midwest farmers are inoculating soybeans even when they are planted on ground that was recently planted to soybeans. Historically, inoculants have been recommended only for new soybean ground or ground that has not recently been planted to soybeans. “Six years ago only a few thousand acres of soybeans were inoculated in Ohio. Now, growers inoculate nearly 2.5 million acres or about half our soybean acreage,” says Ohio State University agronomist James Beuerlein. “That's because we're seeing a 300 to 400% return from applying inoculant.”

Beuerlein's seven years of on-farm tests have shown an average yield advantage of 2.1 bu./acre for inoculated soybeans on rotated soybean ground. With the added yield, growers earn an extra $10.50/acre, while the cost of inoculation ranges from $1.50 to $3/acre. The University of Wisconsin and Purdue University also report yield advantages for inoculation even on ground recently planted to soybeans.

Nitrogen use

Researchers speculate that the newer strains of rhizobia may colonize the root more extensively and fix more nitrogen than the traditional indigenous strains do. Also, new sterile-based and liquid product formulations have up to 10 times more live bacteria than older formulations have, notes Tom Wacek, a researcher with Urbana Laboratories, St. Joseph, IL.

Purdue University agronomist Ellsworth Christmas says growers who want to improve the nitrogen status of the soybean plant should inoculate their seed and forget ideas of fertilizing soybeans with nitrogen. “In eight years of trials, we've seen an average yield increase from inoculants of 1.25 bu./acre, and just a half-bushel increase pays for the inoculation,” he says. “Our studies of nitrogen fertilization of soybeans have rarely shown a yield response and almost never a response great enough to pay for the fertilizer.”

Still, some Midwest universities have not seen a consistent advantage to applying inoculant unless the ground has not been planted to soybeans in the last five years.

Minnesota Extension soybean agronomist Seth Naeve says, “We haven't seen a positive response for inoculants in Minnesota where there's been a long history of soybean production.” However, he acknowledges that inoculants can be a cheap insurance policy, especially if growers plant early into cold soils or if they have coarser soils.

Time factor

Most often seed must be inoculated as it is loaded into the drill or planter and must be planted soon after treatment (within 12 hrs. or less) so the bacterial cells will remain moist and survive long enough to infect soybean roots following germination, according to Beuerlein. New liquid products are more convenient than peat-based products. “Time is sometimes more of a concern than the cost of the inoculant,” Naeve says. “It makes the most sense for growers who buy bulk soybeans and can treat the seed in bulk with liquid inoculants.”

Custom application formulations

Increased use of bulk seed and custom application of both fungicides and inoculant are key trends in the soybean industry. To address these trends, companies are releasing new inoculants that can be custom applied at the retail level up to 30 days, instead of just hours, before planting. Rob Osburn, microbial research and development manager for Liphatech, Milwaukee, WI, says, “Our new Cell-Tech SCI can be custom applied 30 days before planting and offers a real advantage in user convenience.” Similarly, Wacek notes that Urbana Laboratories will offer Nod+30 for custom application this year.

Currently, most inoculants may not be mixed with fungicides and applied to the seed at the same time. One exception is ApronMaxxRTA fungicide, which has been formulated for rhizobium inoculant compatibility. Cell-Tech SCI and Nod+30 can be applied with ApronMaxxRTA up to a week before planting.

“There has been increased use of both inoculants and fungicides, so compatibility issues will be important in the future,” Beuerlein says.