Features Agronomy Weeds
Residual weed control an obvious advantage

In an era where time continues to be a rare and valued commodity, the arrival
of a herbicide which offers improved residual weed control would be greeted
without question. The savings in time, fuel and adverse agronomic impacts would
make that choice an easy one.

But there is more that should be said about the fundamentals of weed control:
a good herbicide is a welcome tool, but part of the toolbox where it is kept
is understanding more about conditions in a particular field.

In the past few years, growers have been encouraged to gain a greater awareness
of the weed spectrum in their fields. Fuelled by weed shifts and a growing reliance
on glyphosate and glyphosate-resistant weed species now on the rise in the US,
weed specialists have been urging growers to pay stricter attention to what
is competing with their crops. Once they have that base understanding, they
can target specific weeds with specific spray programs.

New product to market
In the spring of 2005, DuPont Canada launched Guardian, a revision of sorts
to the chlorimuron-ethyl or Classic molecule. Among its attributes, Guardian
provides solid residual weed control, a particular advantage when considering
options in a glyphosate-tolerant system. "In those fields, you're looking
at a totally non-residual system," says Dave Kloppenburg, a field agronomist
with DuPont Canada, based in London, Ontario. Residual weed control is as important
as ever, he adds, and for two reasons. "One would be the fact that you
may have to make multiple passes in a Roundup Ready system, so having a residual
component marries those two types of technologies."

Another advantage is the shift towards later-germinating weeds such as velvetleaf
and dandelion. Guardian's residual component would provide that extra overlap
effect, beyond glyphosate.

But Guardian holds several other advantages, including convenience and the
potential for better time management. "Guardian also complements glyphosate
on some of the hard-to-kill weeds, like yellow nutsedge, dandelion and sowthistle,"
says Kloppenburg. The impact on prickly lettuce and wild carrot are being investigated,
as well. "Wild carrot is an up and coming weed and I see it more with farmers
that are not only in a Roundup Ready system, but also a lot of no-till, and
that's one particular fit where Guardian has helped."

Timing is becoming more important
For Larry Blaney, Guardian answers two concerns he has regarding his own Milton-area
farm. His first pertains to glyphosate tolerant soybeans and the potential for
resistance developing in the near-term. The problem is creeping closer to Ontario,
as glyphosate-resistant Canada fleabane is now in northeastern Indiana and northwestern
Ohio. "That's why I started to look for something that had an extra component
in there to alleviate the resistance problem," says Blaney.

In 2005, he also switched from seven and a half inch rows for his soybeans
to 15 inch rows. "I wasn't certain how they'd canopy for crop protection
and for crowding out the weeds, so I wanted to spread my risk and attempt to
find a product that might allow me to go through once rather than twice, with
glyphosate applied in-crop with the soybeans," explains Blaney. "The
weed control's excellent and I was really quite happy with the way that it got
the weeds and its residual component."

The timing issue is of particular interest to Blaney. Since he relies on a
custom sprayer, the window for applications becomes that much tighter and for
his operation, Guardian provides that added option. "Here in south Halton,
our windows of opportunity are compressed even more," he notes, adding
that an untimely rain can cause a grower to miss that window altogether.

Know where to grow
But the bottom line on any chemistry is the 'knowledge is power' approach. As
the reliance on glyphosate continues, weed specialists urge greater familiarity
with field conditions, weed species and the right chemistry for the right weeds.

Mike Cowbrough, field crop weed specialist with the Ontario Ministry of Agriculture,
Food and Rural Affairs at the University of Guelph, reminds growers that while
Guardian complements glyphosate, it is but another effective tool. "I've
witnessed demonstration trials where Guardian, especially when you get into
wild buckwheat and lady's thumb, can provide residual value such that you wouldn't
have to go in with a second pass," cites Cowbrough. "But as a producer,
you have to know what you have in your field. If you have massive annual grass
pressure and you're going in with Guardian thinking you are going to get residual
control, you're likely going to end up spending more money."

Cowbrough's concern is that residual weed control is dependent on various factors,
including timing, weed species and climate in a given year. "Let's go back
to the strength of the product from the weed control perspective," he suggests,
pointing to Guardian's impact on wild carrot and nutsedge. On the former, Guardian
has its value, but on nutsedge, the impact can be more of a rate response, leaving
it open to debate that various rates or formulations of glyphosate are more
effective. "But part of the residual value is going to be there on species
that are on the label, where there's added effect."

Again, he stresses the bottom line is knowledge, particularly what weed species
are present in a field, combined with the weeds listed on the label.