By: Joanne Kowalski PAg, -- Regional Crops Specialist Prince Albert
Over the past decade, an increase in the number of herbicide resistant weeds has caused researchers and producers to look for weed management systems that can diminish the rate of occurrence. Right now, there are 461 unique cases of herbicide resistant weeds around the globe:
- The rapid increase in Group 9 (EPSP synthase inhibitor) or glyphosate resistant weeds over the past 20 years is of large concern because of the quick increase from zero in 1995 to 30 in 2015.
- Resistance to Group 2 herbicides, which are protein production inhibitors, has seen the fastest rise from zero species in 1985 to more than 160 in 2015.
- Groups 5, 6 and 7 (photosynthesis inhibitors) have the next highest number of resistant species, increased from zero species in 1985 to 105 in 2015.
- Group 1 (ACCase inhibitors) is next at zero in 1985 to a little more than 45 in 2015.
- Group 4 (synthetic auxins/growth regulator) has increased steadily from zero in 1985 from to 30 in 2015.
Canada has 60 weed species verified as herbicide resistant. In Saskatchewan, there are about 14:
- Group 2 resistance: cleavers, kochia, wild oat, chickweed, shepherd’s purse, wild and ball mustard, redroot pigweed, Russian thistle, stinkweed, and hemp nettle.
- Group 1 resistance: Persian darnel
- Group 1 and Group 3: green foxtail
- Group 9: kochia.
During the adoption of zero-tillage practices from 1990 to 2005, the use of an integrated approach to weed management was an important part of the extension message from the Saskatchewan Soil Conservation Association.
An integrated weed management, or a “many little hammers” approach, is acknowledged as necessary to manage the increase. The hammers include:
- Seeding Rate: increased seeding rate can provide better crop competition, but it is more expensive.
- Row Spacing: narrower row spacing in studies shows lower weed biomass, especially when combined with higher seeding rates.
- Fertilizer Placement: placing the fertilizer into the soils rather than broadcasting results in improved competitiveness of crops and improved yields.
- Seed Size: using larger seeds can reduce weed biomass and seed production.
- Seeding Date: the earlier the better, to allow the crop to get a head start on establishment. This interferes with the weed seed’s ability to germinate and grow, as they are biologically adapted to faster development. A north-south seeding orientation results in shade between rows may reduce weed seed germination and growth.
- Competitive Cultivars: they are better competitors with weeds, especially when combined with other methods.
- Silaging/Green Feed: can reduce weed seed populations. Using perennial crops in rotation leads to greater competition for reducing weed populations.
- Rotating Herbicide Groups: a long-standing integrated weed management tool. However, we now know utilizing herbicides with more than one group in them delays herbicide resistance more effectively than rotating single-group herbicides.
Integrated weed management that recognizes the use of multiple methods of control, both agronomic and chemical, is now a vital function of overall production management. As Eric Johnson of the Crop Development Centre said at the Agronomy Research Update in 2016, “None of the individual control measures provide acceptable control on their own.”
For more information, please contact your local regional office or the Agriculture Knowledge Centre at 1-866-457-2377.