MINNEAPOLIS — Herbicide performance and selectivity are a function of how well the small grains and weeds that receive an herbicide are able to metabolize the active ingredient. The ideal temperature for applying most postemergence herbicides is between 65 and 85 F. However, the temperatures following herbicide application will largely determine crop safety as the plant’s metabolism slows during cool or cold conditions. This extends the amount of time required to degrade the herbicide in the small grain plants. Rapid degradation under warm conditions allows crop plants to escape herbicide injury.
Wild oat is more sensitive to the ACCase herbicide fenoxaprop during cool rather than warm/hot conditions but also may cause some crop injury. Green and yellow foxtail are warm season grasses and their uptake and metabolism is reduced under cold conditions resulting in reduced control. ACCase herbicides provide better grass control in warm weather when grasses are actively growing
Cold temperatures, including freezing conditions following application of bromoxynil may increase crop injury with little effect on weed control. The plant growth regulator herbicides, 2,4-D, dicamba, MCPA, clopyralid and fluroxypyr have adequate crop safety and provide similar weed control, but the rate of weed decline and control is slowed when cold temperatures follow application.
Refer and follow each herbicide label for specific information about weather conditions including adjuvant use during stress conditions.
— Jochum Wiersma, University of Minnesota Extension