Most herbicides become less effective when a plant is under stress. Along with crops, most weeds are also experiencing water stress under current conditions.
Most systemic herbicides perform better under conditions that are ideal for weed growth.
When the plants are free of stress, herbicides are absorbed faster and move better within the plant system, providing more efficient kill. If herbicide application becomes inevitable, application of systemic herbicides early in the morning may be more effective. Spray additives or adjuvants may enhance herbicidal effectiveness based on label recommendations. Therefore use of a lower rate, if the label permits, is advisable under such conditions. I am compiling a comprehensive list of herbicides that you may have applied in the spring, along with the wait periods for crops that may be planted this fall.