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Busting up the myths around weed control

When it comes to weed control, agronomists, weed researchers and company representatives hear plenty of myths and misconceptions in the field when speaking with farmers. Here is a look at some of those myths and truths surrounding weed control.

April 30, 2010  By Top Crop Manager

Getting the most out of weed control means understanding the products. Photo by Bruce Barker.


When it comes to weed control, agronomists, weed researchers and company representatives hear plenty of myths and misconceptions in the field when speaking with farmers. Here is a look at some of those myths and truths surrounding weed control.

It is better to spray in the evening than the daytime
It is a myth that it is always better to spray in the evening. The best time to spray a herbicide depends on what type of weeds a grower is trying to kill and what product is being sprayed.
It can get confusing trying to figure out the best time to spray. For instance, Group 1 herbicides are divided into different chemical families. One of these families, known as the Dims, is best sprayed at night because it breaks down quickly in UV light, particularly on days when UV light intensity is high and herbicide absorption is slow. But not all Group 1 herbicides are the same. The Fop family of Group 1 herbicides are less affected by environmental conditions.


According to Ken Sapsford, research assistant at the University of Saskatchewan, the biggest problem when it comes to timing is that farmers are “always fighting with wind, and the most important consideration is that herbicides be applied when you can minimize problems caused by drift.”

Fast-acting herbicides are preferable to slow-acting ones
Comparing weed control to the fable of the tortoise and the hare, faster is not always better. With some products, weed-control symptoms show up fast. The risk is that the leaves burn quickly but the plant does not die, especially with weeds like dandelion where the chemical needs to get into the root system in order to be effective. Whether fast-acting is better depends on the weed targets.

With other products, it may take longer to see effects but that does not mean the weed has not been controlled. Doug Fehr, senior sales rep for DuPont Canada, suggests that if farmers are uncertain as to the level of weed control they are achieving, they should take a walk through their field with a notebook, a ruler and a handheld GPS or a flag to note the size and location of weeds at the time of application and then again a week later. That way they will know if the herbicide has been effective at stopping the weed or not. “As much as seven days after spraying, the plant may not be dead but it won’t have grown at all, which means it has stopped taking in water and nutrients,” he says. “It can take time for the herbicide to move through the vascular system of the plant to thoroughly kill both what’s above ground and below.”

When spraying glyphosate, less water is better
This is not necessarily a myth, especially if a grower is drawing from a poorer-quality source caused by hard water or contamination with soil or silt. However, as water volume decreases, so does coverage and efficacy. “If you use large droplets, you might miss some of the weeds you’re trying to target,” says Sapsford. “When using lower water volumes and a fine spray, you increase the potential for drift. It’s a fine balance.”

Sometimes growers have to draw water from a slough or dugout where the quality of the water is unknown. Other times growers are spraying in drought, cold weather or other poor growing conditions. In any of these cases, a good solution to improve weed control can be to add a tank-mix herbicide to the tank with the glyphosate.

There is a perfect spray nozzle for all herbicides
According to Tom Wolf, researcher at Agriculture and Agri-Food Canada in Saskatoon, this one is a myth. “Applicators must carefully determine their application goals, consider the target, the mode of action of the pesticide, the environment, and the sprayer features and capabilities before selecting a nozzle.”

A conventional, flat fan nozzle, for example, provides the finest spray and reliable performance but is prone to drift, while a high-pressure, air-induced nozzle can reduce drift by up to 80 percent.

Herbicide resistance is inevitable
It is true that herbicide resistance is happening. There are some weeds that are resistant to certain herbicides. “We can’t stop it but we can delay it,” says Fehr. The majority of herbicide resistance has resulted from the use of the same group of herbicides on the same field year after year. To prevent herbicide resistance, it is important to use different herbicide groups.
New research by Dr. Hugh Beckie, a weed researcher with Agriculture and Agri-Food Canada in Saskatoon, suggests that tank mixing herbicides from multiple groups is the best way to avoid resistance. “The best thing to do is have multiple modes of action within a single spray,” says Beckie. “Appropriate mixtures can more effectively delay resistance than just rotating modes of action from year to year.”

A conventional tank mix will do the trick, but crop protection companies are making it even easier by offering herbicides that contain multiple active ingredients in a single package.

If the product does what it says it is going to do, there is no need to scout my fields
This is definitely a myth, says Fehr. “Farmers should be walking their fields regularly, evaluating weed control, crop health and be on the lookout for signs of nutrient shortage. They should also look for a second flush of weeds, and be scouting for insects, new weeds appearing and signs of disease. Hiring a retail or independent crop consultant or scout is a worthwhile investment for farmers who are facing a time crunch.”

It is important to be on top of your scouting. If the product does not work as expected, “it is a lot harder to diagnose at harvest than 10 days after application,” says Sapsford.  

When in doubt
There is no blanket statement that can be applied to weed control when there are so many variables to consider. For growers trying to determine if they are dealing with a myth or truth when it comes to weed control, the key is to know the weeds they are trying to kill, to follow the product label and to refer to professionals like an agronomist, provincial extension expert or product sales representative. “Ultimately, the reps know the products best,” says Sapsford. n


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