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Maximizing residual herbicide efficacy

Residual herbicides are an effective and convenient weed control tool, say experts. However, given their makeup, growers need to know how to effectively manage them to maximize weed control, minimize or eliminate subsequent crop impact and mitigate risk of weed resistance.

Experts say a key advantage of residual chemistry is early weed removal. And to maximize the effectiveness of these products, they recommend using them in the crops where they will deliver the greatest value.

Eric Johnson, weed biologist with Agriculture and Agri-Food Canada at Scott, Saskatchewan, conducts extensive research with residual herbicides and agrees they offer great benefit. “Certainly the advantage is that they provide season long weed control, including control of late flushing weeds, which is really important in less competitive crops,” says Johnson. “Farmers are also always looking for one-pass systems and, with a residual herbicide, that’s a big advantage in providing a one-pass weed control solution that helps keep their labour and input costs down, and helps with timing.”

Clearfield production system herbicides, for example, offer broad spectrum weed control of grass and broadleaf weeds, as well as flushing weed control which allows for one pass control, says Jeff Bertholet, technical development manager with BASF in Saskatchewan. “The Clearfield herbicides work most effectively when applied early and that’s a key feature of most flushing weed control products: you can apply them early, minimize crop competition and know that you’ll get flushing control of late emerging weeds,” says Bertholet.

Johnson agrees that being able to apply early is a key advantage of residual herbicides. “From a yield point of view, just about every major study shows that early weed removal is important to getting the most out of your weed control dollar,” says Johnson. “This is a major advantage of residual chemistries because growers can apply them before weed emergence, which can also help extend the spraying season for other crops.”

In order to gain full value from residual herbicides however, both Johnson and Bertholet remind growers to understand the product they want to use and the characteristics of the field where they want to apply it. Residual herbicides persist in the soil until deactivated through natural processes, including breakdown by soil microbes, soil moisture, chemical reaction and light, leaching, binding to soil particles or organic matter and escapes into the atmosphere.

“Residual herbicide interaction and breakdown is somewhat more complex because the means by which the product is deactivated is based on a number of variables,” says Johnson. “In Canada, weather and temperature are crucial to the breakdown because the majority of herbicide breakdown occurs by microbial action, so anything that stimulates microbial activity will increase breakdown speed and anything which slows it down will cause the herbicide to persist longer.”

This is also why organic matter (which holds moisture), soil texture (which can have variable water holding capacity) and particularly precipitation are crucial, says Johnson.

Use residual herbicides in crops where they will provide the highest return
To maximize the efficacy, minimize carryover and mitigate risk of weed resistance, there are a number of best management practices growers should consider and employ when using a residual herbicide, including following label and product directions; following recommended application guidelines; planning rotations carefully; monitoring environmental conditions, including weather and soil conditions; practicing good agronomics, including seed, seeding practices and fertilization; and keeping good herbicide and crop rotation records.

In addition, Bertholet says the most important stewardship guideline is to make no more than two applications of a Group 2 herbicide on the same field in a four year period. “This may have implications on growers’ crop rotations, so what they need to think about is saving, or using their two applications of Group 2 herbicides in the crops that make the most sense and where they can draw the greatest value from the use of that Group 2, like in Clearfield canola, Clearfield lentils, Clearfield wheat or field peas, for example,” explains Bertholet. “In addition, a sound strategy for helping manage weed resistance is to use a Group 2 product that includes multiple modes of action, such as Odyssey DLX or Altitude FX.”

Johnson agrees, adding that “Growers need to look at what crops they are growing and what herbicide options they have for that crop. In the case of field peas where growers have limited options for broadleaf control, they should not use up that Group 2 or residual herbicide in a competitive barley crop where there are a number of herbicide options from a number of different groups.”

Both experts also point out that for growers who are looking for some of the benefits of flushing weed control with less residual activity, there are herbicide options. These products are a good option for growers who tend to be in dryer areas, who know their specific field conditions cause slower residual herbicide breakdown, who want to rotate to a more sensitive crop, or who are concerned about managing potential carryover.

Many growers specifically choose a herbicide with residual activity because they want extended weed control.

And these herbicides certainly fit the bill; early, one pass weed control saves time and does the job throughout the growing season. One of the primary strengths of these products, as their name implies, is their extended activity in the soil. For growers to get the most out of these products, they need to be actively involved in their management to maximize the in-season benefits and to minimize or eliminate their potential impact on subsequent crops.