Manganese deficiency in soybeans appears to be a growing issue in areas with high-pH soils (6.5 and up) and/or higher organic matter content and where glyphosate-resistant soybean varieties have been planted. Deficiencies have been reported in Indiana, Michigan, Kansas and Wisconsin.

One of the most limiting factors to high yield in glyphosate-resistant soybeans is a suspected micronutrient deficiency resulting from applications of glyphosate to soil, weeds and to glyphosate-resistant soybeans, report Shawn Conley and Carrie Laboski, soil specialists, University of Wisconsin (UW). They are conducting a study after numerous inquires about the issue last year.

“It's important to know that their research applies most appropriately to soils that have borderline or deficient levels of manganese (Mn),” writes George Rehm, University of Minnesota nutrient specialist, retired. “This issue is not one we can paint with a broad brush. It's important to understand that soils differ greatly across the Corn Belt. It is difficult to take data from one state and transfer it to another.

“The response to Mn in Indiana might be expected because soils in the northwest and northeast part of that state have low Mn levels,” Rehm says. “In Kansas, the responses were reported in production environments with yields of roughly 70 bu./acre.”

Rehm did not find a response to Mn among glyphosate-resistant soybeans in Minnesota.

THE UW RESEARCHERS will post preliminary soil and leaf tissue test data from this year's large-scale field experiments on Conley's Web site (www.coolbean.info) this fall. This is the first year of a two- to three-year study funded by the Wisconsin Soybean Marketing Board.

Glyphosate changes the soil microflora affecting Mn availability and is a strong micronutrient chelator to make Mn less available in soil, explains Don Huber, professor emeritus, plant pathology, Purdue University, who researched the issue for 16 years before he retired last fall. The herbicide inhibits Mn uptake by plants through inhibition of several enzymes necessary for micronutrient uptake, Huber says. He adds that glyphosate immobilizes Mn and other micronutrients in plant tissue. This reduces the efficiency of Mn.

Huber found that the presence of the glyphosate-resistance gene also significantly reduced Mn efficiency by the plant even without glyphosate being applied.

Mn deficiency symptoms can emerge in the V4-V6 growth stages. It can be easily confused with other micronutrient deficiencies, disease or weather stress (accentuated by drought) or can be missed entirely. Tissue analysis, however, can help.

Mn levels in southwestern Ontario soil have dropped from 30 to 40 ppm to single digits over the last five years, notes Greg Patterson, president, A&L Canada Laboratories, London, Ont., whose company conducts soil and tissue analysis throughout Canada. This is tracking where glyphosate is used the most, he says.

Growers can manage the issue by applying foliar Mn at least eight days after glyphosate application or by applying Mn sulfate at planting, report Conley and Laboski.

Changing cultural practices also can have a large impact on Mn availability (firm seed bed, crop sequence, source of nitrogen, etc.), Huber says.

“Quite often, a better response to Mn deficiency is achieved when Mn is applied with zinc, since the latter is more mobile in the plant and more effective in physiologically detoxifying glyphosate in glyphosate-resistant plants,” Huber says.