By Brian Jenks, Ph.D.
Farmers are in the business of producing food and sustaining their families. They make decisions that will bring in the most income, just like every non-farmer has to do in their walk of life. However, farmers face a distinct challenge in that what is best financially (at least in the short term), may not be what’s best agronomically in the long run. That’s because of Mother Nature’s rules. One of her rules is that biological species, such as weeds, adapt to what farmers do.
We all like a routine that is convenient and easy. Farmers are no different in that they want to control weeds in the most convenient and economical manner. If they find something that works well once, it’s understandable that they choose that method again and again. However, weeds are notorious for finding ways to survive and proliferate. They adapt to anything a farmer does. If a farmer tills the soil at the same time every year to take out early-emerging weeds, the weeds find a way to germinate later. If a farmer uses the same herbicide frequently, weeds find a way to make that herbicide ineffective.
All U.S. states now deal with herbicide-resistant weeds. In North Dakota, where the most canola acres are grown in the nation, farmers deal with several herbicide-resistant weeds such as glyphosate-resistant kochia, horseweed (marestail), common ragweed and waterhemp. Many wild oat and green foxtail populations are resistant to mode of action (MoA) group 1 herbicides such as Puma®, Axial®, Assure II® and Select®. Unfortunately, some wild oat and green foxtail populations are also resistant to MoA group 2 herbicides such as Raptor®/Beyond®, Everest®, GoldSky® and Varro®.
Canola growers have the goal of increasing acres to meet consumer demand. To do so, canola must be grown by more farmers and/or grown more frequently. If growers are going to increase acres and stay ahead of resistant weeds, they must have diverse crop rotations and use herbicides with different MoAs.
Growers should consider the following suggestions for delaying or managing weed resistance:
- Use proper crop rotations. A two-year crop rotation such as Roundup Ready® (RR) canola-wheat may lead to resistant weeds in 5-7 years. A three- or four-year crop rotation would be more effective for managing weeds and preventing herbicide resistance. Consider cool versus warm and short versus long season crops. Consider fall versus spring planted crops. Alternating planting dates makes it harder for weeds to adapt.
- Don’t grow RR canola more than once every four years.
- Don’t grow LibertyLink® canola more than once every four years.
- If you choose to grow a RR crop more often, such as corn or soybeans, be sure to use soil-applied herbicides (pre-plant or pre-emergence) as well as other post-emergence herbicides that have a different MoA. Just because you grow a RR crop doesn’t mean you can only use Roundup herbicide with it.
- Rotate between RR, LibertyLink® and Clearfield® canola. Don’t use the same biotech trait over and over.
- Avoid weed control programs where you rely solely on post-emergence applications. This is difficult in canola because of limited options, but some spring canola growers could consider using a fall Sonalan® to supplement post-emergence weed control.
- Include a small grain crop in rotation to break up pest cycles and allow for use of different herbicide MoAs.
- Don’t cut recommended pesticide rates; always follow label instructions. Cutting rates will increase weed escapes and the chance of them developing resistance.
- Spray weeds when they are 1 to 4 inches tall.
- Scout fields. Hand-pull weed escapes, especially waterhemp and Palmer amaranth.
- Learn about the MoAs of different products. What are group 1, 2, 9 and 27 herbicides? Just alternating trade names won’t protect you against weed resistance. But changing up MoAs will.
- Consider fall herbicide applications, especially for winter annual weeds like horseweed.
Brian Jenks, Ph.D., is a weed scientist at the North Central Research Extension Center of North Dakota State University in Minot, N.D.