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Group 2 herbicides: where do they fit?

While there is no one way to manage Group 2 resistance, perhaps the answer lies in individual farmer approaches.

November 28, 2007  By Bruce Barker

Two different farms. Two different ways to use Group 2 herbicides. One focus
on managing herbicide resistance. "Every year, most farmers have a problem
child. One field that doesn't pull its own weight or measure up to all the other
fields. NW-17 has been my field. Poor wild oats control two years in a row,
and I was starting to wonder if I had an issue with Group 1 herbicide resistance,"
farmer and agrologist Blair Harris of Yorkton, Saskatchewan, explains as a preamble
to why he tried CDC Imagine Clearfield wheat in 2005 and 2006. "Adding
pressure is that I am an agrologist and the field is beside Highway #16. Everybody
sees it and everybody knows who farms it."

Harris uses Group 2 herbicides for wild oats control in cereals.
Photos Courtesy Of Bruce Barker.

Harris says the Clearfield system with CDC Imagine wheat helped him manage
a potential Group 1 herbicide resistance problem by introducing a different
chemistry group into his rotation for wild oats. He believes it can be a valuable
option for other producers who do not grow pulses or other Clearfield crops.

Further south, a short distance from the Manitoba and US borders, a Carievale,
Saskatchewan, farmer has a different approach to using Group 2 herbicides. "I
try to save Group 2 herbicides for high value pulse crops. That means I use
it one in five years in my rotation," says Maurice Berry, who cautions
that what works for him might not work for others in different areas. Berry
does not use Clearfield crops on his farm, although he sees potential for Clearfield
lentils and Clearfield sunflowers, once they are commercialized.


Two farms offering two different approaches to Group 2 herbicide management.
Both illustrate the thought farmers are putting into managing herbicide resistance.

Bruce Murray, a weed specialist with the Crops Branch of Manitoba Agriculture,
Food and Rural Initiatives at Carman, Manitoba, says farmers have good reason
to put a lot of thought into how they use Group 2 herbicides. Group 2 includes
four herbicide families, the imidazolinones, sulfonylureas, sulfonamides and
triazolopyrimadines. Some of these herbicides control grassy weeds, others broadleaves
and some both.

Group 2 herbicide families and herbicide trade names.
Imidazolinones Absolute, Adrenalin, Assert, Odyssey, Pursuit, Solo
Sulfonylureas Accent, Ally, Escort, Express, Muster, Option, Prism,
Refine Extra, Sundance, Triton, Ultim, Unity
Sulfonamides Everest, K2
Triazolopyrimadines Frontline, Spectrum, Prepass, Trophy

Murray explains that the first Group 2 herbicide was released in 1982 and the
first resistance cases were confirmed only five years later on kochia and prickly
lettuce. By 2002, there were 70 species of weeds with Group 2 resistance worldwide
and by 2005, there were 93 different species showing Group 2 resistance. "Resistance
is the Achilles heel of Group 2 herbicides. I am a big fan of Group 2 herbicides,
they are a good fit for farmers and there is a laundry list of benefits, but
they need to be used judiciously," says Murray.

Murray explains that Group 2 weed resistance appears to be evolving faster
than any other previously documented herbicide group. Currently, in western
Canada there are at least 11 species of weeds that have evolved resistance to
a Group 2 herbicide; two grasses, including green foxtail and wild oats; and
nine broadleaves, including cleavers, chickweed, hempnettle, kochia, ball mustard,
redroot pigweed, Russian thistle, spiny annual sowthistle and stinkweed. In
Manitoba, it is estimated that more than 90 percent of the kochia populations
are resistant to Group 2 herbicides.

"There are a number of reasons for this. Group 2 herbicides are highly
effective and exert a high selection pressure towards resistance. Also, there
are a number of mutations in the ALS enzyme that will confer resistance, and
so the chances of having an initial rare resistant biotype in your field may
be higher than for other herbicide groups. Plus, many of the Group 2 herbicides
have residual properties and the extended weed control serves to increase selection
pressure. Many western farmers have also switched to Group 2 products like Everest,
Assert and Sundance to control Group 1 resistant wild oats," he explains.

