Resistant
Weeds!
The list of herbicide resistant weeds, and counties where they can
be found, continues to grow! Since 1997, redroot pigweed, green pigweed,
common ragweed and Eastern-black nightshade resistant to Group 2 herbicides
(such as Pursuit, Pinnacle, Classic, Ultim, etc) have been positively
identified in Ontario soybean fields. Are your fields part of this
dilemma?
If you had weed escapes, which should have been controlled by your
herbicide program, and you cannot find a good reason for the lack
of control, the possibility of herbicide resistance should be explored.
The most common sign of resistance is when all the weed species are
controlled except for one, a weed that should have been easily controlled
by the herbicide program. Resistant weeds often begin in patches or
streaks corresponding to the harvest pattern of the combine. Over
time, resistant weeds will spread across the entire field.
Before declaring a weed resistant, make sure that other explanations
for weed escapes and misses are investigated. Weeds that emerge after
application with non-residual herbicides can confuse the diagnosis.
Some species are naturally more tolerant to some herbicides. Improper
equipment setup, missed adjuvants, poor spray pattern, poor canopy
penetration, improper weed stage at time of spraying or weather issues
can all lead to misses that can be misdiagnosed as weed resistance.
The following guidelines will help manage and/or prevent weed resistance:
- Use crop rotation. Rotate crops and herbicides.
- Avoid the using the same herbicide or herbicides from the same
grouping in the same field, in consecutive years.
- Avoid using herbicides with the same mode of action more than
once per season.
- Use tank mixes where 2 or more products give effective control
against the target weed, and the products are from different mode
of action groupings.
- Keep accurate records of crop rotations and weed control programs
used in all your fields, including weather conditions at time of
application. Map the location of resistant weeds so they can be
found in future years.
- Use clean seed and clean equipment when moving field to field.
- Scout your fields after herbicide application to detect weed escapes
or weed shifts. If a potentially resistant weed or weed population
is detected, use available control methods to avoid weed seed production
in the field. Rescue sprays must be from a different herbicide group.
| Top of Page |
