If you perform a Google search for “manganese deficiency in soybeans,” a lengthy list of articles pops up, including several research papers examining the possible antagonistic relationship between manganese (Mn) and glyphosate.

Yet despite the many scientific efforts to understand micronutrient deficiencies, university soybean researchers and soil fertility experts find it difficult to provide simple guidelines on when and how soybean growers should apply micronutrient supplements.

“We don't really have a good recommendation system for micronutrients,” says Fabian Fernandez, an assistant professor of soil fertility and plant nutrition at the University of Illinois. Fernandez says Mn deficiency is the most common concern among soybean growers, but other micronutrients like zinc, iron, copper, boron and molybdenum are also essential for maximum soybean yields.

Researchers seem to agree that foliar supplementation works better than soil-applied Mn. But it's difficult to draw conclusions on the best strategies for foliar applications because Mn concentration levels in soybeans are impacted by a wide range of factors including soil pH, moisture and plant development.

“You might have a spot in the field that's deficient under certain conditions, such as when it is very dry or very wet. You might see a deficiency for a short period of time and then it disappears,” Fernandez says.

Mn problems can occur, “from the V2 stage, anywhere on through the rest of developmental cycle,” says Tim Boring, a research technician and doctorate student studying Mn deficiency at Michigan State University (MSU).

THE CRITICAL LEVEL for the youngest mature trifoliate leaf and petiole (leaf stem) after first-pod formation is 20 parts per million, according to Fernandez. Below this value soybeans are considered deficient in Mn and may suffer chlorosis, or the yellowing of leaves, with dark green veins, stunting, leaf drop, reduced flowering and severe yield loss.

George Zmitko, Owosso, MI, farms 8,000 acres and plants about 3,000 acres each year in commercial and seed soybeans. Michigan's prevalent lakebed and muck soils provide a bounty of organic matter, but they are often severely lacking in Mn. Approximately a quarter of Zmitko's soybean crop typically shows signs of deficiency.

“We usually just watch the beans and when they start getting a little yellowish-green you've got to get them sprayed quickly,” says Zmitko. “Otherwise, they'll be too far gone and you'll lose your potential for yield.”

In the late 1990s when glyphosate-tolerant Roundup Ready soybeans first became available, Zmitko tried to tankmix Mn with Roundup in order to save foliar application costs.

“Sometimes Roundup wouldn't work well and other times the Mn wouldn't work,” says Zmitko. “It was never consistent.”

Zmitko now applies Mn separately from Roundup applications and treats his susceptible soybeans up to four times per season. He says the $5-6/acre treatment cost is an economical alternative to the severe yield losses that occur when extreme deficiency strikes.

Research by retired Purdue University researcher Don Huber observed that postemerge foliar treatments of glyphosate herbicide may actually cause or exacerbate Mn deficiencies or limit yield in glyphosate-tolerant varieties grown in Mn-deficient soils.

Huber's research prompted several additional trials, but not all results have been consistent. “We did not see any antagonistic relationship with Roundup and Mn in terms of yield loss in some trials in southern Illinois,” says Fernandez, who plans to conduct more studies in the near future.

PURDUE'S CURRENT cropping systems Extension Specialist Tony Vyn has conducted Mn studies for the past three years. His team's research, supported by the Indiana Soybean Alliance, looked at foliar vs. starter fertilizer applications of Mn (foliar appeared superior to soil-applied), co-applications of Mn with zinc and boron (yield results were more likely to be positive at the same rate of Mn when the three micronutrients were applied together) and how various foliar Mn products and timing situations affect upper trifoliate leaf Mn concentrations.

Vyn's results also differed somewhat from Huber's earlier work. “Postemergence applications of glyphosate had no significant impact on the soybean's ability to take up Mn when those soybeans were grown in weed-free situations,” says Vyn. However, only one formulation of glyphosate (Roundup WeatherMAX) and a single rate of glyphosate were used in each application at Vyn's three research sites. Vyn says at certain glyphosate rates and formulations, Mn response in soybean fields may still be affected.

Questions on interactions between glyphosate and Mn when they are mixed together in spray tanks also persist. Last summer, Boring ran several trials at Zmitko's farm and one other location comparing yield responses from various protocols of Mn applications. He compared various rates of supplementation and tankmixing glyphosate and Mn (both chelated Mn-EDTA and the salt form MnSO4) together vs. applying Mn three days before glyphosate.

Regardless of the formulation, Boring saw better yields and Mn absorption when the Mn and glyphosate were applied separately.

But not all researchers are convinced it is worth it to growers to make separate trips across the field to apply the two materials.

Robert Mullen, Ohio State University Extension soil fertility specialist, has evaluated various foliar Mn applications on glyphosate-tolerant soybeans at two different Ohio sites over the past three years. (See table for results.)

“If a producer has had a confirmed Mn deficiency in the past — with a tissue test — and symptoms are becoming visible, then we recommend foliar Mn,” Mullen says. “At this point, I am not differentiating between the two formulations, with regard to which is better. I would recommend tankmixing over delaying application, assuming the producer has a good pre-emerge herbicide program and is not dealing with serious weed issues.”

Purdue's Vyn says the jury is still out on tankmixing vs. separate applications. “I think, bottom line, we need much more research on tankmixing different Mn formulations with different glyphosate formulations for soybeans at different stages of development in order to ascertain the pluses and minuses of tankmixed applications,” he says.

Vyn says the best way to avoid Mn deficiencies is to avoid creating areas in fields with high pH levels (above 6.6) by using variable-rate technology for applying lime. “The best way to avoid Mn deficiencies on soybeans, whether they are Roundup Ready or not, is to avoid over-application of lime. It is those high-pH situations that lead to deficiency symptoms, especially as soil-available Mn declines,” concludes Vyn.

GLYPHOSATE WAS CHELATING AGENT

Michigan State's Tim Boring explains that glyphosate was first developed as a chelating agent, so its molecules are quick to bind with other free cations, including non-chelated Mn molecules. “When you put the Mn with the glyphosate, it renders both of those products less efficient to the plant,” he says.

“Based upon previous work, if growers are concerned with glyphosate efficacy due to tankmixing, a chelated form of Mn (EDTA) would be a better choice than a salt formulation along with conditioning the water with ammonium sulfate,” says Ohio State's Robert Mullen.