Figure 1. Weed resistance risk triangle. Source: Beckie, H.
J., 2006 Herbicide Resistance Weeds: Management Tactics and Practices; Weed
Technology Vol. 20 Issue 3 (July-September) pp. 793-814.

Careful crop rotation planning required
At Carievale, Berry's rotation reflects his careful consideration for both economics
and herbicide rotations. He works with a five year rotation, starting with an
oilseed (InVigor canola or sunflower), followed by a cereal (barley), a pulse
(peas, pinto beans or lentils), another cereal (wheat or oats) and finally flax.
He also likes soybeans, white millet and canaryseed, and fits them into the
rotation when economics are favourable.

"It is a real challenge to find a profitable crop some years and I admit
straying from this rotation when pure economics dictate what crops I feel will
pay the bills. Generally, I believe this rotation is sustainable for our farm
with the weed and disease control programs that we use," he says.

Berry's main use for Group 2 herbicides has been Odyssey on peas and Pursuit
on pinto beans. He previously used Absolute on Clearfield canola, but he has
taken that out of the rotation so that he can save Group 2 herbicides for pulses.
He would like to use Pursuit on soybeans, but it is registered in Manitoba for
soybeans only, and foresees the potential for Clearfield lentils and Clearfield
sunflowers in the future.

"We have cut out all other Group 2 herbicides in the rotation. We feel
that their application in special crops is very important, and take seriously
the notion to do what we can to delay herbicide resistance," explains Berry.
"On our farm, this meant stopping the use of Group 2 broadleaf sprays on
our wheat, oats and barley. And with Clearfield lentils coming on stream, it
is going to be very important to be careful how we use Group 2 herbicides in
order to maintain their long-term use."

Murray cautions that growers need to do more than think about rotating the
imidazolinone family of Group 2 herbicides, or 'imi's' as many people refer
to them. The 'imi' family includes Absolute, Adrenalin, Assert, Odyssey, Pursuit
and Solo herbicides.

Berry saves Group 2 herbicides for his high-value pulse crops.

"It is not just 'imi' use that we need to be careful with. Growers need
to consider all of the Group 2 herbicides together. If you use Everest, in the
sulfonamide family of Group 2 herbicides, for wild oats control, you could be
selecting for not only wild oats resistance, but wild mustard resistance, as
well. That resistant wild mustard could be resistant to some or all of the other
members of the remaining Group 2 herbicide families," cautions Murray.
"The same can be said for the sulfonylureas like Refine Extra and Sundance.
Weeds resistant to the sulfonylureas may also be resistant to some of the 'imi's'."

For Harris, who does not grow peas or Clearfield canola, the CDC Imagine wheat
Clearfield system has become a valuable way of introducing Group 2 herbicides
into his cereals-canola rotation so that he does not have to rely as much on
Group 1 herbicides for wild oats control. His typical rotation is driven by
canola, followed by barley, wheat and then back to canola. He has started to
cut back on barley, replacing it with either oats or winter wheat. In any one
year, he has one-third of his acres in canola and hard red spring wheat. "The
way I see it on my farm, CDC Imagine is a way to get Group 2 herbicides into
the rotation for wild oats control. I was starting to get backed into a corner
without it," he says.

A side benefit of the Clearfield wheat crop is the volunteer barley control.
Harris receives a premium from Saskatchewan Wheat Pool because there is no volunteer
barley in the crop. It can add up to $10 per tonne, depending on the level of
inputs he buys from SWP. He does not give up any yield either, with his CDC
Imagine wheat yielding about 53 bushels per acre of #2 CWRS wheat in 2005: the
same as two other varieties of wheat that do not carry the Clearfield gene.
His 2006 crop was just as successful with his CDC Imagine wheat running the
same as Splendor wheat and with protein levels of at least 15.5 percent.

Still, Harris has to be careful not to get Group 2 herbicides into his rotation
too often. For example, with a rotation heavy in cereals, he has to ensure he
does not use Group 2 broadleaf herbicides too often, either.

