Top Crop Manager

Features Agronomy Weeds
In herbicides, timing is everything, too

General rules may not always apply.

November 12, 2007  By Ralph Pearce

37aIn 2000, Dr. Peter Sikkema of Ridgetown College, University of Guelph, joined
with Dr. Al Hamill of Agriculture and Agri-Food Canada at Harrow, Ontario, to
begin work on defining optimum timing for herbicide applications. After four
years of field work, with 20 plots in corn and 16 plots in soybeans, their data
suggests there may be no general rule of thumb when it comes to defining the
optimum timing for herbicide application.

This work is part of Sikkema's recent research into defining yield loss and
the associated dollar values as a function of herbicide timing. But this research
has found that weed management, and the appropriate herbicide application timing
is highly field specific. "The whole objective when we started was that
we wanted a simple rule of thumb, where we could go to growers and say, 'If
you're going to be growing Roundup Ready corn or Roundup Ready soybeans, you'd
better get your herbicide on by 'this' stage'," relates Sikkema, adding
that in reality, every field is different and the optimal application timing
is therefore different, as well. "It's crucial that farmers realize that
there's a range of when they should get their herbicide on, and even more important,
if they delay application too long, it can result in significant yield losses."

When these experiments were initiated, Sikkema did not appreciate the impact
of a delay in herbicide application timing on crop yield. Growers need to know
the weed species composition and density in each field. "In some fields,
you can wait up to the seven or eight leaf stage of corn and not incur any yield
loss at all," says Sikkema. "But there has to be a significant education
effort in terms of helping growers understand the factors that influence the
appropriate herbicide application timing."


Why so flexible?
Asked why there is such flexibility, Sikkema points to three key factors. At
the top of his list is the relative time of weed and crop emergence. If weeds
are up at the time of planting or emerge with the crop, competition will be
high, necessitating a pre-emerge or early post-emergence program. "In contrast
to that, let's say you planted your corn on April 25: typically, a lot of our
weeds are not going to emerge until the latter part of May and into June because
they're temperature dependent," says Sikkema. "In that situation,
the corn is going to be up far ahead of the weeds, and with that competitive
advantage, timing isn't going to be as critical."

The next two factors he cites are weed density and species composition. In
the case of weed density, the more weeds there are per square metre, the earlier
the need for spraying. In terms of species composition, it has been determined
that broadleaf weeds like lamb's quarters are more competitive than annual grasses
like green foxtail. "If you have predominantly broadleaf weeds, they will
result in a far greater yield loss than if you have predominantly annual grasses,"
says Sikkema. Determining the appropriate timing for each specific field requires
scouting on the part of growers.

A fourth factor which plays a role is environmental conditions. Good soil fertility
and adequate soil moisture result in a less-competitive environment since there
is enough moisture and nutrients for both the crop and the weeds. "But
if you have dry conditions, those weeds are sucking up the moisture and there'll
be less left for the crop," cautions Sikkema. "That's another time
when you have to be in there spraying earlier."

Although the project's results are not broad in their findings, their impact
is nonetheless remarkable, especially in terms of yield loss and the associated
loss in value. In some soybean fields, the research showed a 1.9bu/ac yield
loss by delaying application by one day (from 23 to 24 days after emergence),
costing the grower $15.20 per acre (based on a price of $8 per bushel). In corn,
a day's delay could cost a grower 0.46bu/ac and $1.61 per acre (based on a price
of $3.50 per bushel). But while Sikkema acknowledges early herbicide application
as a 'good insurance policy', it may not be a wise use of glyphosate, or other
herbicides, depending on conditions from year to year, as well as from field
to field. "We've tried to over-simplify, and a lot of those general rules
of thumb were good in terms of education, as well as from understanding weed-crop
competition," explains Sikkema. "But our data say that in some fields,
you have to be spraying by the first leaf stage of corn, but in others, you
literally don't have to spray until the 10 leaf stage."

A careful balance must be struck
Somehow, the middle ground between sweeping statements like 'spray early' and
'watch your specific field' must be found. Time availability being such a huge
issue for growers, advising they do more scouting or better acquaint themselves
with their weed spectrum on a field by field basis may be easier said than done.

Mike Cowbrough, field crop weed specialist with the Ontario Ministry of Agriculture
and Food at the University of Guelph, agrees that time is a large factor in
establishing that delicate balance. He breaks it down to 'who' is doing the
spraying. "Your considerations are different if you're the person that
does your own spraying, then your approach has to follow Sikkema's advice,"
states Cowbrough. "But if you're getting custom work done, it's maybe a
bit more tricky in that you still have to know those fundamentals like weed
spectrum, density and time of emergence. However, because you're at the mercy
of a custom applicator's schedule, it requires more planning ahead in order
to ensure that the correct application timing is met."

Cowbrough also acknowledges the difficulty in coming up with a simple 'rule
of thumb' when sorting through such a large database of information. "If
you average it out, there's a story to tell, but if you step back from it for
a second and really look at each trial individually, it is all over the map,"
says Cowbrough, agreeing that applying herbicide early as an insurance policy
is becoming a bit overstated. "All of those things like environment and
type of weed and time of emergence have the greatest impact on timing rather
than just the corn leaf stage."

Cowbrough also expresses concern about the use of terms like 'pre-emergent'
and 'post-emergent' and the confusion that occurs differentiating between the
emergence of the weed and that of the crop. He says that if weeds are up at
the time of planting, a herbicide program should be applied as soon as possible.

In closing, Sikkema says in almost every field there is a positive return on
investment in herbicides for weed control in corn and soybeans. To maximize
the return on investment, growers must select the appropriate herbicide and
correct application timing. "We have to get away from making these all-encompassing
blanket statements simply because every field is different, and you have to
manage each field differently." 


Stories continue below