Top Crop Manager

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Weather stress impacts herbicide tank-mix performance

Flexibility may be required.

November 15, 2007  By Top Crop Manager

Unusually cool and wet conditions in 2004 gave many producers some serious
headaches with herbicide performance. "No matter what herbicide product
you used in 2004, crop safety and weed control were likely issues," says
Denise Maurice, manager for agronomic information extension with Westco in Calgary.
"It was a huge challenge."

The problem in short: plant metabolism, which is sensitive to environmental
conditions, got out of whack in the cooler-than-normal conditions. In these
conditions plant metabolism is slower, which can result in reduced weed control
or crop injury because the plant is not actively growing at the time of application.

Maurice says, "On herbicide labels, the words 'actively growing' ensure
herbicide performance. Actively growing means the plant can metabolize the herbicide.
In some cases, slow plant metabolism can lead to crop injury."


"You want the least amount of herbicide to enter the crop. If it does,
you want the crop to metabolize it very quickly so it doesn't harm the crop,"
says Brent Flaten, Agricore United ACE specialist for southern Saskatchewan.
"Crop damage can occur before the herbicide can be fully metabolized."

Tolerance issues
Spring frost in 2004 also threw a wrench into herbicide applications. When spraying
season arrived in west-central Saskatchewan, so did light frosts and cold weather.
This created issues with crop tolerance and weed control.

"If you have a frost event, you have to wait at least three days to spray,"
says Maurice. "The waiting period allows the crop to recover from tissue
damage; it is critical in terms of herbicide performance.

"Some labels say to spray when conditions are above 15 degrees C, otherwise
control will be reduced and crop injury can occur," she says. "In
the 2004 season, many herbicide manufacturers advised farmers to wait three
days on either side of frost."

In the same way, too much heat can pose similar problems. A very fast-growing
crop can have increased crop injury if it fails to metabolize active ingredients
as quickly as needed.

Flaten says to be careful in adverse weather conditions when trying to remove
weeds from a crop that has similar characteristics, such as wild oats in wheat
or broadleaf weeds in legume crops. "These herbicides depend on slight
metabolic differences under normal growing conditions. Performance is less predictable
if the plants are under high stress," says Flaten.

In the adverse growing conditions of 2004, some farmers may also have had a
few problems with tank-mix performance. As an example, "If a grower planned
to mix a wild oat herbicide into a tank of Frontline (a broadleaf weed herbicide),
any question of performance probably came up in view of late timing and unfavourable
conditions," says Maurice. "Dow AgroSciences was pro-active in responding
to questions. They recommended other tank-mix options."

It is the journey
Brian Wintonyk, customer agronomist with Dow AgroSciences, describes the application
of a herbicide as a journey from the spray tank to the site of herbicide action.
He says it is important to manage as many aspects as possible. That means using
the correct rates and water volumes, as well as staging both the weeds and crops

Check weather forecasts. "If weather conditions are unfavourable for herbicide
applications, and field conditions dictate that a herbicide must be applied,
then performance expectations need to be adjusted," says Wintonyk. Given
those conditions, the decision to apply the herbicide comes down to a management
decision, and making certain tradeoffs.

Regardless of the herbicide, good decisions are key for optimizing herbicide
performance. "For example, if a field has heavy wild oats populations and
weather forecasts indicate cooler temperatures, it may be advantageous to apply
a grass herbicide alone followed by a sequential broadleaf application,"
explains Wintonyk. "This can maximize grass control, since in most situations
any grass/broadleaf weed tank-mix provides three to five percent less wild oats
control than the grass herbicide alone."

Some grass herbicides, though, must be mixed with broadleaf weed herbicides,
so it is important to always check the label and crop.

Those growers who recognized the timing problem in 2004 chose to make separate
applications, rather than use a tank-mix. "Strong benefits such as time
management support tank-mixing herbicides, but we sometimes recommend split
applications to sharpen weed control, especially for wild oats. Wild oats is
a strong competitor and well adapted to cool conditions," says Maurice.

Flexibility also becomes an important consideration. Maurice says in 2004,
InVigor canola was a classic example. The canola was hardly growing when the
wild oats needed spraying, but no other weeds had emerged. "In this case,
farmers were better off with an early wild oats herbicide application such as
Select, and then coming back later with Liberty to ensure performance from each

Given that product labels and tank-mix advice in the Guide to Crop Protection
books published by provincial agricultural departments are based on favourable
growing conditions, growers should seek additional advice under adverse conditions.
A good idea is to access very specific product application advice. Growers can
call agronomists or company sales representatives, or use product manufacturers'
1-800 call centre guidelines. -30-


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