How often is too often?
With the ability to use Group 2 herbicides every year for many different weeds
and crops, and in the Clearfield system of canola, wheat and lentils, researchers
and manufacturers are looking at recommendations for Group 2 usage. BASF, the
developers of the Clearfield technology, has laid out its Clearfield Stewardship
program, which is designed to help ensure the sustainability of the technology.

Robert Hornford, a BASF technical specialist at Winnipeg, Manitoba, explains
that the company recommends an integrated approach to managing Clearfield technology
and Group 2 herbicides, which includes resistance management, control of herbicide
tolerant volunteers, and managing out-crossing to non-Clearfield crops and weeds.

BASF's recommendations are to not exceed a maximum of two exclusive Group 2
herbicides on any one field in any four year period. Hornford also says that
growers can separate out grassy and broadleaf Group 2 herbicides when setting
up rotations. Controlling Clearfield volunteers and choosing herbicides with
multiple modes of action are also key strategies.

"In the stewardship program, we define exclusive use depending on the
weed you are targetting. If you are going after wild oats, you could use a Group
2 on wild oats two in four years, for example," explains Hornford. "But
if you were targetting a broadleaf weed in the other years, and that broadleaf
herbicide doesn't control grassy weeds, then that would be counted separately."

Herbicide rotations for Group 2 herbicides become difficult to manage when
many of the products control, or have activity, on both grassy and broadleaf
weeds. Murray says a farmer has to keep in mind that the use of a Group 2 herbicide
could be selecting for both grassy and broadleaf weed resistance. For example,
Odyssey has activity on both grassy and broadleaf weeds, and can select for
herbicide resistance in both weed types.

Previously, weed experts had recommended no more than a one-in-three herbicide
rotation. More recently, weed scientists have moved towards a risk assessment
when considering herbicide rotations. In provincial herbicide guides for 2007,
rather than focussing on the number of applications in a rotation, the new approach
is looking at the relative risk of a herbicide group developing resistance.
Group 1 and Group 2 herbicides are both rated with the highest risk of developing
herbicide resistance with 10 or fewer applications before resistance may be

Murray says he likes the new risk rating system because it puts more knowledge
into the hands of growers, rather than just picking a number out of the air.
"Is it one-in-four, one-in-three, who's to say for certain? I remember
those discussions and it is difficult to pin down a certain number," he
explains. "I like the new system because it allows growers to assess their
own risk based on their herbicide use and crop rotations."

And that is exactly what Berry and Harris have done. For Berry, saving Group
2 use in his high-value pulse crops helps him benefit from these technologies.
For Harris, using Group 2 herbicides for wild oats control adds another dimension
to wild oats resistance management. However the technologies are used, implementing
good herbicide rotations is critical in managing herbicide resistance.

Farming with Group 2 herbicides
Maurice Berry provides these tips on getting the most out of Group 2 herbicides
on his farm.

Spray early: Odyssey is to be sprayed on peas at the one to six
above ground node stage. Peas can be at the six above ground node stage
and not appear very tall. We have sprayed on the later side and experienced
crop yellowing. As Odyssey has soil uptake, earlier spraying seems to
be most effective. That will most likely be the same on Clearfield lentils.

Seed heavy: We feel that in peas and pinto beans, a full crop
canopy makes weed control easier.

Use full rate: Odyssey is weak on kochia, but when we seed fairly
heavy and use full rates, we do have acceptable control.

Necessary use of fall glyphosate: A fall burnoff for pea and lentil
crops make weed control easier. Canada thistle can be a real problem in
peas and lentils, and the 'imi' is not going to control them. Because
beans are planted in late May, a mid May burnoff works well for pinto

Pursuit on beans: Pinto beans are not a very competitive crop
and early spraying is a must. Normally we use Basagran on our pintos.
If we had wild buckwheat appear in the beans, Pursuit worked well when
the buckwheat was small in size. We would spray the beans with a grass
killer separately and usually have to do it twice, as there can be a lot
of volunteer cereals appearing late in the bean's open canopy.



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