Features Agronomy Corn
Mn antagonism with glyphosate uncertain

Ask Mike Cowbrough about manganese antagonism with glyphosate, and he shakes
his head. He has run side-by-side test strips in fields around the province,
and discussed it at length with growers wanting some direction. But there is
little definitive proof the relationship has any consistency.

In 2003, Cowbrough, field crop weed management specialist with the Ontario
Ministry of Agriculture, Food and Rural Affairs at the University of Guelph,
began his tests in response to growers' frequent questions about mixing the
two. Another reason was the accepted use of manganese as a quick-fix solution
for yellowing soybeans.

The test strips have compared glyphosate alone versus glyphosate plus dry manganese,
with a chelated manganese formulation added to the strips in 2005. "Every
time we've added a dry manganese product, we've seen a reduction in control,"
relates Cowbrough. As for the chelated formulation, he added it to the demonstration
in response to the industry's assertion that it will not antagonize to the same
degree as the dry formulation. "To a certain extent, that's true, and we've
only had one year to experience it, but our results show there's still some
level of antagonism."

Not a scientific study per se
In terms of precise percentages, Cowbrough cannot answer definitively. To start,
the test strips are not part of a funded research project. But in some cases,
the reduction in control is well above 20 percent while in others it is not
noticeable.

At different demonstration sites, he has asked growers to comment on the appearance
of weed control without telling them the specifics of each treatment. "They
always say the dry manganese treatment seems to be a little weaker than the
other," says Cowbrough. "Even on species you would think would be
very sensitive, like pigweed and wild mustard, especially if you push staging,
allowing them to get larger to six inches tall, that certainly makes it worse."

What makes it an interesting phenomenon is the default mentality of growers
when they see yellowing in the leaves. In 2004, Purdue University began researching
the cause, but nothing conclusive has come of it. Cowbrough concedes it may
be a matter of a manganese deficiency in the soils, but it could also be the
result of a doubling or tripling of the rate of glyphosate on overlaps and in
corners. He cites anecdotal evidence of growers who have sprayed manganese and
seen their yellowing soybeans green-up shortly after. "All I know is that
since 2003, we've had more and more calls from growers asking if they can mix
manganese with glyphosate," says Cowbrough. "And my approach is to
see what there is in the literature and do some demonstrations to see what the
possible pitfalls are."

There is little doubt of the variability of the results. Cowbrough notes there
is always a risk of weed escapes when tank-mixing, even in species that are
not normally sensitive. "Does it happen every time? I don't know definitively,"
he adds. "Has it happened every time I've tried it? Yes. But I can't say
for certain if someone adds it, it won't be an issue, all I can draw on is what
I've seen."

And he will continue to run the test strips so long as growers continue asking
questions. The work is generated by growers, and Cowbrough says he believes
he is obligated to show them the impact.

Monsanto seeing much the same thing
For the most part, Brian Legassicke's findings mirror much of what Cowbrough
states. There is evidence of antagonism, but it is variable, and certainly manageable.
"We have witnessed some variability on how much antagonism you might see
between products, but it's been hard to pinpoint and draw any conclusion,"
says Legassicke, technology development specialist for Monsanto in central Ontario.
"In general, we're still talking to growers about the pros and cons."

Legassicke concedes antagonism can be higher in tougher weed species like velvetleaf,
and in the more controllable species that might grow to six inches or higher.
"If it's an easy-to-kill grass or pigweed, it's probably not an issue,
but if it's a ragweed or one of the tougher species, it might show up,"
says Legassicke. "And if it's stressed, like in drought or dry conditions,
it might show up."

One method of overcoming the antagonism is to increase the rate of glyphosate.
In trials, antagonism has been reduced with an extra 0.33L/ac of Roundup WeatherMAX
or 0.5L of the regular glyphosate formulation, but not exceeding the maximum
label rates of 1.33L/ac or 2.0L/ac of the respective formulations.

Ammonium sulphate (AMS) is another way to compensate, helping the glyphosate
form better in hard water by tying up the cations. Legassicke says adding a
rate of 1.0L/ac to 1.5L/ac of liquid or 1.0kg/ha to 1.5kg/ha of dry ammonium
sulphate has reduced antagonism caused by the manganese products in research
trials. He is not certain whether it works as well as increasing the rate of
glyphosate, but he is very certain of the order in which things should be mixed.
Done in the wrong order can negate the impact of the glyphosate. "You have
to put in the manganese fertilizer and then the ammonium sulphate and then put
the glyphosate in last."

The Bottom Line
About five percent of our acreage of soybeans needs to be sprayed with
manganese.

We have some sandy loam fields with areas of high organic matter, muck
soils where we know soybeans suffer from manganese deficiency, and we
can check these quickly. If we see leaves turning white, we spray and
they green right up.

The application cost is not high to spray the areas we need, at $8.50/ac,
so we prefer to use a separate pass rather than risk damage or needing
to increase the rate of glyphosate. Lennie Aarts,
Wainfleet, Ontario.