Weed resistance is Topic Number One on this year's
winter meeting circuit. The onset of triazine
resistance is serious enough, let alone resistance
to other herbicide families.
But weed resistance may be overplayed. All those
weed escapes after harvest you see on the back
roads may be 'weed misses' rather than resistant
weeds.
Weed misses are weeds that have escaped because the
wrong herbicide was used. Using that herbicide
again the following year allows all the weeds that
got away last year to seed and spread even more.
In the 1960s, farmers used the miracle product
atrazine to clean corn fields. On our own farm at
home, I remember the year when we had all sorts of
weeds in corn field we thought had been clean the
year before. We had annual grasses escape. And in
the 1980s, when I was called to a farm because
Bladex did not control the foxtail, I was pretty
certain that most times I would find that barnyard
grass, not foxtail, had escaped. Bladex still does
not control barnyard grass well.
Some of the new herbicides control certain weeds
better than others. If you continue to use the same
herbicide year after year, the weeds that are only
partially controlled will build up. Add to this the
dramatic change to less tillage, more perennial and
biennial weeds, and there is a great chance for
weed escapes with continued use of the same
herbicide.
Continuous cropping with the same herbicides is
inviting disaster. A recent article featured a U.S.
grower who had resistance develop to a
"newer-to-Canada" herbicide. But when interviewed,
he revealed he had planted soybeans on the same
field for four years and used the same herbicide
each year. In some ways that grower got what he
deserved. He had created a beautiful situation for
weed resistance to develop. It was the weed
resistance that got him all the press, but if the
field had been taken over by weed misses, there was
no story. And that is what is occurring on many
fields. Guess what his solution to his weed
resistance was? Crop rotation.
For Ontario growers, the solution is careful
scouting of fields, noting in particular weed
escapes that are not on the label of the product
you are using. These weeds are more apt to cause
grief than the weeds that are on the label and not
being controlled, especially as you practise more
monoculture as in soybean production.
Applying tank-mixes of herbicides in case
resistance develops is too costly. Besides, if your
neighbours aren't following the same strategy you
will get their seeds anyway.
Pat Lynch is a consulting agronomist with Cargill.

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