Manganese Fertilizer and Glyphosate Antagonism

Manganese (Mn) deficiency in corn and soybean is common in high pH soils, particularly under drought conditions.¹ High organic matter soils are also associated with Mn deficiencies.¹ Under high moisture situations, Mn deficiency symptoms may improve or disappear. Soybean tends to be more susceptible to reduced yield potential compared to corn when Mn is deficient.

In soybean, Mn deficiency symptoms appear as interveinal leaf yellowing with dark green veins and are most pronounced on the newest leaves (Figures 1 and 2). However, if fields were not scouted when symptoms first occurred, lower leaves may show symptoms. Because of this, Mn deficiencies can be confused with magnesium (Mg) deficiencies, which usually appear on lower leaves with the upper leaves remaining green. Magnesium deficiency also produces interveinal leaf yellowing with darker veins (Figure 3).³

Management of Manganese Deficiency

The best procedure for managing Mn deficiency is applying a foliar application of an Mn micronutrient solution. The best option is applying manganese sulfate with 28% Mn, which can provide 1 to 2 lbs actual Mn in 30 gallons of water.³ Research has shown soybean yield is decreased about 5% for each week an Mn application is delayed after deficiencies appear.¹

Since foliar Mn application timing coincides with herbicide weed management, many producers consider tank mixing Mn micronutrient solutions with an herbicide, which in most soybean fields is a glyphosate herbicide product.⁴ In practice, tank mixing Mn products and glyphosate may reduce weed control. Michigan State University research showed weed control may be antagonized when glyphosate and Mn products are tankmixed.⁴

Common lambsquarters and velvetleaf control were greatly reduced when glyphosate was mixed with ethylaminoacetate-manganese, and less so with lignin sulfate-manganese and MnSO4 powder (by 10 to 30% compared to glyphosate alone).⁴ An EDTA-manganese formulation enhanced glyphosate efficacy (25% in giant foxtail and 40% in velvetleaf).⁴

Antagonism Avoidance Methods

1. Apply Mn foliar fertilizers separate from glyphosate. Michigan State research demonstrated that separating the product applications enhanced giant foxtail control. However, velvetleaf control was reduced by 15% when the Mn application occurred two days before the glyphosate application.⁴ In comparison, the 15% reduction was less than the 30% reduction when ethylaminoacetate-manganese and glyphosate were tank mixed.
2. Add an adjuvant to the spray tank that helps prevent antagonism. An excellent option is adding ammonium sulfate (AMS) at 17 lbs per 100 gallons of water. The AMS prevents Mn from binding to the glyphosate molecule. Another option is using chelates such as EDTA or citric acid; however, depending on the Mn product, weed control efficacy varied.⁴

In summary, tank mixing AMS with an Mn product and glyphosate is advisable when weed pressure is low. If weed pressure is moderate, the use of AMS plus a chelated Mn product may be considered. Under heavy weed pressure, Mn and glyphosate should not be mixed.⁴

Sources:

¹Camberato, J., Wise, K., and Johnson, B. 2010. Glyphosate – Manganese interactions and impacts on crop production: The controversy. Purdue Extension Weed Science. Purdue University. https://ag.purdue.edu/btny/purdueweedscience/wp-content/uploads/2021/02/Camberato-Glyhosagte-Manganese-Interactions-2010.pdf

²Holshouser, D. 2015. Manganese deficiencies in soybean. Virginia Ag Pest and Crop Advisory. Virginia Cooperative Extension. Virginia Polytechnic Institute and State University. https://blogs.ext.vt.edu/ag-pest-advisory/manganese-deficiencies-in-soybean/.

³Silva, G. 2013. Magnesium versus manganese: Supplemental sources and application methods – Part II. Michigan State University Extension. Michigan State University. https://www.canr.msu.edu/news/magnesium_versus_manganese_supplemental_sources_and_application_methods_par

⁴Thelen, K. 2008. Tips to avoid weed control antagonism when applying manganese fertilizer with glyphosate. Michigan State University Extension. Michigan State University. https://www.canr.msu.edu/news/tips_to_avoid_weed_control_antagonism_when_applying_manganese_fertilizer_wi/.

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