Moisture-stressed plants and herbicide applications

Clark Brenzil, weed management specialist with Saskatchewan Agriculture, provides the following consideration for spraying weeds after weather conditions have switched from very dry to very wet:

1. Rainfall may have released the residual herbicides that may have been present in soil. This means that symptoms, if they are to be seen will show up about 10 to 14 days after the rain.
2. Even though rain has fallen, the leaves present prior to the rain will still have thickened cuticles and may resist herbicide uptake. As time progresses with consistent moisture, new growth will have less cuticle and allow better uptake of herbicides.
3. On the plus side, the health of both crop plants and weeds will be better allowing herbicides to work more effectively.
4. Recent rains will mean another flush of weeds, after many producers have already applied a herbicide. This is where the multi application uses are effective.
5. Evening temperatures have been on the cool side, which could have an impact on herbicide tolerance.
6. New rain means multiple stages in the field which could impact in-crop herbicide timing and harvest management decisions. Often times the new crop is the larger percentage of the population.