Soybeans After Soybeans Can be Problematic — DTN

If you want to ruin an agronomist’s day, ask him for the best management practices for planting soybeans back into soybeans.

“They’re basically asking, what’s the best strategy for doing something we shouldn’t be doing?” University of Wisconsin agronomist Shawn Conley told DTN, with undisguised frustration. “My number-one recommendation is don’t do it!”

Such questions are on the rise among Wisconsin growers as the markets tip in the favor of soybeans, Conley said.

“We’re going to see more soybean acres out there,” he predicted.

While market factors are tempting some growers to plant soybeans for the second year in a row, the practice comes with a host of agronomic issues, such as increased weed pressure, higher disease risk, and more fertility requirements, experts told DTN.


For better or worse, 2015 does promise to be the humble bean’s year to shine, confirmed DTN Grain Market Analyst Todd Hultman.

Last week, Informa Economics estimated that U.S. soybean acres will rise 5% to 88 million in 2015, as corn acres contract 2% to 88.6 million.

“The specific numbers will be debated in the months ahead, but the general direction of less corn and more soybeans is generally accepted, as most measures of profitability currently favor soybeans,” Hultman explained.

The soybean’s place as the preferred commodity is far from permanent or stable, however, Hultman warned.

“Anyone considering planting soybeans in 2015 should be aware new-crop prices have plenty of bearish risk, especially if South America comes through with another record crop,” he pointed out.

If growers plant the predicted 88 million acres of soybeans and the weather cooperates, the U.S. could produce a 4-billion-bushel crop, he explained. The result would be another 240 million bushels in ending supplies that are already estimated at 410 million bushels in 2014-15. “Another year of good weather has the potential to bring November 2015 soybeans down to $7,” Hultman concluded.


There is one creature out there who would love to see growers plant soybeans back-to-back, Iowa State University plant pathologist Greg Tylka noted: the soybean cyst nematode.

The nematode first emerged in U.S. fields in the 1950s and since then has made itself at home, particularly in the soils of Soybean Belt states like Illinois, Iowa, and Indiana. The pest reproduces on the roots of soybean plants, robbing them of yield and plant health in the process.

In 2014, Iowa growers saw surprisingly high SCN reproduction, most likely because the most common source of resistance — PI 88788 — continues to erode, Tylka noted.

A second source of resistance, known as Peking, is rare and accounts for only 2% of all SCN-resistant soybean varieties. As a result, growers with a history of SCN who want to plant beans again will find few good options come seed selection for 2015, Tylka said. “The best solution in that situation is don’t do it,” he said.

PI 88788 is composed of four or five genes, so soybean varieties can sport different combinations of this type of resistance, but there is no way for growers to know which genes are in their soybean selections.

The best bet is to pick different soybean varieties from last year to increase the chances of getting a different arrangement of genes, Tylka said. However, since multiple companies can market identical soybean varieties under different names, this tactic will only work if you stay within one seed company, he added.

Growers can also select from the small pool of soybean varieties with the Peking source of resistance. Iowa State University releases an annual assessment of soybean varieties’ performance against SCN, along with the source of resistance in each one.

Some companies also sell seed treatments that are designed to give additional protection against the nematode, such as Syngenta’s Clariva Complete, Bayer’s Poncho Votivo, and Bayer’s new seed treatment, ILeVO, a product originally designed to protect against SDS.

Tylka said Clariva did show some slight yield improvements and nematode suppression in small plot trials from Iowa State last year. Also, ILeVO consistently knocked nematode reproduction on susceptible soybean varieties down to the level of resistant varieties in greenhouse trials, Tylka found.


Foliar soybean diseases such as white mold and frogeye leaf spot can also build up inoculum in fields that stay in soybeans, warned Iowa State plant pathologist Daren Mueller.

If conditions are right for these diseases in 2015, bean-on-bean producers will find themselves at increased risk for an outbreak.

Ironically, growers who saw sudden death syndrome (SDS) in their fields in 2014 might get a break from a bean-on-bean rotation. Preliminary results from a study by Iowa State plant pathologist XB Yang showed that the SDS fungus can build up its numbers on corn residue as much, if not faster than, on soybean residue.

“It suggests the pathogen has evolved to thrive in a corn-soybean rotation,” Mueller noted.


University of Missouri weed scientist Kevin Bradley is all too familiar with soybean-on-soybean farming, a common practice in the Show Me State, at a cost to weed control.

“We have to be that much more vigilant from a weed control standpoint,” he told to DTN in an email. “The pigweeds (waterhemp) are even more likely to proliferate in continuous soybean systems, and I believe other weed shifts can occur as well if it is a continuous Roundup Ready soybean system.”

Weeds such as toothed spurge and Asiatic dayflower, which emerge late and tolerate glyphosate, can become much more troublesome in a soybean-on-soybean situation, he warned.

Since growers will be dealing with the same crop, they can also expect to see the same weed pressure and the same line-up of pre- and post-emergence herbicide products. The risk of weed resistance and the difficulty of controlling already resistant weeds skyrocket, Bradley noted.

“These are the situations where we must have herbicide mode of action rotation,” he said. If possible, growers should even try to rotate between different cropping systems like Liberty Link and Roundup Ready soybeans, he added.


Growers who plant back into soybeans should be prepared to take soil tests and supplement accordingly, Conley said.

“Do not skimp on potassium,” he warned. “Soybean is a high-demand user of K, [and] fertility deficiencies often exacerbate disease incidence and severity.”

A soybean plant can also remove nutrients such as phosphorus, calcium, magnesium and sulfur from a field. Soybeans are able to fix nitrogen from the soil, but deficiencies can still occur, particularly later in the season with high-yielding beans.

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