Crops and Weeds – How to Spray Variable Stages

Crop growth and development is variable within many fields in western Canada. A common question being asked is when is the best time to spray? Each field should be considered on a case-by-case basis. Keep in mind a few important points:

  • The first step is to scout. Scout the crop for signs of stress (moisture) and the weeds to determine what weed species are present and at what stage.
  • Weeds that emerge ahead of or at the same time as the canola crop will cause the most yield loss.
  • The following link provides information on time of weed removal:
  • Generally it is best to wait for recovery before spraying crops under stress. However, crop tolerance is less of a concern if weed pressure is high (and impacting yield).  Evaluate the field to determine what is causing the greater stress.
  • Stressed canola plants will begin reproductive growth earlier than normal (less vegetative biomass produced before initiating buds). Crop damage (flower blast) and yield loss can occur when herbicides are applied to a crop in bud or bolting stages. However, weed pressure also causes yield loss. Scout the field thoroughly to determine average crop stage and to determine which stress is most limiting (weeds vs. compromised crop tolerance). Keep in mind that herbicide injury can also delay crop maturity, which is a definite concern this year.
  • Some growers are noticing that the grassy weeds are present and need to be controlled, whereas broadleaf weeds are slower to emerge. While research has shown that sequential applications are rarely economical, in this situation, it may be practical to split apply herbicides. This is especially true in drier areas where weeds are competing for precious moisture.
  • If herbicide dollars are limited, spray early with the first application and re-assess the need for a second application later. Remember that some weeds (e.g. wild buckwheat) get more expensive and much tougher to control as they get bigger.

The following link has more information on weed control in canola: