Weeds that emerge before the crop can be highly damaging to crop yield potential because they out-compete the crop for moisture, nutrients and sunlight. A small number of weeds (even just a few per square foot) emerging before or with the crop can be much more damaging to yield potential than a larger number of weeds flushing later. AAFC weed scientists Neil Harker and John O’Donovan estimated that the yield effect of one weed emerging a week before the crop is equivalent to that of 100 weeds emerging three weeks after the crop. When the crop emerges, all early weeds need to be dead. Cool, wet conditions that are holding up seeding can also reduce herbicide efficacy, but early weed control with lower efficacy is generally preferable to no control at all or late control with higher efficacy — as long as weeds are present and not frost damaged. Read more.
Tips for spraying weeds in dusty conditions. Most products are not strongly affected by dust, but glyphosate is an exception. Read this article for tips to improve glyphosate activity when leaves are covered in dust.
Tips for spraying in the wind. Use low-drift nozzles, lower the boom and drive slower can help. Read more.
Tips for spraying in cool temperatures. Herbicides tend to work best in warm sunny conditions when weeds are actively growing and cycling nutrients into their growing points. In these conditions, weeds will take in herbicides most efficiently. The Canola Encyclopedia provides two scenarios to explain the efficacy risk of spraying in cool temperatures and the value of spraying early (when cool) versus waiting for warmer conditions. When to spray weeds after a frost.
Post seeding/pre-emergence window. If choosing to seed before spraying, weeds present will have a minimum five days — usually more — before the crop emerges. These weeds can advance very quickly in good conditions, which is why growers who seed before spraying may choose to apply in the narrow post-seeding pre-emergence window. Consider the following risks with this practice: (1) If a burnoff doesn’t get done before the crop emerges, the crop is in a significant yield loss situation. In-crop weed control needs to occur as soon as possible and may have fewer options than prior to crop emergence. (2) The longer the weeds persist from the point of emergence until they are dead, the greater the yield loss. The larger the weed at the time of crop emergence, the greater the speed at which yield is lost following emergence. (3) The later into May, the greater the chance that air and soil temps will increase quickly. Warmer soils mean quicker crop emergence. The faster crops emerge, the more likely the pre-emerge weed control window will be missed altogether.