Using glyphosate is complicating the uptake of some minor nutrients

1. By Larry Reichenberger
MISSING MICRONUTRIENTS Using glyphosate is complicating
the uptake of some minor nutrients
T
he use of glyphosate herbicide soil pH or organic matter are high, table on the following page. With no
and Roundup Resistant crops or where soils are sandy or poorly manganese application, the conven-
has made it easier to control drained. A 60-bushel soybean crop tional soybean type out yielded the
weeds, but it may also be mak- removes only a handful of manga- Roundup Ready type by 12 bushels
ing it harder to manage crop nese—about .6 pounds per acre. per acre. Applying five pounds per
nutrients. That’s because this Recent studies in Kansas and Indi- acre of manganese to the Roundup
‘major’ herbicide doesn’t fit well with ana have found that some Roundup Ready soybeans erased this yield lag.
many ‘minor’ nutrients—the trace ele- Ready soybean varieties may be even The application had no positive im-
ments that are part of the delicately more susceptible to manganese de- pact on the conventional soybeans.
balanced system of crop growth. ficiency than conventional soybean “We repeated this study last season
“Glyphosate is an economical and types. “We think the problem lies and found similar results. We got a 4-
effective herbicide, but it can have ex- with the gene that was inserted into 6 bushel per acre increase from man-
tensive non-target effects on nutrient soybeans to give them resistance to ganese applied to Roundup Ready
availability, the soil environment and glyphosate. This gene also changed soybeans grown under irrigation, but
agricultural sustainability,” says Don the composition of a substance ex- no response to the application on the
Huber, professor of botany and plant uded from the roots that helps to solu- conventional type,” says Gordon.
pathology at Purdue University. bilize manganese in the soil so plants Ties that bind. Gordon’s study mea-
Manganese deficiency. Research- can utilize it,” says Barney Gordon, sured the concentration of manga-
ers are just beginning to understand agronomist at Kansas State University. nese in the upper leaves of soybeans
the relationships that exist between In research at the North Central and found there was less than half as
glyphosate and micronutrients. Com- Kansas Experiment Station that he much in the resistant variety when no
plications have been most evident for manages, Gordon compared the re- fertilizer was applied. This difference
manganese. Manganese is the most sponse to manganese of the conven-
common micronutrient deficiency tional and Roundup Ready versions Above: Manganese may be a limiting factor
in soybeans in the eastern cornbelt (isolines) of the same soybean variety. for growers trying to maximize the yield of
and is common in other areas where Results of that study are shown in the Roundup Ready soybeans..
THE FURROW 23

2. MANGANESE BOOSTS BEANS
Variety
KS4202 KS4202 RR
Mn Applied Yield Concentration Yield Concentration
lb./acre bu./acre ppm bu./acre ppm
0 76.9 75 64.9 32
2.5 76.1 80 72.8 72
5.0 74.9 92 77.6 87
7.5 72.6 105 77.6 95
micronutrients into usable forms. Above: Research at Kansas State indicates
“Glyphosate moves readily from Roundup Ready soybeans may benefit more
“Glyphosate can have ex- plant leaves to the roots and into the from an application of manganese fertilizer.
tensive non-target effects on soil. Once there, it immobilizes solu-
nutrient availability.” ble manganese in the soil and can also tilizer when weeds are small (2 inches)
—Don Huber reduce the population of organisms and conditions are good. When weeds
needed to make manganese and iron are larger and conditions less favor-
available for plant uptake,” he says. able, use only a chelated form of the
quickly narrowed with the addition of Huber points out one study that micronutrient fertilizer to minimize
only 2.5 pounds of manganese. showed the number of manganese- the antagonism. Under tougher condi-
Huber says the glyphosate resis- reducing organisms present in a gram tions, the herbicide and fertilizer ap-
tance in genetically modified corn of soil was reduced by nearly 90% plications should be separated. In all
and soybeans reduces manganese up- following an application of glypho- cases, ammonium sulfate should be
take and efficiency by 10 to 50%. “The sate. In another, where only 2.5% of used with the application.”
impact varies by variety or hybrid and the recommended application rate of Management strategies. Researchers
the uptake of iron and other nutrients glyphosate made its way into the soil, are finding that, not only should mi-
can also be reduced,” he says. manganese uptake was cut by 80% cronutrients and glyphosate often not
Researchers suspect several other and iron uptake by 50%. be combined, their application should
reasons for the micronutrient uptake Yet another conflict between even be separated by more than a
problems associated with Roundup micronutrients and glyphosate week. “We found that normal uptake
Ready crops. One of those hinges can exist in the spray tank. Be- and translocation of foliar applied
around glyphosate’s role as a che- cause deficiency symptoms for manganese didn‘t occur until at least
lating agent—a chemical that binds micronutrients like manganese eight days after glyphosate was ap-
tightly to other mineral elements. tend to appear about the time plied. In one study with zinc on corn,
“It was first patented as a chelating growers would be applying glypho- the highest yield wasn’t obtained until
agent, not as a herbicide,” says Kurt sate on Roundup Ready soybeans, the micronutrient was applied 15 days
Thelan, agronomist at Michigan State many would prefer to apply a foliar after glyphosate,” says Huber.
University. “It wraps around other fertilizer in the same operation. How- Huber says the greatest conflicts be-
molecules or cations and makes them ever, recent research has shown that tween glyphosate and micronutrients
unavailable. On soybeans, an applica- the effectiveness of both the glypho- occur where soil fertility levels are
tion of glyphosate can temporarily tie sate and some micronutrient fertilizers low or other conditions limit nutrient
up manganese that is in the leaf tissue are reduced when they’re tank-mixed availability. “Crop sequence, tillage,
and cause the ‘yellow flash’ symptoms and applied together. soil pH and the form of nitrogen used
often reported by producers.” “The antagonism between glypho- are also factors. Some varieties or hy-
Other complications. Huber adds that sate and manganese fertilizer can brids are also more efficient in nutri-
glyphosate can also pose problems be- impact weed control,” says Thelan. ent use. However, it may be necessary
cause it is toxic to soil microbes—tiny “Generally, there’s no problem tank- to abandon glyphosate and seek alter-
organisms whose job is to transform mixing most forms of manganese fer- native methods of weed control.” m
22 THE FURROW