By Brian Jenks, Ph.D.
It’s now been 24 years since herbicide-tolerant canola was first approved in the United States. The ability to control weeds in canola with post-emergence broadleaf herbicides revolutionized canola production. Prior to 1998, soil-applied herbicides Treflan® and Sonalan® were able to provide decent control for many weeds; however, many canola fields struggled to compete with tough weeds like kochia. With no post-emergence herbicide options other than Stinger®, growers had to save canola for their cleanest fields and hope the crop would outcompete the weeds.
With canola demand increasing in the late 1990s and the improved ability to control weeds, canola acres nearly doubled from 1997 to 1999 (460,000 to 855,000) and nearly tripled from 1997 to 2001 (460,000 to 1,300,000). Roundup Ready® canola varieties dominated the market after herbicide-tolerant canola approval because Roundup® was easy to use and controlled most weeds, including North Dakota’s most troublesome kochia, Canada thistle, wild oat, wild mustard and others.
Liberty Link® varieties were excellent, but it took some time for growers to understand its strengths and weaknesses and how best to use it. Some people perceived Liberty® as equivalent to Roundup because of the similar common names (glufosinate vs. glyphosate) and their uses in canola, but growers quickly learned that they are very unique herbicides. Liberty works best in sunny, warm and humid conditions on small weeds less than 4 inches tall. Liberty does not control perennial weeds like Roundup. Liberty works better at higher spray volumes (15-20 gpa), while Roundup works better at lower spray volumes (10 gpa).
Just because you grow a Roundup Ready crop doesn’t mean you have to use only Roundup.
Over time, Liberty acres increased due to the popular pod shatter reduction technology in InVigor® varieties as well as the increase in weeds becoming resistant to Roundup. In North Dakota, there are five broadleaf weeds known to be resistant to Roundup: kochia, horseweed (marestail), common ragweed, waterhemp and Palmer amaranth. Roundup-resistant kochia is probably the biggest challenge in counties where canola is primarily grown. Liberty can control kochia and other Roundup-resistant weeds, but good control is best achieved when spraying small weeds in warm, sunny conditions.
Liberty controls some grasses when small but struggles with yellow foxtail and larger grasses in general. It should be applied with a grass herbicide such as clethodim to control annual grasses like wild oat and foxtails. Many wild oat and green foxtail populations are resistant to Group 1 herbicides such as Puma®, Axial®, Assure II® and Select®. Unfortunately, some wild oat and green foxtail populations are also resistant to Group 2 herbicides such as Raptor®/Beyond®, Everest®, GoldSky® and Varro®. Liberty tank-mixed with clethodim has controlled these Group 1 and 2 resistant populations.
Currently, nearly all canola growers in North Dakota rely on either Roundup or Liberty applied post-emergence to control weeds. The question that needs to be asked is: Given the rapid development of weed resistance over the past 20 years, does canola have enough weed control options to stay on top of current weed issues as well as those that will develop over the next 20 years? We are fortunate that no weeds in North Dakota have developed resistance to Liberty. However, we know it is just a matter of time before resistance occurs in canola-growing areas. In 2021, a Palmer amaranth population in Arkansas was confirmed to be resistant to Liberty. It is certainly likely to happen in canola-growing areas in the near future.
What can farmers do to delay resistance to Liberty, Roundup or other herbicides? Consider the following suggestions, which are primarily focused on spring canola production, but may apply to winter canola as well:
- Use proper crop rotations. A two-year crop rotation such as RR Canola-Wheat may lead to resistant weeds in 5-7 years. A three-year or four-year crop rotation would be more effective for managing weeds. Consider cool vs. warm season crops. Consider short vs. long season crops. Consider fall vs. spring planted crops. Alternating planting dates makes it harder for weeds to adapt.
- Don’t grow Roundup Ready or Liberty Link canola more than once every four years.
- If you choose to grow a Roundup Ready crop more often, such as corn or soybeans, use soil-applied herbicides (pre-plant or pre-emergence) as well as other post-emergence herbicides with a different mode of action. Just because you grow a Roundup Ready crop doesn’t mean you have to use only Roundup.
- Rotate between Roundup Ready, Liberty Link and Clearfield canola. Don’t use the same trait over and over.
- Avoid weed control programs that rely solely on post-emergence applications. This is difficult in canola because of limited options, but some spring canola growers can 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 the use of different herbicide modes of action.
- Don’t cut rates. Always use the recommended label rate. Cutting rates will increase weed escapes and the chance for developing resistance.
- Spray weeds when they are 1-4 inches tall.
- Scout fields. Hand-pull weed escapes, especially weeds like waterhemp and Palmer amaranth.
- Learn about the different herbicide modes of action. What are Group 1, 2, 9 and 27 herbicides? Just alternating trade names won’t protect you against weed resistance.
- Consider fall herbicide applications, especially for winter annual weeds such as 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.