Herbicide Resistance

In current cropping systems, we rely heavily on chemical herbicides to control our weed populations. These herbicides are divided up into groups indicating their mode of action (MOA); all products that fall in the same group kill plants by interfering with the same enzyme or biochemical pathway in a plant. These products have served us well since the 1940’s when 2,4-D, the first modern herbicide, was discovered.

Herbicide resistance is quickly becoming one of the biggest concerns facing grain farmers today. This concern became very real in 1996 when the first case of glyphosate resistance was discovered in Australia in 1996. Previous to this, many believed weeds would not be able to develop resistance to this herbicide MOA. By 2008 glyphosate resistant giant ragweed was first discovered in Ontario and by 2011 glyphosate resistance was discovered in Alberta. Weed scientist suspect the next weed in Western Canada to develop glyphosate resistance may be wild oats or cleavers.

This issue should be of serious concern to all grain farmers. It is estimated that herbicide resistance already costs Western Canadian farmers between $20-50/acre, imagine the cost once glyphosate resistance becomes common! Hundreds of millions of dollars are spent each year by crop protection companies looking for new MOA’s, but none have yet to be discovered. Dr Hugh Beckie, research scientist for AAFC, has developed 10 resistance management strategies that can be applied to your farm:

1. Maintain a Database
   Tracking herbicide MOA, cultural/mechanical practices and weed populations will be an invaluable future reference.
2. Strategic Tillage
   Deep tillage may reset the resistance clock but only use if, where, or when needed.
3. Field & Site Specific Weed Management
   One size does NOT fit all; treat each field and situation separately.
4. Weed Sanitation
   We may be able to slow herbicide resistant seed dispersal by tarping trucks and cleaning equipment between fields.
5. In Crop Wheat-Selective Herbicide Rotation
   Group 1 wild oat resistance is a common problem, mostly due to the over use of wild oat herbicides in wheat. Rotate between a wheat-selective herbicide (e.g. Axial) when growing wheat and herbicides not selective in wheat (e.g. Centurion) in subsequent crops, or use herbicides that are non-selective period (e.g. Roundup or Liberty).
6. Herbicide Group Rotation
   It is important to know the risk of weeds developing resistance to the herbicide mode of action you are using. For example, group 1 and 2 products are the highest risk so avoid back to back in crop applications of these groups.
7. Herbicide Mixtures/Sequences
   Mixing multiple MOAs in one tank is much more effective at preventing herbicide resistance than rotating groups as long as all MOAs target the weed of concern.
8. Pre- & Post-Herbicide Scouting
   Know your enemy! Weed resistance is often shrugged off as non-performance of herbicide; often by the time we realize we have resistance it’s two years to late and it has already spread extensively throughout your field.
9. Competitive Crops
   Practices that promote crop competitiveness act as natural biological weed control. Keep seeding rates up, narrower row spacing, sufficient nutrition, using seed treatments, etc.
10. Crop Diversity
    Rotation, Rotation, Rotation! The risks facing grain producers from issues such as herbicide resistance, Clubroot in canola, Fusarium Head Blight and Stripe Rust in cereals and root rots in pulses can all be reduced by extending rotations. We have to start looking at our farm economics over four or five year cycles instead of just one year at a time. Our ability to continue to grow the crops that traditionally are the most profitable could be at risk if we don’t improve our crop diversity.

We can not afford to continue to keep doing things the way we’ve always done them; by doing so we are effectively selecting for our next big problem. Recognize the importance of managing risk, take a look at your weed management strategies, improve them where you can and keep those weeds guessing!