Production Resources

Your weed management plan shouldn’t end after you’ve sprayed! Scouting after a post-emergence herbicide application is an important step in ensuring adequate weed control and identifying potential problems early.

When evaluating the efficacy of a herbicide application, timely scouting is important. The expected symptoms, and the time it will take for symptoms to develop can vary based on herbicide mode of action and whether weather conditions are conducive to active plant growth and herbicide uptake. Table 9 in the Weed Control section of the Guide to Field Crop Protection describes weed symptoms and timing for different herbicide groups. For some contact herbicides, symptoms may be expected within a few days, while some systemic herbicides may take up to a few weeks for death to occur. Identifying problems as early as possible allows for a quick response when problems do arise.

When scouting, pay particular attention to:

• Are there weed species that should have been controlled by the herbicide, but survived?
• Are there any field- scale patterns associated with uncontrolled weeds?

The efficacy of a herbicide can be affected by many factors. When a herbicide fails to control weeds, it is important to identify the cause. Some factors to consider:

Herbicide Choice

• Was the herbicide a good choice based on the weed species present? Are the surviving weeds on the label?
• If the weeds are on the label, is it labelled for control or only suppression?

Weather

• Was there rain soon after application? The rain-free period for different herbicides can vary significantly. Some are as short as 15 minutes, while others may require up to 8 hours.
• Were conditions hot and dry? This can make it harder for the herbicide to enter the leaf, but when weeds are stressed for any reason, herbicide efficacy can be reduced. On the other hand, cool weather can slow weed growth and cause the weeds to take in the herbicide more slowly. Foliar applied herbicides are most effective under warm temperatures, high humidity and when soil moisture is adequate.

Weed Characteristics

• What size were the weeds when they were sprayed? Generally, smaller weeds are more effectively controlled.
• Were there a lot of weeds? Weed control when there is heavy weed growth can be challenging. Some herbicides (ex. AAtrex, Steadfast IS) recommend increasing the rate for dense weed infestations. Using higher water volumes and appropriate adjuvants can improve weed control when weed density is high.

Application Timing

• Was application early, allowing significant numbers of weeds to emerge after herbicide application?
• Was application late? As mentioned above, reduced control can be expected for larger weeds with most herbicides.

Application Problems

• Was the correct herbicide rate used? Rate can be dependant on crop, weed species and weed size.
• Did you try a new tank-mix? Tank-mixes of some herbicides or foliar fertilizers can result in reduced weed control compared to using the products separately (known as herbicide antagonism). In addition, was an appropriate adjuvant used?
• Is there a pattern that might suggest equipment problems such as plugged nozzles, misses or drift?
• Was an appropriate water volume used? Especially for contact herbicides, thorough coverage is key. Nozzle selection, spray pressure and water volume are all important considerations.
• Was water quality considered? The pH, hardness or presence of soil particles in spray water can all negatively impact the efficacy of some herbicides.

Herbicide Resistance

If these other factors have been considered and ruled out, its time to consider whether the weeds survived because of herbicide resistance. There are many weed species in Western Canada with known herbicide resistance, and new cases are being discovered on a regular basis. In particular, look out for:

• Irregular patches of uncontrolled weeds when the rest of the weeds have been well-controlled.
• Healthy weeds next to dead weeds when both are on the herbicide label. When weeds that should be controlled are affected differently by the same herbicide, that’s a sign to investigate further.

If you do suspect herbicide resistance, it is important to get those weeds tested. Once herbicide resistance has been identified and confirmed, it is possible to develop a plan to manage those problem weeds.

Additional resources

International Herbicide-Resistant Weed Database

Prairie Weed Monitoring Network

Managing Herbicide-Resistant Kochia

Managing Herbicide-Resistant Wild Oats