Organic regrowth from prior seed deposits presents ongoing difficulty for Canadian farmers who implement close crop rotations. The growing dependence on intensive cropping systems such as cereal-oilseed rotations makes volunteer crop competition an increasingly significant issue. These unwanted volunteers, such as canola in cereals or cereals in broadleaf crops, can act much like weeds: Volunteers consume essential resources like nutrients, light, and moisture while reducing crop yield and creating harvest difficulties as well as pest and disease control challenges.
Growers who depend only on post-emergent herbicides during the growing season face expensive and unreliable results while increasing the risk of developing herbicide-resistant weeds. Growers who implement a proactive approach based on residual herbicides can suppress volunteer crops early to maintain seedbed cleanliness and promote initial crop strength.
The Volunteer Crop Challenge
Volunteer crops develop when seeds from an earlier crop survive in the soil to sprout during the next growing cycle. The Canadian Prairies face this problem because canola-cereal rotation systems are common in the area. Volunteer canola tends to sprout in multiple waves during the growing season when weather conditions vary. The ongoing competition from volunteer crops decreases cereal production levels and presents herbicide selection challenges, especially when fields contain various herbicide-resistant canola strains such as those tolerant to glyphosate, glufosinate or imidazolinone.
Volunteer cereal plants in broadleaf crops such as peas or lentils create comparable difficulties. In addition to competing for resources, these plants enhance the risk of diseases exclusive to cereals like rusts and leaf spots, damaging crop quality and reducing profitability.
Volunteers that emerge concurrently with crops or in successive flushes often escape effective control with post-emergent herbicides, particularly when environmental factors delay treatment or limit herbicide options.
Residual Herbicides: Proactive Suppression from the Start
Residual herbicides work by remaining active in the soil after application to help manage weeds and volunteers. Growers can target and reduce the first flushes of volunteer crop emergence by applying these products prior to or during seeding time. The method grants the intended crop a competitive advantage while lowering early-season weed pressure and reducing the probability of multiple in-crop herbicide passes.
Key Advantages of Using Residual Herbicides:
- Extended Weed Control: The effectiveness of residual herbicides lasts several weeks after application to suppress weeds during essential early crop development stages, depending on the product and rate.
- Multi-flush Control: Volunteers like canola often germinate in waves. Using residual herbicides correctly helps manage early flushes.
- Reduced Post-Emergent Pressure: Residual herbicides that manage early growth lower selection pressure on post-emergent herbicides, which aids in resistance management.
Selecting the Right Residual Herbicide
Growers must assess target volunteer species and their herbicide tolerance traits, as well as soil type, crop rotation plan, and moisture availability, to select the right residual product. Growers situated in western Canada who deal with volunteer canola in cereal fields should evaluate the use of pre-seed and soil active herbicides that incorporate multiple modes of action and are designed for prolonged residual effects.
Flumioxazin, pyroxasulfone, and sulfentrazone, herbicides provide effective residual control for broadleaf weeds and some grasses. Farmers should apply these herbicides in early spring before seeding or in combination with a burnoff herbicide in a tank mixture.
Always consult the product label for:
- Rainfastness and activation moisture requirements
- Crop and rotational restrictions
- Re-cropping intervals
- Soil type compatibility
Application Timing and Technique
The optimal performance of residual herbicides depends on soil disturbance and soil properties.
Key timing considerations include:
- Soil Moisture: A specified quantity of rainfall following application is necessary for many residual herbicides to become active. Drought conditions may delay or reduce effectiveness.
- Temperature: When soils remain cold, root activity and herbicide uptake both experience a slowdown. Monitor forecast trends to ensure optimal conditions.
- Residue Cover: Residues from previous crops create a barrier that captures herbicide spray droplets. Use enough water to help the herbicide reach the soil surface. If heavy trash exists, consider harrowing before application.
- Calibration and Equipment: Set up sprayer calibration and choose nozzles to achieve uniform soil coverage. Uniform application of residual products leads to their highest effectiveness.
- Type of residual product: Some soil active properties require no till and minimum disturbance drills. Some products need to be worked into the soil. Know how the product works, and the best practices to promote maximum efficacy.
Integrating Residuals into a Weed Management Strategy
Residual herbicides are not a standalone solution. Their greatest value is realized when they are integrated into a larger Integrated Weed Management (IWM) plan that includes:
- Scouting for weed and volunteer crop species pre-season
- Using competitive crop varieties
- Adjusting seeding rates and row spacing to enhance crop competition
- Timely in-crop herbicide applications when needed
Farmers in Canada benefit from an early residual herbicide application because it helps maintain clean fields throughout short growing seasons and protects crop yields.
Growers who implement residual herbicide strategies can take the pressure off early-season weed competition and safeguard crop yield potential. The optimal results from these tools are achieved when growers choose the appropriate crop protection in Canada and use it at the right times within an integrated weed management strategy.
