Drastic changes in temperature usually have a greater impact upon reducing glyphosate activity than consistently cool temperatures.
The reduced glyphosate activity usually occurs when fields are sprayed within 1 to 3 days of a change in temperature of greater than 15 to 20 degrees.
The greater the temperature change — and the longer the cool period extends beyond the temperature change — the more likely reduced weed control will occur with glyphosate. A drastic change in temperatures likely negatively affects other translocating herbicides as well.
When spraying can’t be delayed during these periods, do everything possible to maximize glyphosate activity. Intensively scout fields for weed control after application during large temperature changes and consider tank-mixing herbicides with glyphosate that can improve control of those weed species most likely negatively affected by the temperature changes.
If weed control is negatively affected, re-treat fields no sooner than 10 days after the previous application, unless weed control was a complete failure.