By Ross H. McKenzie PhD P.Ag.
Herbicides have become very important for weed control. However, frequent and repeated use of the same herbicide groups has gradually resulted in development of herbicide-resistant weeds to the point that resistance has become a very serious problem for many farmers.
When herbicide resistance is a relatively minor problem, growers tend to pay less attention to managing it than they should. Once resistance starts affecting a major weed or a major herbicide used on the farm, then growers pay more attention to the herbicide group number on the label. In Canada, growers who have consistently used best integrated weed management practices have fewer herbicide-resistant weed issues on their farms. What these farmers are doing differently to avoid the development of weed resistance versus other farmers, is that most are using a combination of integrated weed management to try to combine chemical, cultural, mechanical, biological or other practices in a proactive cropping system to enhance long-term sustainable crop production.
#1 Understand the mode of action (MOA) and rotate herbicide use by group number
For each herbicide used on your farm, be sure you are aware of and understand the mode of action. This is the way an herbicide controls susceptible plants. Specifically, it is how the plant processes are affected by the herbicide – which biological process or enzyme in the plant the herbicide interrupts, and how this affects normal plant growth and development. In some cases, the mode of action may be a general description of the injury symptoms observed on susceptible plants. Each mode of action has a unique herbicide group number.
Simply rotating herbicide active ingredients is not enough to prevent the development of herbicide-resistant weeds. Rotating herbicides based on modes of action is necessary to prevent or delay development of herbicide-resistant weeds. For each herbicide used, be sure to pay attention to the mode of action (herbicide group number). See table 1.
Rotating herbicide active ingredients means more than just rotating herbicide brands. Products with different names, or from different companies, may have the same mode of action and herbicide group number. Growers must take great care to ensure that changing from one herbicide manufacturer to another doesn’t result in a continued application of herbicides with the same mode of action. Different herbicides with the same chemical group number may not control the same weeds. Products may have a combination of herbicide chemistries or registered tank-mix combinations that allow them to control additional weeds, but if the group number is the same, the basic herbicide chemistry is the same and repeated frequent use will result in increased risk for developing herbicide resistance.
Each provincial department of agriculture provides detailed information on herbicide group classification by mode of action, which lists the herbicide groups (chemical family), active ingredients and the herbicide product names in which the active ingredients are found. The information is available on department websites and crop protection publications. For more information on herbicide group numbers and herbicide rotation, contact your provincial crop specialist or ag information centre. Herbicide companies and private agronomists are also excellent sources of planning information.
#2 Weed assessments in all fields
Identify all weeds present on your farm. For each field, assess which weeds are present and their relative populations. Determine the biology and life cycle of each weed (e.g. annual, biennial or perennial).
Have herbicide-resistant weeds been identified in any of your fields? If so, determine the group number of the resistance and note all herbicide products with that group number to ensure use is restricted.
#3 Assess the herbicides at risk on your farm
For each field, determine the number of applications of each herbicide group in the past 10 years (or more, if you have good field records).
Identify any herbicides that have been frequently, or over-used on your farm. Are any of theses herbicides in the moderate- or high-risk groups for developing resistance (Groups 1, 2, 3, or 8)?
Develop a plan to avoid applications of frequently and over-used herbicides in the future.
#4 Crop rotation
Use at least a four-year crop rotation including cereal, oilseed and pulse crops to disrupt weed growth and life cycles. More diverse rotations also allow a wider range of herbicide groups to be used for weed control.
Using both winter wheat and spring wheat (or other winter crops) with different life cycles in a diverse crop rotation helps disrupt the life cycles of weeds and therefore can aid in weed control. Keep in mind that winter annual weeds can become a greater problem if the frequency of winter crops in the rotation increases.
Long-term crop rotations that include annual crops and perennial forage crops in the rotation are ideal. Forages are excellent for competing with weeds and interrupting weed life cycles. Forage crops are also very beneficial to improve soil quality and build soil organic matter. Ideally, continuous cropping is best for effective weed competition.
#5 Seed sources
Ensuring seed is meticulously cleaned is very important to prevent importing new weed species onto your farm.
Using certified seed is a very good practice to consider, but be aware of the types of weed seeds in the certified seed. Check the certificate of analysis, which shows any weed seeds present.
Using high-quality seed is also important. Test your seed for germination and vigour – plump seed is often more vigorous and competitive.
#6 Seed and seeding
Seed each crop as early as is reasonable to give the crop a head-start over weeds.
Review the seeding rates you use for each crop. Consider shifting to a slightly higher seeding rate to increase crop competition with weeds. Also consider using seeding equipment with a seedrow spacing as narrow as is reasonable to be more competitive with weeds.
Seed each crop as shallow as is reasonable for rapid germination and emergence.
Place fertilizer with seed or near seed at safe rates to give crops easy access to nutrients and promote healthy, vigorous plant growth.
Zero tillage will minimize weed seed incorporation into soil, which is a good aid to assist with cultural weed control.
In situations where considerable weed pressure occurs, tillage may be appropriate for weed control, but always keep in mind the importance of soil conservation!
Removal of weeds by hand may be useful for removing small weed patches or weeds that are hard to control.
#9 Control weeds early with herbicides
Extensive research in Canada has shown yields of most crops are optimized when weeds are removed earlier with herbicides versus later.
#10 Utilize herbicide mixtures
An herbicide mixture is more effective in delaying development of herbicide resistance when the less resistance-prone herbicide controls a similar spectrum of weeds as the more prone herbicide. Both herbicides should have similar persistence to control the same weed flushes, but the two mixed herbicides must have different modes of action.
#11 Use of herbicide-resistant crops
Planting an herbicide-resistant crop like canola has become a common technique for managing Group 1- and 2-resistant grassy and broadleaf weeds. Varieties with stacked traits provide a different mechanism for resistant weed control.
#12 Harvest weed seed management
At harvest, consider using a chaff wagon to collect weed seeds blown out the back of the combine and prevent spreading weeds back onto the field.
These are just some of the practices to consider in order to improve integrated weed management on your farm. Herbicides will continue to be the dominant method to control weeds, but keep in mind the importance of preventing new weeds from being imported onto your farm and the various cultural practices you can utilize for weed management. To keep herbicide resistance in check, Prairie farmers must use the highest possible level of integrated weed management. The challenge for our industry is to encourage all Prairie farmers to become very conscientious of using multiple control practices to manage weeds on their fields.