By Alberta Agriculture and Forestry
Early weed control has many benefits as weeds compete with crops for nutrients, water, and light. “Research on weeds germinating before the crop emerges as compared to crop emerging before the weeds shows a very significant drop in yield loss when the crop emerges prior to the weeds,” says Harry Brook, crop specialist with Alberta Agriculture and Forestry. “A pre-seed burn-off with a herbicide or final cultivation should be as close to the seeding activity as possible to prevent weeds getting the jump on the crop.”
All crops have a critical weed control period, which is the time when the crop is 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,” Brook says. “Corn’s critical period depends more on nitrogen availability than anything else. If you can keep the weed pressure down until the critical period is passed, you minimize yield losses from weed competition.”
Field scouting is essential to giving an edge battling weeds, notes Brook. “Field scouting tells you what weeds are present and their density. Once a field has been scouted and a weed problem identified, the degree of threat needs to be assessed. 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, Brook says 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. 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, yet not too large to have already affect crop yield from competition. Low spray volumes and coarse sprays can lead to insufficient herbicide landing on the plants. Best temperatures for application should ideally be above 12 to 15 C, when the plants are actively photosynthesizing. If it was frosty in the morning, waiting until a warm afternoon will improve efficacy.”
Another tool in the weed control toolbox is the competitive nature of the crop itself. “Highly competitive crops can reduce the effects of weeds on yield. Once a crop canopy has covered the soil, sunlight no longer can penetrate to the ground and weeds stop germinating,” adds Brook. “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.”
For more information, contact the Alberta Ag-Info Centre at 310-FARM (3276).