Know your enemy and its destructive potential, says provincial crop specialist

There’s a critical window for controlling weeds, but it depends on both the crop 
and the threat posed by individual species

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All crops have a critical weed control period when they are most susceptible to significant yield loss from weed competition.

The critical weed control period for canola is around 17 to 38 days after emergence. Peas can be as early as two weeks after emergence.

“Other, more competitive crops, like the cereals, have a less defined critical period,” said provincial crop specialist Harry Brook. “If you can keep the weed pressure down until the critical period is passed, you minimize yield losses from weed competition.”

Start by scouting.

“Once a field has been scouted and a weed problem identified, the degree of threat needs to be assessed,” said Brook. “An example of an early, non-yield threatening weed is whitlow grass. It’s a very slow-growing, small plant that bolts and goes to seed, usually before seeding. It’s not a direct threat to the crop.

“However, if other weedy plants are also present in sufficient numbers and are a threat to yield, you can choose an appropriate control measure.”

Winter annual weeds like stinkweed, narrow-leaved hawk’s-beard, shepherd’s purse, scentless chamomile, and many others can start growing in the fall. They overwinter as a small rosette but are then quickly able to go to seed once spring arrives.

“Control of them in the spring requires very early action. You need to know the weeds present to choose the best control method. Crop volunteers from previous years are also an increasingly problematic weed obstacle. Volunteer canola is one of our top weed control issues every year. These and other problem weeds will require additional products when applying a spring burn-off with glyphosate.”

To get the best result from any early herbicide application, the herbicide must be applied when the weeds are actively growing.

“Under cool or cold conditions you can expect poor results from the spray as the target weeds are either dormant or growing too slowly,” said Brook. “They cannot absorb and translocate enough active ingredient to kill them. Weeds also have to be large enough to absorb enough herbicide to be killed.

“Low spray volumes and coarse sprays can lead to insufficient herbicide landing on the plants. Best temperatures for application should ideally be above 12 C to 15 C when the plants are actively photosynthesizing.”

Another tool in weed control is the competitive nature of the crop itself.

“Highly competitive crops can reduce the effects of weeds on yield,” he said. “Once a crop canopy has covered the soil, sunlight no longer can penetrate to the ground and weeds stop germinating.

“Heavier seeding rates can also squeeze out weeds. Hybrid canola and barley are our two most competitive crops. You still have to choose a competitive variety. Semi-dwarf barleys are less competitive than regular barleys. Heavier seeding rates always increase the crop’s competitive nature against weeds. Thin crops allow light to hit the ground, stimulating more weed growth.”

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