Pre-emergent herbicides are an essential component of early-season weed control programs. When used properly and under the right conditions, these herbicides can provide effective control of weeds before they emerge and compete with the crop for moisture, nutrients and sunlight.
However, in some parts of Canada, growers may experience inconsistent results from pre-emergent herbicides. This can include poor weed suppression or control, patchy application or early escapes. Identifying and correcting any issues with application or product compatibility is key to ensuring long-term success with pre-emergent herbicides.
In this article, we will discuss the most common causes of pre-emergent failure on Canadian farms and offer solutions and best practices to help growers get the most out of their herbicide programs.
Factors that Influence Pre-Emergent Herbicide Performance
Weather Conditions: Adequate rainfall or irrigation is essential to activate many pre-emergent herbicides. Temperature can also affect activation rates, as well as soil microbial activity and volatilization losses.
Application Timing: Pre-emergents must be applied before the target weeds have germinated. The best timing will depend on local conditions and weed species present, but may include factors such as soil temperature, growing degree days (GDD) and scouting.
Crop and Soil Types: Different crops have different tolerances and growth habits, which can affect pre-emergent performance. Soil texture and organic matter content can also influence herbicide activity, depending on the specific active ingredient used.
Spray Equipment and Calibration: Improper calibration or poor spray coverage can lead to missed areas or uneven application, reducing the overall efficacy of the treatment.
Weed Species and Resistance: Not all pre-emergent products are effective against all weed species. Resistance or shifts in weed populations can also impact performance.
Variations in Soil Properties, Moisture, and Texture
Inconsistencies in soil properties, moisture and texture are common issues that Canadian growers face when using pre-emergent herbicides. These factors can impact the activation, efficacy, and longevity of pre-emergent herbicide products and, ultimately, lead to performance problems or failures on the farm.
One of the most important considerations for pre-emergent herbicide use is soil moisture at the time of application and in the weeks following treatment. Without adequate soil moisture, pre-emergent herbicides may not activate properly, or may degrade before having an opportunity to do so.
Poor distribution of soil moisture can also contribute to patchy performance or variable efficacy in treated areas. In areas with inconsistent rainfall or irrigation, growers may find that their pre-emergent herbicides fail to provide consistent control of weeds across the field.
In some cases, soil moisture issues can be improved through cultural practices such as adjusting planting depth or timing, using surface residue to conserve moisture or selecting more drought-tolerant crops or varieties. Growers can also consider using irrigation systems to provide a more consistent supply of moisture when rainfall is not available.
Variations in soil texture and composition can also influence pre-emergent herbicide performance. In soils with high levels of clay or organic matter, certain active ingredients may have reduced activity or availability due to adsorption or binding with these soil components.
For example, herbicides in Group 3, 8, 14 or 15 may require rate adjustments or different formulations to achieve control in heavy or organic soils, and using products with lower adsorption potential is one way to mitigate this challenge.
Inconsistent soil properties within a field can also lead to variability in pre-emergent herbicide performance. Localized areas of different soil types or properties may require different management strategies for optimal pre-emergent herbicide activity.
Some growers may benefit from using precision soil sampling or mapping tools to identify areas of variability and target their management practices accordingly.
Failure to Achieve Sufficient Soil Coverage
Uneven or insufficient coverage of the soil surface with the pre-emergent herbicide product is another common challenge that can lead to early-season weed escapes or failures. This can occur due to equipment issues, incorrect application rates, improper spray techniques or application in high-residue or uneven fields.
Improving coverage with pre-emergent herbicides often involves equipment calibration and adjustments. Maintaining proper boom height and pressure, nozzle spacing and wear can help ensure more uniform coverage with the product.
In addition, growers can also consider strategies such as cleaning residue from the soil surface or using a residue-managing tillage system to help ensure adequate product contact with the soil surface.
Spray drift is another challenge associated with coverage issues and pre-emergent herbicides. Drift occurs when small droplets of the herbicide are carried by the wind away from the target area and can result in variable coverage or damage to non-target areas.
Growers can take several steps to prevent spray drift, including using nozzles and pressure settings that produce larger droplets, applying herbicides when wind speeds are low and using shields or other drift-reducing technologies.
It is also important to use care when applying pre-emergents near sensitive areas, such as waterways or other crops that may be susceptible to drift damage. In some cases, growers may need to adjust their application timing or use buffer zones to protect non-target areas.
Resistance Management
Resistance to pre-emergent herbicides is an increasing concern for Canadian farmers. When the same mode of action is used repeatedly on a field, there is a risk that weed populations will evolve to become resistant. Symptoms of resistance include surviving weeds that were initially controlled by a pre-emergent, an increase in the size of the weed seed bank or a shift in the weed spectrum to more difficult to control species.
Resistance management strategies include using multiple modes of action and herbicide families, rotating herbicides with different chemical groups, mixing products or using tank mixes of pre-emergents and post-emergents. Cultural practices such as changing seeding rates, rotations or tillage systems can also help to break the cycle of resistance and prolong the effectiveness of existing products.
Timing Application for Optimal Activation
The application of a pre-emergent herbicide product is only one step in a successful program. The active ingredient in the product must also activate, which is often driven by moisture and temperature. If pre-emergent herbicides are not applied at the right time, or if environmental conditions do not activate them after application, weeds can escape control or the product can degrade before ever having an opportunity to work.
Timing can be key to activating pre-emergents, especially in areas with irregular weather patterns or where rainfall is not guaranteed after application. In addition to using weather forecasts and monitoring local conditions, growers can also consider using moisture sensors or other tools to help ensure that soil moisture is at the right level for activation before applying pre-emergents.
Growers may also need to consider application timing and duration when selecting a pre-emergent product. In some cases, a longer-lasting pre-emergent may be better suited to the environmental conditions or target weed spectrum.
Dry Weather Conditions and Lack of Rainfall
Dry conditions or a lack of rainfall can be a significant limiting factor for pre-emergent herbicides, as moisture is required to activate many pre-emergent active ingredients. In areas where rainfall is irregular or unreliable, there is a risk that the pre-emergent will not have enough moisture to activate and control weeds.
Growers can try to work around this by adjusting their timing or application methods to target periods of expected rainfall or irrigation. In some cases, it may be possible to apply pre-emergents earlier and then activate them later with irrigation, for example.
Crop Seeding Depth
Crop seeding depth can also influence pre-emergent herbicide efficacy, as seeds planted too deep may not be affected by the herbicide, while seeds planted too shallow may germinate before the product has an opportunity to activate.
Growers can help to ensure that seeding depth is optimal for their crop and herbicide program by using precision seeding equipment and taking care to maintain consistent planting depths across the field.
Water Quality and pH
Water quality is another factor that can impact pre-emergent herbicide performance, as high pH or “hard” water conditions can inhibit activity. Growers using water sources with poor quality should test their water regularly and consider using conditioners or other treatments to improve pH and water quality.
Integrating Crop Protection Solutions
For consistent early-season weed control, growers must approach pre-emergent herbicide use as one element of a broader crop protection strategy. Combining timely application, correct product selection, and good agronomic practices ensures that herbicides work as intended.
As part of the broader framework of crop protection Canada standards, farmers are encouraged to maintain detailed spray records, regularly test soil and water conditions, and stay updated on local resistance patterns through provincial agronomy networks. Pre-emergent herbicides remain a powerful tool in the fight against early-season weed competition. However, achieving consistent performance in Canadian conditions requires understanding and managing the many factors that influence herbicide efficacy—from moisture availability to timing, soil interaction, and weed pressure.
