State of weed resistance in Western Canada and future outlook

Hugh J. Beckie, PAg
Weed Scientist
Agriculture and Agri-Food Canada (AAFC), Saskatoon, Saskatchewan

Canada ranks third behind the U.S. and Australia in the number of herbicide-resistant (HR) weed biotypes. Since 1975 in Canada (when Hoe-grass was introduced), there have been on average 1.5 new biotypes per year. The total number of biotypes in Canada is over 60, equally split between Western and Eastern Canada. There are 23, 20, and 22 HR biotypes in Alberta, Saskatchewan, and Manitoba, respectively.

The 2014/2015 Saskatchewan weed survey (2,242 fields) ranked the top 10 most abundant weeds:

1. green foxtail;

2. wild oat;

3. wild buckwheat;

4. volunteer canola;

5. Canada thistle;

6. spiny annual sow-thistle;

7. cleavers;

8. lamb’s-quarters;

9. narrow-leaved hawk’s beard;and

10. dandelion.

All of the annual weeds have a number of HR biotypes across the prairies. The top 3 weeds have remained in the top 3 for nearly 50 years, despite all the new herbicides introduced in the 1970s and 1980s (Gp 1 and Gp 2s). The three weeds that have moved up the ranking the most since the last survey in 2003 are spiny annual sow-thistle (34th to 6th place), narrow-leaved hawk’s beard (20th to 9th place), and cleavers (14th to 7th place).

AAFC has conducted HR weed surveys since the mid-1990s. Since the baseline surveys (2001-2003) where 10.9 M acres were found infested with HR weed biotypes, we now estimate that over 50% of cultivated land on the prairies (38 M acres) has an HR weed. This will need to be verified with a new round of HR weed surveys: Saskatchewan in 2014/15, Manitoba in 2016, and Alberta in 2017.

The estimated cost to producers ranges from $1.1 to 1.5 billion dollars annually in terms of increased herbicide use and decreased yield and quality. About 75% of cases (2007-09 surveys) are wild oat (Gps 1, 2, and/or 8), which is our most economically important weed. There are also a number of Group 2 broadleaves, such as cleavers, wild mustard, and shepherd’s purse.

In Canada, there are four glyphosate-resistant (GR) weeds. In Western Canada, kochia is the only GR weed confirmed to date. We currently estimate over 100 cases across the prairies, usually associated with chem-fallow, but also found in wheat, canola, soybean, and lentil fields. The last prairie surveys of GR kochia were 2012-2013. We plan to conduct the next round of GR kochia surveys beginning in Alberta in 2017, Manitoba in 2018, and Saskatchewan in 2019. In 2015, we confirmed another HR weed – Gp 2 + 4 (dicamba, fluroxypyr)-resistant kochia in a wheat field in southern Saskatchewan (see photo). Soon, we can expect kochia with 3-way resistance: Gps 2+4+9.

So what do I recommend to producers to delay or manage resistance in their weed populations? My top 10 best management practices (BMPs) are:

1. sound record-keeping, including keeping track of weed populations in your fields over time using GPS;

2. strategic tillage;

3. field and site-specific weed management (e.g., patch management),

4. weed sanitation – preventing weed immigration and dispersal within fields;

5. in-crop wheat-selective vs. non-selective herbicide rotation – to manage metabolic resistance in grass weeds selected by wheat-selective herbicides (so if a herbicide is registered in wheat, it can select for metabolic resistance in wild oat that uses the same enzyme system to break down herbicides);

6. herbicide group rotation – avoid back-to-back in-crop Gp 1 or 2 products;

7. herbicide mixtures/sequences – pre, in-crop, post-harvest;

8. pre- and post-herbicide scouting (I am thinking of drones!),

9. competitive crops and practices that promote competitiveness – fast ground cover; and

10. crop rotation diversity.