One of the rules for good weed control is that weeds must be actively growing. When you have cool weather without sunlight weeds are not growing as well as when the sun is shining and the temperatures are above 10 oC. This makes them harder to kill.
If you are spraying a soybean burn down and have perennial weeds, control of these weeds will be improved under warm sunny spray conditions. Normally you should wait 72 hours between spraying perennials and planting beans and 24-48 hours between spraying annuals and planting. If you sprayed your burn down under less than ideal conditions increase these wait times by 24-48 hours.
As an aside if you are tank mixing Touchdown/Roundup with clay-based products such as Boundary or Conquest for soybean burn downs, you must increase the rate of Roundup/Touchdown to offset the negative effect of the clay based herbicide tying up some of the Touchdown/Roundup.
Temperatures also affect the crops’ ability to break down herbicides. It is the 12 hours after application that is most critical for crops to break down herbicides. If the weather is cold and there is no sun the crop is less actively growing. It will be harder for the crop to break down the herbicide. Under these conditions you are more apt to have herbicides effects that can cause yield loss.
Some herbicides are more damaging to crops than others. When spraying bromoxynil/MCPA (Buctril M) the damage can be very noticeable but generally no yield loss. When you spray hormone herbicides under less-than-ideal conditions the effect is not seen immediately but can cause a yield loss. Try to avoid spraying wheat if the weather will drop below +5 oC in the first 12 hours after spraying. This temperature gives some leeway for error between predicted and actual temperatures.
When in doubt about spraying issues regarding the weather ask your herbicide supplier and always read the label. BF