Using Cover Crops to Manage Winter Weeds with Herbicides

Cover crops are becoming more common in Canadian farming systems to manage weeds, moisture and nutrients. When implemented along with residual herbicides, pre-seed burndown herbicides, in-crop herbicides, fungicides and seed treatments cover crops provide another option for farmers to reduce weed pressures, disease pressure and increase nutrient availability for their principal crops.   

Why Use Cover Crops to Control Winter Weeds?

From the dry cold of the Prairies to the more humid coastal climates of the Maritimes, there are many winter annual weeds that can emerge in fields after the main crop has been harvested, like chickweed, shepherd’s purse, cleavers and various types of mustards. If left unchecked, these winter annuals can germinate, produce seed and make future herbicide use more intense.

Cover crops provide a biological solution to winter weeds. By quickly establishing a canopy on the soil surface, cover crops prevent many winter weeds from emerging by:

• Outcompeting many winter annual weeds for light, water and nutrients — especially as days get shorter and temperatures drop
• Forming a physical barrier to light
• Containing natural chemicals that prevent weed seeds from germinating or halt early seedling growth

Using cover crops in an annual rotation can help minimize winter annual weed emergence and decrease spring herbicide applications.

Residual Herbicides and Cover Crops 

Although cover crops help suppress winter annual weeds, they may not completely prevent them from emerging. After terminating the cover crop and before planting or crop emergence, a residual herbicide can help further control weeds and create a chemical weed barrier while the crop establishes. The combination of cover crop suppression and chemical control can lead to cleaner seedbeds, improved crop stand and economics and potentially a reduction in herbicide applications.

Considerations for Residual Herbicides in Cover Crops 

There are a few factors to consider when using a residual herbicide in conjunction with cover crops:

• Select a residual herbicide that fits in your crop rotation and check the label to ensure you’re using it correctly.
• Products for crop protection in Canada will have planting or re-cropping timelines on the label if there is a possibility of a herbicide persisting in the soil.

Planning & Selecting Cover Crop Species 

When selecting cover crops, be sure to select species based on climate, crop rotation and your management plans.

Rye is a popular choice on the Prairies because it grows quickly, tolerates low temperatures and suppresses winter weeds throughout fall and spring with strong growth. Rye also offers an allelopathic benefit. 

Winter wheat provides a benefit for nitrogen scavenging, although wheat might not have as vigorous an establishment as rye.

Oilseed radish and mustard provide excellent suppression and biofumigation benefits, although the standing plant is often less durable than rye and often requires earlier termination.

Fast-establishing cereals with a legume or brassica to enhance biomass and maximize benefits.

It’s important to select a species that provides the right amount of coverage and growth to reach the desired management goals. This can be anything from optimizing winter suppression, collecting nitrogen for residual uptake, building up soil structure through increased biomass or producing spring mulch through termination.

Seeding & Termination Timing 

Seeding times may vary depending on cover crop type, primary crop staging and climate. Rye is typically seeded between August and early September to give the roots and canopy time to develop.

Termination is usually done in the spring, but timing depends on the climate and crop plan. Farmers aim to terminate the cover crop before it emerges as a weed or causes challenges for spring seeding.

Termination can be done mechanically (mowing or rolling-crimping) or chemically. Some farmers will use a pre-seed burndown spray to terminate the cover crop and any emerged weeds followed by, or in combination with, a residual herbicide to provide extended weed control..

Using Residual Herbicides After Cover Crops 

Following termination of the cover crop, apply a residual herbicide to help prevent winter annual weeds from emerging before the main crop. Here are some best practices: 

• Select a residual herbicide that fits in your crop rotation and allows for timely planting without carryover concerns.
• Understand how your soil residual herbicide will work best. Knowing if the residual product needs to be incorporated in the soil or if it works best in a no till or minimal disturbance system will impact how you manage the end of your cover crop.
• For example, when integrating a cover crop with an early spring application of Fierce EZ or Valtera EZ, you want minimal disturbance to the soil. Heavy soil disturbance disrupts the herbicide barrier near the soil surface. Where as a product like Rival (trifluralin) needs to be worked into the soil for maximum efficacy.

Managing and Measuring Success 

Success comes with regular monitoring: 

• Determine how much cover crop is established and growing to estimate weed suppression levels.
• Compare fields with and without residue to determine the effect of combined management.
• Thicker residue may suppress weeds but also create cooler soil temperatures and delayed seedbed warming.

Measuring cover crop biomass, counting weeds and tracking results over multiple years will help farmers optimize the cover crop species, termination timing and residual herbicides for consistent results.

Common Challenges 

Cover crop and herbicide programs are not without their challenges:

• Heavy amounts of residue may make seeding difficult and reduce soil warming during spring.
• Termination failures can cause competition with the cash crop.
• Some residual herbicides can adhere to residue or behave differently in cool, moist conditions. Read the label for guidance on pre-plant interval and residue cover issues.
• Farmers should manage these challenges with proactive measures, like scouting and backup plans (e.g., spraying after planting if necessary).

Costs and Benefits of Cover Crops and Herbicides

There are additional costs associated with cover crops (seed, seeding, termination, etc.) but there are also benefits which may include:

• Reducing weed seedbank buildup
• Fewer spring herbicide applications
• Improved crop stand and yield
• Healthier soil through enhanced soil structure
• Resilience against soil erosion from stormwater runoff

A return on investment can be seen in all these benefits, especially in regions where the field remains open after harvest in the fall and crop establishment can be delayed in the spring.

In Canada, cover crops and herbicides offer a powerful and effective solution to weed control. By integrating cover crops with targeted pre-seed burndown and residual herbicide programs, growers can protect fields from post-harvest and early-season weed emergence and create healthier crops.

When well planned and executed — from the species selection through to controlled termination — this integrated program improves agronomic performance while laying the foundation for ongoing weed suppression. Although not a perfect system, it represents a flexible approach to sustainable weed control in Canada that can deliver the results growers expect.