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.