Controlling Weeds with Pre-Emergent Herbicides | P&H Agriculture

Controlling Weeds with Pre-Emergent Herbicides

April 1, 2024 | Eastern Canada, Western Canada

Pre-emergents are a foundational part of a successful herbicide-based weed management program.

Timing and environmental factors will affect efficacy, but the appropriate herbicide applied at the right time can have significant benefits to overall crop health and quality. Developing a pro-active pre-emergent strategy is one way to effectively control weed populations and improve crop yields.

Why are Pre-Emergent Important?

As weed resistance becomes more wide spread, pre-emergents are one way producers can combat weed populations and reduce nutrient competition early. They can help spread work load and ensure that your protection products are applied before weed populations explode.

Here are Some Tips to Control Problem Weeds With Pre-Emergent Herbicides:

1) Identify the target weeds: Crop Scout early and often to identify the problem weeds in your field, and gauge your level of concern for herbicide resistance. This will help you choose the right pre-emergent herbicides that are effective against those weeds.

2) Timing is key: As the name suggests, pre-emergents are most effective the closer products are applied to seeding date. Results of applying directly before or after seeding vary, but generally most pre-emergents are best applied within a seven day window of seeding. A 2020 article from the University of Iowa 1 has an excellent breakdown of the varying pros and cons of applying pre-emergents before, during, and after seeding date.

Application timing is critical for effectiveness. You may need to apply the herbicide before the target weeds appear. This timing will vary depending on the crop and the weed species, so consult with your local agronomist for specific recommendations.

3) Select the appropriate herbicide: Once you have identified target weeds and determined the optimal timing for application, you can choose the appropriate herbicide to match problem weeds and resistance challenges. Be sure to consider what (if any) extended control you might need.

4) Tank mixing and herbicide layering: These methods can be an effective way to increase the efficacy of weed control and reduce the risk of herbicide resistance. However, not all herbicides are compatible for tank mixing, so check the label instructions carefully.

5) Monitor and adjust: Scout your crops frequently to monitor the application results and adjust as needed. If you notice there are still some weeds present, you may need to consult your agronomist and apply an in-crop herbicide or other weed management technique.

Contact your local P&H Agronomist for help building your custom crop care plan for 2024.


University of Iowa Extension and Outreach. Preemergence Herbicide Application Timings: Pros and Cons. April 9, 2020.,pressure%20for%20herbicide%20resistant%20weeds.