The ideal time to control weeds is a shifting target but by following several key principles growers can help ensure they hit the ideal weed control bulls-eye, says Christine Mardell, Canola Council of Canada agronomist for the Peace.

"Number one principle is to concentrate on controlling early emerging weeds--at the one- to four-leaf crop stage--and worry less about the later emerging weeds-at the four- to six-leaf crop stage. Spraying later emerging weeds is for aesthetics," she says.

Research by the Alberta Research Council (ARC) on wild oats has shown weeds that emerge before or with the crop cause greater yield loss than weeds that emerge after the crop. Fewer weeds tend to emerge after the crop has reached the four-leaf stage and those that do are usually weaker and spindly. Therefore, control weeds at the one- to four-leaf stage of canola to maximize profitability, she notes.

For good early season weed control, consider using a pre-seed burn-off treatment like glyphosate. Canola Council Canola Production Centre trials found significantly higher yields for a pre-seed burn-off compared to no burn-off. For optimum weed control, wait as long as possible to apply the burn-off (to maximize weed seed germination), but be careful to not go beyond the ideal seeding date, says Mardell.

The more competitive the weeds, the fewer needed to cause a yield reduction. "This illustrates another key principle of good weed control--field scouting. Know what is growing and refer to previous weed records to get an idea of what weeds may be an issue later in the season," says Mardell.

If a herbicide-tolerant canola will be grown, growers can opt to wait until more weeds emerge and apply an in-crop herbicide. Growers have the option of one in-crop application or applying a sequential treatment. If relying on one herbicide application, recognize that weed competition is greatest at early crop stages. Therefore, time single herbicide applications early to keep the crop weed free at its most vulnerable stage. Later emerging weeds may contribute to the seed bank and not look attractive, but they have much less impact on yield than weeds present early in the season.

In addition to herbicides, pre-seeding tillage can aid weed control. Winter annuals including cleavers, stinkweed and shepherd's purse are 'canola unfriendly' broadleaf weeds because they either have no control methods in conventional canola or are expensive to control in the crop. Light tillage in the fall will give 80 to 90% control of fall-germinated weeds, says Mardell.