Tips to avoid weed control antagonism when applying Mn fertilizer with glyphosate

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Manganese deficiency is the most common crop micronutrient problem in Michigan, particularly in soybeans and wheat. The availability of manganese to the plant generally decreases as soil pH levels increase. Consequently, soybeans grown in the calcareous soils of Michigan’s Thumb area and in the lake-bed soils in Michigan’s Southeast Lower Peninsula, historically have been found to be deficient in manganese. When possible, producers in these areas prefer to tank-mix manganese micronutrient solutions with foliar applied herbicides to eliminate an extra trip across the field. As glyphosate is estimated to be used on approximately 80 percent of Michigan soybean acreage, reported reductions in weed control on some of these acres caused by tank-mixed manganese applications is an important issue. The cold, wet weather experienced in May may have exacerbated manganese deficiency problems due to poor soybean root growth. In a project funded by the Michigan Soybean Promotion Committee, MSU researchers have shown that weed control can be antagonized when some manganese fertilizers are tank-mixed with glyphosate.

Field experiments showed a significant antagonism in common lambsquarters and velvetleaf, when glyphosate was tank-mixed with ethylaminoacetate- manganese, and slight antagonisms in tank-mixes with lignin sulfate- manganese and MnSO4-powder. Greenhouse studies have shown that an EDTA- manganese formulation enhanced glyphosate efficacy (by 25 percent in giant foxtail and 40 percent in velvetleaf). However, tank-mixes of each of the other manganese products caused significant antagonisms, reducing weed control by 10-30 percent when compared to glyphosate alone.

One possible method for avoiding the antagonism is to apply the manganese fertilizer at a different time than glyphosate. In a greenhouse study, the ethylaminoacetate- manganese fertilizer was applied six, four and two days before-, two days after-, and at the same time as glyphosate. There were no differences in herbicide efficacy in giant foxtail for any of the two-pass timings. In velvetleaf, manganese sprayed two days before the glyphosate application reduced weed control by 15 percent. But this was not as severe as the 30 percent reduction when glyphosate and ethylaminoacetate- manganese were tank-mixed.

A second method for avoiding the antagonism is to add an adjuvant to the spray tank that will prevent the antagonism, either by chelating the manganese tightly (with chelates such as EDTA or citric acid) or by preventing the manganese from binding to the glyphosate molecule (by adding ammonium sulfate). In greenhouse experiments, ammonium sulfate and EDTA improved herbicide efficacy in all glyphosate- manganese tank mixes except for EDTA- manganese, where efficacy remained the same. Citric acid antagonized the manganese -EDTA-glyphosate tank-mix, but improved the efficacy of all other manganese formulations. The degree that each adjuvant improved the different manganese -glyphosate tank-mixes varied, and will be the subject of future studies.

In summary, if conditions are optimal for glyphosate control of weeds, i.e. weed pressure is low and weed size is small, you may get adequate weed control with glyphosate tankmixes with commercially available manganese formulations. However, even under these optimal conditions it is still advisable to add ammonium sulfate (17 lbs per 100 gallons of water) to minimize the antagonism imposed by the manganese. If weed control conditions are marginal, i.e. moderate weed pressure or moderate weed size, a chelated form of manganese such as an EDTA- manganese formulation, plus ammonium sulfate is recommended with glyphosate tankmixes. Finally, if weed control conditions are poor, i.e. heavy weed pressure (especially lambsquarters and velvetleaf) or large weed size, manganese should not be tankmixed with glyphosate. Rather, in these tough weed control conditions, manganese should be applied in a separate application following the glyphosate application.

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