Resistance is a naturally occuring, inherited ability of some weed biotypes to survive a herbicide treatment that should, under normal use conditions, effectively control a weed population. Some herbicide resistant weeds have naturally developed one or more mechanisms that allow them to survive a herbicide treatment.

For a weed to be considered resistant it must:

• Normally be controlled by the herbicide
• Survive a usually lethal dose of the herbicide
• Be heritable, meaning it is passed from generation to generation

Herbicide-resistant individuals are naturally present within a weed population at very low frequencies.

Did you know that there is no known resistance to Group 6 herbicides in Western Canada?

Why is it important to you?

The management issues and yield concerns with herbicide resistance are very real, especially now with an increasing number of herbicide resistant weeds spreading across Western Canada.

Loss of viable herbicide options

• Have to rely on currently available herbicides for the foreseeable future
• No herbicides with new modes of action are in advanced trials

Would require changes in weed and crop management practices

• Zero or reduced tillage rely heavily on herbicides for weed control

Reduced return on investment

• Increase the cost of weed management
• Loss of yield potential

The number of weeds with herbicide resistance continues to rise

• Including weeds resistant to multiple modes of action

How does resistance develop?

The image below demonstrates the interaction between a weed species and a particular mode of action and the development of herbicide resistance at different speeds. It’s clear to see that over time the use of a herbicide quickly changes from being very effective to becoming extremely ineffectual. In fact, by the fifth year of application the herbicide is failing to eliminate herbicide-resistant weeds. It is particularly alarming that there is a dramatic increase in failure rate from the fourth to fifth season.

What can you do to delay the onset of herbicide resistance?

• Don’t rely solely on herbicides for weed control
• Develop field-specific long-term weed management plans
• Rotate herbicide groups year over year

General guidelines for herbicide rotations:

• Avoid repeated use of the same herbicide or herbicides having the same mode of action in the same field year to year
• Limit the number of applications of a single herbicide or herbicides having the same site of action in a single growing season
• Use mixtures or sequential treatments of herbicides having different modes of action which are active on the same target weeds whenever possible
• Use non-selective herbicides pre-seed or pre-emergence to control early flushing weeds (prior to crop emergence) and/or weed escapes

1. CROP ROTATION

Rotate crops as often as possible to utilize the unique modes of action available within the crop.

2. TIME FERTILIZATION

Time and place fertilizer to benefit your crop, not weeds.

3. TARGET PROBLEM WEEDS

Systematically target problem weeds and respond quickly to changing weed populations.

4. SCOUT OFTEN

Scout often and take note of individual weed response to all applications (pre-emergent, post-emergent and pre-harvest).

5. REMOVE ALL WEEDS

Make sure you remove all weeds before they set seed – even if you have to pull by hand. Remember the old English proverb: One year seeding, seven years weeding.

6. PERIMETER WEED MANAGEMENT

Leaving a 1m wide weed-free zone around your cropping fields (so new weeds can’t propagate and expand throughout your fields).

7. OUTCOMPETE WEEDS

Decrease your row spacing and increase seeding rates to optimize plant populations, maximize crop competitiveness and minimize time to crop canopy closure.

8. MAXIMIZE YOUR HERBICIDE’S EFFECTIVENESS:
• Match selection of herbicide(s) with your most difficult weeds.
• Use full-labelled rates.
• Include the best tank mixes and adjuvant(s).
• Use correct spray volume (follow label directions).
• Use correct nozzle spacing and droplet size.
• Don’t spray at excess travel speeds, follow label directions.
• Apply during the best weather conditions (warm, humid, and sunny conditions with minimal wind).
• Maximize rain-fastness intervals.
• Time of day (herbicides are most effective when sprayed between 8:00am and 9:00pm).