Early season weed control is key

Spring spraying can be challenging. Look for the right weather and moisture conditions

Early season weed control is a top priority for canola growers. Weeds that emerge before the crop are much more competitive than those coming up with the crop, and herbicide can’t undo the damage done by weeds left uncontrolled before emergence.

Clark Brenzil, provincial weed specialist with the Saskatchewan Ministry of Agriculture, says research shows crops are susceptible to competition from weeds the moment they emerge until they begin to elongate.

Further research conducted at the University of Saskatchewan shows that herbicide application timing is critical and can be beneficial to yield. Basically, earlier is always better.

Steve Shirtliffe, plant science professor at the University of Saskatchewan, agrees. “When plants start growing it’s a race to capture the most nutrients,” he says. “If you don’t control your weeds before it can be bad. Most herbicides work best when the weed is small.”

Not all weeds emerge early. If this happens, Brenzil recommends a second application. “For crops that prefer warmer soils for germination the application of two burn-off treatments may be the preferred approach to weed control prior to seeding,” he says.

Problematic spring weeds include winter annuals like shepherd’s purse, narrow-leaved hawk’s beard and stinkweed. According to Nicole Kimmel, a provincial weed specialist in Alberta, winter annuals can remove a lot of spring soil temperature as they begin to grow very early. Large, fast-growing weeds are also a problem.

“Not only do they maximize space, but they also easily dominate water and nutrient resources,” says Kimmel, who mentions that lamb’s quarters, kochia and Canada thistle can be particularly problematic. Some weeds, though, aren’t as bad as one might think.

“Green foxtail, for example, isn’t the end of the world,” says Shirtliffe.

Before spraying it’s always a good practice to scout fields to see which species are present, especially since a lot of crops are more competitive than weeds.

Spring spraying

Most growers know that spring spraying can be challenging, especially if the soil is saturated. Kimmel reminds growers that it’s important to keep in mind that big, new sprayers carry a lot of weight, especially when the tank is full. “Ideally, you don’t want to go into a low spot with a full tank,” she says. “Luckily new sprayers are also equipped with GPS to allow producers to start with the high areas and move to the low areas once the tank has been emptied some.”

Field conditions aren’t the only hindrance. Adverse weather conditions can mess with herbicide timing. According to Brenzil, the best weather conditions for good herbicide efficacy are the same conditions that are good for crop growth. The best conditions are bright and sunny with warm temperatures between 20 C and 25 C and nighttime lows of 10 C or better. To minimize off-target movement, Brenzil also recommends spraying when there’s a light breeze and relatively unstable air conditions.

“The problem with stable air conditions is that they will likely result in a temperature inversion where warm air is trapped near the ground by cooler air above, creating a physical boundary between the two layers,” explains Brenzil.

Applications made during an inversion may become suspended above that barrier and then travel a distance before being intercepted by another object, like a tree. The situation could be much worse if applying a volatile herbicide on days where there is little to no wind.

“The sun heats up the soil’s surface and an inversion is created after spraying is long done,” says Brenzil. “The warm soil enhances the volatility of the herbicide and it continues to volatize under the physical inversion barrier, building up concentration to injurious levels throughout the evening.”

When the sun comes up the following day, he continues, the air begins to destabilize the concentrated layer and creates a shift of several hundred meters before dissipating. The direction of this movement is difficult to predict and may change several times.

Nicole Kimmel encourages growers to use a pre-seed herbicide treatment when needed. Of course, she recognizes that Mother Nature sometimes has her own agenda, which may not allow growers to get into the field. She recommends that growers avoid spraying when foliage is still wet from rain or dew.

“A leaf retains only a limited volume of water, and therefore a limited amount of herbicide,” she says. “Rain or wet foliage can cause spray to simply runoff. If the foliage is wet or if water can be shaken from it, spraying should be delayed until conditions are drier.”

Narrow windows of herbicide application can cause producers to push outside of ideal spray conditions. Missing a window of opportunity with herbicides can often result in greater yield losses since the weeds will have a chance to cause damage and will be tougher to control later, says Kimmel.

About the author


Melanie Epp is a freelance farm writer based in Guelph, Ont.


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