Paul O. Johnson
Weed Science Coordinator
Early application of post-emergent chemicals before weeds are too large will insure good control of that flush of weeds. The thing that has changed with resistant weeds is when old chemistry or non-glyphosate control is used large weeds are not as likely to be controlled. The weeds will compete with the crop for nutrients, moisture, and sunlight.
In the past with glyphosate type products some people waited for all weeds to be up before spraying. If the weeds were not heavy this worked and did not have a large effect on yield. However, with non-glyphosate chemicals, weed size and crop size both have to be considered when making spraying decisions. The rule of thumb that is used now is “early is just about always better than late.” Even if you have some late emerging weeds they will not have a large effect on yield but could still increase the weed seed bank for next year. With just using other chemistries there may be a few weeds left in the field because the chemical used may not cover all types of weeds in the field.
The last thing to remember is if it is wet and humid in the field avoid using the methylated seed oils as they could cause more crop response than desired in wet conditions. Usually if plants are growing well with moist conditions a surfactant is desirable. Always read the label to determine the right additives and also remember some of the old chemistries do not require the addition of any additives.
Source : SDSU