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Crop rotations with herbicide tolerant crops

Save your best herbicide option for the toughest problems.

November 29, 2007  By Donna Fleury

Herbicide tolerant (HT) crops offer growers a greater number of options for
weed control. However, HT crops add some complexity to rotations, since there
are crop and herbicide rotation interactions that need to be considered before
and after herbicide tolerant crops are planted. The key is careful planning
of both crop and herbicide rotations to get the best control of weeds without
any surprises in terms of volunteer crops the subsequent year, subsequent crop
injury or increasing the risk of developing herbicide resistance in weeds.

"About 95 percent of the canola grown in western Canada today is herbicide
tolerant, so growers are obviously finding it to their advantage," says
Keith Topinka, research associate with University of Alberta. "HT crops
can be used to clean up a weedy field, whereas before, canola had to be planted
on the cleanest fields. Now growers have good herbicides that will clean up
a field and with systems like Roundup Ready canola, growers can also control
more perennials in-crop as well."

Roundup Ready or Liberty Link canola are good choices after Clearfield

The first step in determining whether or not herbicide tolerance is needed
is to assess the weed spectrum on individual fields. If a HT crop is selected,
the next step is to decide which system is the best choice for each situation.
"Growers also need to look down the road as to whether or not the herbicide
they apply in-crop will lead to any problems in the future," explains Topinka.
Normally there are not any problems, but producers need to be aware of them.
"You need to look ahead to make sure the crop you plant in one year won't
become a volunteer problem the next."

For example, with a Roundup Ready canola system, the volunteers will not be
controlled with a pre-seed glyphosate burnoff the following spring. By adding
a Group 4 product, such as 2,4-D or MCPA as a tank-mix to Roundup, or other
products such as Pardner or Pre-pass, then the Roundup Ready volunteers can
be easily controlled. "Products such as 2,4-D or MCPA are very inexpensive
to apply and will provide control as a pre-seed burnoff," says Topinka.
"Many growers rotate to a cereal crop after canola, which have inexpensive
in-crop controls for volunteer Roundup Ready volunteers as well."

With the Clearfield canola system, growers have to be concerned with herbicide
residues the following year, particularly if using Pursuit. "Sticking to
the label recommendations is the best practice," says Topinka. "If
growers are applying Odyssey or Pursuit, they have to make sure they are not
planning to rotate to a sensitive crop in the subsequent year. Following Clearfield
canola with peas is not a good rotation, as Clearfield volunteers will not be
well controlled in the subsequent crop."

Aside from residues, there are a number of other issues that will affect a
grower's choice of crop and herbicide rotation. "The main thing growers
should think about with herbicides is changing the mode of action to help reduce
the risk of weeds developing resistance," says Murray Hartman, oilseeds
specialist with Alberta Agriculture, Food and Rural Development (AAFRD). Rotating
herbicide groups, or using tank mixtures of different groups can help reduce
the chance of developing herbicide resistance.

"Changing the modes of action and applying herbicides at different times
helps to keep the weed community more off balance," says Hartman. "It's
really the diversity of practices, such as different timings and methods that
keep the weeds off balance that improve control and help reduce the risk of
herbicide resistance."

There are other crop management tools that assist with weed management, such
as timing of seeding, seeding rates and types of crop rotations. Including other
crops such as winter cereals, forages or silage in rotation changes the selection
pressure on weed species and allows for the use of different herbicide groups
and timings.

Considering which herbicides will give the best advantage in terms of crop
rotations is also important. For example, Group 2 herbicides are important in
cereal crops for weeds such as cleavers, chickweed and others. "If you
want to have peas and canola in a very short rotation, then you need to determine
where the Group 2 is going to be the most critical," explains Hartman.
"If using the Group 2 is most critical in your pea crop, then instead of
using a Clearfield canola, select another system so you can avoid using Group
2 in your canola as well."

