Seed & Chemical
Prevent glyphosate resistance now
Glyphosate and Roundup Ready canola crops are valuable tools for growers,
November 20, 2007 By Donna Fleury
Glyphosate and Roundup Ready canola crops are valuable tools for growers, and
it is important to protect this technology. Although globally the risk of glyphosate
resistance is considered to be low, the trend to direct seeding systems and
herbicide resistant canola crops has increased the reliance on glyphosate use.
Although no glyphosate resistant weeds have been found in Canada to-date, some
of the resistant weeds confirmed in parts of the US are the same weeds found
in western Canadian cropping systems. So although the risk of glyphosate resistance
remains low, planning ahead for proper herbicide and crop rotation management
Also, direct seeding and reduced tillage have changed weed communities and
weed control practices. "Glyphosate has worked its way into so many aspects
of our production systems, from pre-seed to in-crop, pre and post-harvest, managing
the use of glyphosate will be the key to the longevity of this valuable herbicide,"
says Denise Maurice, technical development manager for crop protection, Agricore
United at Calgary, Alberta. "It's important to keep track of all herbicides
used in a cropping system and that every application counts."
Glyphosate has brought a lot of benefits to cropping systems and growers should
work towards conserving its use. "The addition of herbicide tolerant canola
crops to our cropping mix has meant big benefits in terms of herbicide options
and rotations," says Murray Hartman, oilseed specialist with Alberta Agriculture,
Food and Rural Development (AAFRD) at Lacombe. "Before herbicide tolerant
canola crops, we were stuck with Group 1 for grass herbicides and Group 3 in
canola, and we have known weed resistance to those products. Now we have other
However, it is important to not rely on just one cropping and herbicide system.
"The key to good management is to throw lots of curve balls at our pests,"
Weeds, for example, get used to one system and certain weeds like to keep occupying
certain niches, whether one seeding date or a certain crop. Therefore, the priority
is to throw diversity at them by changing seeding dates, types of crops, fall
and spring seeded crops. There are various options for preventing glyphosate
resistance, depending on the weed that is the biggest problem in a particular
field. Rotating herbicide groups, or using tank mixtures of different groups
can help reduce the chance of developing herbicide resistance.
"There are alternatives that can be used, integrating other herbicide
groups and tank-mixes that can be good options," says Maurice. "Consider
using tank-mixes like Pre-Pass or Express plus glyphosate to mix up the modes
of action and change the control strategy. Even fall 2,4-D could be explored
as an option when appropriate."
Depending on the weed populations and pressure, change crop rotations and herbicide
tolerant canola system selections. "There are so many different seeding
operations and cropping systems in western Canada, there isn't any way to be
as prescriptive as in the past," says Maurice. Growers must develop the
best crop and herbicide rotations to match their individual cropping systems
and weed control needs. "Save your glyphosate application for where you
need it most."
There are various management options depending on the weed that is the biggest
problem. "If you tend to use glyphosate every year, perhaps there may be
a way to skip a year such as seeding an early seeded barley crop into Roundup
Ready canola stubble," explains Hartman. "You may just need a 2,4-D
pre-seed application for broadleaves and volunteer canola, then hopefully the
barley will compete enough until an in-crop application. Although it's so easy
to just spray with glyphosate, it is important to start thinking about it now
so you don't have to face solving a resistance problem down the road or losing
this valuable technology."
Monitoring and field scouting are key
"Always scout a field after spraying and look for escapes," says Hartman.
"A patch pattern without an obvious linear spray miss is a strong signal
that there could be a problem."
Maurice agrees it is important and adds even if it is not a resistance problem,
it could be a timing issue. "We're finding that the populations of foxtail
barley for example are increasing, which has more to do with timing of application
than anything else. Walk your fields, know what is going on and see where it
could use some adjustments." Nothing else can replace monitoring and field
"One of our key recommendations for managing weed resistance is to use
the right herbicide product at the right rate and at the right time in weed
development," explains Rob Neyedley, technology manager, chemistry and
Roundup Ready crops, with Monsanto Canada. "This is by far the most critical
action growers can take to minimize the risk of weed resistance." In fact,
he says Roundup brand herbicides and Roundup Ready canola have actually helped
manage weeds that are resistant to other herbicide groups for more than 10 years
now in Canada.
Neyedley adds that glyphosate is actually a low risk herbicide for developing
herbicide resistance. "Plus, in western Canada, Roundup brand herbicides
used in Roundup Ready canola are rotated with other herbicide groups in other
crops, which could also lower the risk of weed resistance developing."
When Roundup Ready canola is grown in rotation with cereals, pulses and other
oilseed crops, herbicides with alternate modes of action are used. Neyedley
adds that weed resistance to any herbicide should be taken seriously, but encourages
growers to not over-react. It is still best to get the facts before making final
cropping and herbicide use decisions. He adds that "Although glyphosate
can be applied at multiple times throughout the year, these applications often
target different weed species and populations, which reduce overall selection
pressure for glyphosate resistance."
Maurice notes that although herbicide resistance management used to focus on
the number of years a herbicide was used, Dr. Hugh Beckie, Agriculture and Agri-Food
Canada, has developed a new way of looking at resistance. "He suggests
growers should be counting the number of applications, not the number of years
of use," says Maurice. For example, Group 1 herbicides are classified as
high risk herbicides and resistance has developed after 10 or fewer applications.
Group 9 products, Roundup brand products and other glyphosate products, are
classified as low risk herbicides and resistance has developed after greater
than 20 applications.
Maurice reminds growers to put things in perspective and not overlook the herbicide
resistant management of Group 1 and Group 2 herbicides, where the probability
is much greater. "The potential for glyphosate resistance remains low,
but it's still important to be vigilant." Remember to count every use and,
yes, pre-seed applications count. -30-