The Importance of Herbicide Timing

There are few decisions that can be more important in today’s crop production than timing on spray applications. The timing of an herbicide application is key because weeds can affect the crop’s moisture, nutrients and sunlight right from the time of emergence. When a herbicide is applied to a growth stage outside of the target weed’s ideal time of control, effectiveness is greatly reduced. Over time, this can also lead to a higher risk of herbicide resistance. For the Canadian producer, knowing this window for control and striking when it’s open is one of the best ways to help protect yield and the life of your weed management tools.

Why Timing Matters

Each herbicide is formulated to have a certain mode of action and a target weed growth stage at which it is recommended to be applied. Growers often question the recommendations but they are there for a reason. Years of testing and field trials have shown this to be the time when the herbicide will achieve maximum uptake, translocation and overall performance.

Take the example of systemic herbicides that are most effective at a stage when weeds are small and actively growing. At this stage the active ingredient will move within the plant easier. Contact herbicides are more effective when they have tender leaf tissue to cover, again this is harder to achieve with a larger, more mature weed with a thicker cuticle. Apply outside of this stage and the weed is more likely to survive.

The Consequences of Late Applications

Spraying too late is, by far, the most common weed control mistake. Not only are large weeds more difficult to control, they are also likely to have already caused irreversible yield loss. Weeds which are at a robbing size, such as wild oats or cleavers in cereals for example, can steal nutrients from the crop in critical early stages, even if they are then sprayed out.

In addition to yield loss, late applications also increase the selection pressure for herbicide resistance. When only part of a weed population is controlled (small plants being more susceptible), the survivors pass on their herbicide resistant traits to future generations. Over time these resistant populations become much more expensive and difficult to control.

Early Control is Key

Research throughout the Prairies indicates the first three to six weeks after crop emergence are the most critical period in which to control weeds. This is commonly referred to as the “critical weed-free period” and is the period in which the crop is most sensitive to competition and when herbicides provide the most value. Herbicides applied in this window reduce the need for a second application, by starting clean, the crop canopy will close more quickly and provide natural suppression of later emerging weeds, along with the chemical suppression provided by the herbicide application.

Recognizing Weed Growth Stages

Accurately identifying weed growth stages is essential for correct herbicide timing. Broadleaf weeds are usually most vulnerable from the cotyledon to the four-leaf stage, while grasses are best targeted before they tiller. For example:

●        Wild oats: best controlled before the two- to three-leaf stage.

●        Kochia: most susceptible before four leaves, particularly with a broadleaf herbicide in Canada programs.

●        Cleavers: easier to manage before they begin to branch and climb.

When weeds pass these stages, they not only become harder to control but also increase the risk of herbicide failure, especially if resistance is already present.

Environmental Factors and Timing

The growth stages of weeds aren't the only thing to consider when timing your sprays. Weather and soil conditions also play a role in how well an herbicide works. Cold snaps, drought stress, or heavy soils can all slow down weed metabolism, limiting how much herbicide is taken up and moved around the plant. Applying a spray during these conditions can lead to spotty control, even if the weeds are at the perfect growth stage.

Growers should look for periods when weeds are actively growing and conditions are favorable for absorption. Sometimes waiting a few days of good weather before spraying can be the difference between great control and a disappointing result.

Resistance Management Through Timely Applications

Herbicide resistance is one of the biggest challenges facing crop protection in Canada. Weeds that survive late or poorly timed applications are more likely to evolve resistance, making future management more difficult and costly.

Timely applications, particularly when combined with multiple modes of action in a tank mix, reduce the chance of survivors. By targeting weeds when they are most vulnerable, growers can maximize the effectiveness of each mode of action and limit the opportunities for resistance to develop.

Practical Tips for Farmers

To make the most of herbicide timing, Canadian farmers can follow several best practices:

1.       Scout fields early and often. Walk fields before and during the critical weed-free period to identify emerging species and assess their growth stage.

2.       Prioritize timely applications over acreage. Focus on getting into the most weed-dense or vulnerable fields first, rather than spreading resources thin.

3.       Use residual herbicides where appropriate. Soil-active products can hold back weed flushes and extend the window of control, providing flexibility during busy seeding and spraying periods.

4.       Work with agronomists. Local expertise can help tailor herbicide programs to field history, weed pressure, and environmental conditions.

The application timing of herbicides is far more critical to the successful control of weeds than most of us think. We all know the rule that, if at all possible, you need to apply your herbicides when weeds are small and actively growing. Applying at this time will generally give you the best performance, best yield protection and it will also help reduce the rate of development of herbicide resistance. In contrast, late applications or poorly timed applications have a huge negative impact on performance, and they can significantly damage the long-term efficacy of our most important weed control tools.

For Canadian farmers, recognizing the narrow window for effective control and planning accordingly can make the difference between a clean, high-yielding crop and a season of chasing weeds. By combining careful scouting, timely action, and strategic use of multiple modes of action, growers can ensure their herbicide programs continue to work well into the future.