Best Management Practices to Maximize Herbicide Efficacy

By Tyce Masich, AAg, Crops Extension Specialist, Outlook

June 2024

When it comes to herbicide use, producers must consider various agronomic, environmental, mechanical and economic factors to achieve maximum returns on their investment. Given the complexity of herbicide application, there are many poor practices that reduce herbicide efficacy. Ignoring water quality, weather conditions, and herbicide rotations are associated with reduced weed control and the development of herbicide resistant weeds

Many producers rely on dugouts and wells for spray water. These sources often have contaminants such as mineral ions, silt and organic matter that diminish efficacy of some herbicides. Water testing is recommended to measure the hardness, alkalinity, pH and conductivity of potential spray water sources. This allows producers to identify potential limitations to herbicide performance. The best way to mitigate the risks of poor water is using treated water that is free of these contaminants and at neutral pH. This option can be expensive and less accessible, which makes it less popular with producers. Other measures can be taken to ensure spray water from dugouts and wells provides effective weed control. When working with hard water, producers should:

• Use herbicides that are less affected by hard water;
• Use maximum recommended herbicide rates;
• Spray when weeds are most susceptible to herbicide damage; and
• Consider using non-ionic surfactants.

Environmental conditions during product storage and application affect herbicide performance. Storage requirements can vary between herbicides; always follow specific label instructions and make sure your storage facility meets these requirements. For example, storing pesticides in freezing or hot temperatures, direct sunlight or humid conditions will reduce efficacy and create potential fire or spillage hazards. It’s best to store pesticides in a facility that is well-ventilated, dry and where the temperature can be monitored.

Prior to spraying, producers should assess weather conditions to determine if they will provide effective weed control. Herbicides are most effective when applied to weeds that are actively growing, which mainly occurs during warm, sunny days. The optimal temperature to spray is when daytime highs are 20-25 C and nighttime lows are 10 C or higher. Herbicides are still effective at lower temperatures; however, weed control may be reduced. Weed control from herbicides is reduced or halted in cloudy, cool and wet conditions. Therefore, avoid spraying when nighttime temperatures are near or below 0 C and daytime highs are 10 C or lower. Additionally, avoid spraying in hot (28 C or higher) and dry conditions as herbicide performance is typically lower and the risk of drift is higher.

Herbicides also require a rain-free period after application; producers should avoid spraying immediately before rainfall. Always consult the product label for rain-free periods after spraying as rain fast times vary between products.

Since herbicides are frequently used in conventional cropping systems, poor herbicide management can lead to resistance developing in weeds. The best ways to prevent herbicide resistant weeds from developing in your field include:

• Tracking and rotating herbicide groups sprayed in your fields. Herbicide resistance develops from repeated applications of the same herbicide group.
• Testing commonly resistant weeds such as wild oats and kochia in your field for herbicide resistance by submitting a sample to the Crop Protection Lab. Instructions on submitting a sample can be found on the Crop Protection Lab webpage. Early identification of herbicide resistant weeds allows producers to select appropriate herbicide groups that best manage resistant weeds.
• Tank mixing multiple herbicide groups reduces the selection pressure of each mode of action being sprayed. This provides better weed control and reduces the risk of herbicide resistant weeds developing

Herbicides are a large investment for producers as multiple applications each year are usually required for sufficient weed control. Using best management practices when using herbicides is crucial for achieving high returns on investment and maintaining the profitability of your operation. For more information on herbicide use and best management practices, please contact the Agriculture Knowledge Centre at 1-866-457-2377 or your local Ministry of Agriculture regional office.