Concerns over herbicide resistance are increasing, with Group 1 and 2 herbicides
considered high risk. "There are already a number of weeds in Alberta that
have developed resistance to Group 2 herbicides," explains Topinka. Group
2 resistance seems to happen very quickly, often after only six or seven applications.
The Clearfield canola system is the only one that uses a Group 2. "If you
need to use a Group 2 more frequently in rotation, then hopefully you can spray
that Group 2 in a tank-mix with another group to reduce the risk of developing
resistance." For example, a Group 2 resistant kochia will require a tank-mix
with a Group 4 product to be controlled.

Group 1 herbicide resistance is particularly a problem in wild oats. "We
have a lot of herbicide resistance in wild oats, especially in Manitoba, so
growers require other options in-crop for wild oats and volunteer cereals. In
cereals, the Group 2 product Assert offers control, however, some wild oats
are also resistant to it." Repetitive herbicide use can eventually result
in a build-up of resistant weed populations. "The key is to look three
or four years down the road, and reduce your reliance on Group 1 and Group 2

The risk of glyphosate resistance is lower than for Group 1 or 2 products and,
so far, glyphosate resistant weeds have not been confirmed in Canada. "However,
because we are using glyphosate so frequently, logically it's only a matter
of time before we do have some glyphosate resistant weeds appearing," says
Topinka. There are three or four species of weeds in the US where glyphosate
resistance has been documented. "We have some similar weed management practices
in Canada and, therefore, can eventually expect to see the same thing here with
our widespread use of glyphosate in zero-till, pre-seed, in-crop and pre-harvest
applications. As with other herbicides, mixing glyphosate as a tank-mix with
another herbicide will delay the development of any herbicide resistant patches."

Clearfield wheat adds to the puzzle
Adding to the complexity of HT crop rotations, Clearfield (CL) wheat has become
another option for some producers. The Clearfield system includes the herbicide
Adrenalin, which is a combination of Imazamox, a Group 2 herbicide, and 2,4-D,
a Group 4 herbicide. Because Adrenalin cannot be applied until the wheat is
at the four leaf stage, a pre-seeding application, typically glyphosate, is

"While a tank-mix of the two herbicide groups is beneficial, it's important
to note that Adrenalin will still not control all Group 2 resistant weeds, as
some weeds on the label are only controlled by Group 2, not Group 4," explains
Topinka. For example, wild oats, green foxtail, cleavers, hempnettle and chickweed
are not controlled by 2,4-D, and if the field contains patches of those resistant
to Group 2, then they will not be controlled by the Clearfield system.

CL wheat is another herbicide tolerant crop that will give producers some options.
For example, it is a good option for controlling barley in wheat. Feed or silage
barley may follow CL wheat, but only glyphosate or a Group 1 herbicide will
provide volunteer CL wheat control. This crop would not be suitable prior to
malt or seed barley production, since some wheat volunteers will emerge after
the pre-seed burnoff. If the crop following CL wheat is peas, then any in-crop
CL wheat volunteers will have to be controlled in-crop with a Group 1 product.
There are registered mixes for Group 1 on peas, however they add an additional
$15 per acre to the herbicide bill that producers need to consider.

Topinka adds, "If you use CL wheat and then follow with CL canola or peas,
the canola and peas will not be injured by the residue, but if you continue
to use these Group 2 products, you will have to carefully consider the crop
rotation for the following year. The key is to save your best herbicide for
your least competitive crop. Liberty Link or Roundup Ready canola, or silage
barley would be better options following CL wheat."

Herbicide tolerant crops are giving producers an advantage, however, they have
added to the complexity of herbicide and crop rotation planning. Other HT crops
may be added to the mix in the future, with Clearfield lentils a possibility
within five years. "Growers can manage HT volunteers through adjustments
to pre-seeding and in-crop applications, good integrated management techniques
and sometimes with the use of additional herbicides," says Topinka. "Proper
management of crops and rotations will be key to avoiding volunteer HT surprises
in subsequent crops, and to reduce the risk of developing herbicide resistance.
Plan your rotations ahead so you can save your best herbicide for your least
competitive crop." 


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