Cases of wild mustard resistant to Muster are starting to appear.
A number of growers in Manitoba’s Red River Valley noticed persistent wild mustard infestations in 2000 after one, and in some cases two, Muster applications.
Even though environmental conditions were variable, herbicide resistance was considered possible.
As a result, Muster manufacturer DuPont Canada collected seed samples from suspicious wild mustard patches in 23 fields.
University of Manitoba tests showed that 17 of the samples had some level of resistance to ethametsulfuron, the active ingredient in Muster.
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Most of the resistant samples were collected between Morris and Roland, although resistance was also confirmed around High Bluff, Crystal City and Altamont.
The level of resistance was variable, with preliminary tests confirming that some samples were more resistant when the herbicide was applied at a later leaf stage.
There has been some investigation into whether the plants are also resistant to other Group 2 herbicides. So far, one sample was found to be resistant to thifensulfuron-methyl, found in products such as Refine, Pinnacle and the Plus part of Triumph Plus and Champion Plus.
Unlike green foxtail and wild oats, wild mustard outcrosses readily, so there is potential for rapid spread of resistance via pollen as well as through seed movement.
The fact that numerous wild mustard samples have been confirmed resistant is not surprising.
Mathematical models that have assumed the likelihood of naturally occurring resistant plants, outcrossing rates and other variables have been used to predict outbreaks of noticeable resistant weed patches in a field. These models show that a long period of low-level buildup is followed by a rapid increase in the proportion of resistant plants.
Such a pattern was evident in the evolution of trifluralin-resistant green foxtail that covered large areas of southwestern Manitoba in a fairly short time.
It is possible that there are more resistant wild mustard populations in Manitoba than those reported.
Any grower who noticed wild mustard that was difficult to control with Muster in 2000 should consider the effect of local environmental conditions as well as the possibility that these populations are resistant.
This is especially true for growers who noticed variable control of wild mustard, but good control of other broadleaf weeds listed on the Muster label.
The confirmation of Muster-resistant wild mustard deserves the consideration of growers and extension specialists.
Even though 70 percent of canola acres were herbicide-tolerant varieties in 2000, this meant that 30 percent were still conventional varieties, and many of these acres would have been sprayed with Muster.
Canola samples with greater than five percent wild mustard contamination are downgraded to sample. At least three fields inspected in 2000 were in this category.
Growers who have Muster-resistant wild mustard confirmed in a particular field could consider replacing conventional canola with a herbicide-tolerant variety. Liberty and glyphosate should be effective, while Clearfield herbicides such as Pursuit and Odyssey are also thought to control Muster-resistant populations.
In cereals and other rotational crops, Muster-resistant wild mustard can be controlled by tank mixing Group 2 herbicides with other commonly used herbicides from Grou-10-p 4 (2,4-D and MCPA), or by using herbicides from Grou-10-p 5 (Sencor) or Grou-10-p 6 (Basagran, Buctril M).
The use of good crop and herbicide rotations, recommended sanitation practices and other integrated management techniques will help ensure Muster will continue to be an effective weed control tool in conventional canola systems.
