Temperatures following herbicide application influence crop safety and weed control. Crops metabolize herbicides but metabolism slows during cool or cold conditions, which extends the amount of time required for plants to degrade herbicides. Rapid degradation under warm conditions allows plants to escape herbicide injury. Herbicides may be sprayed following cold night-time temperatures if day-time temperatures warm to at least 60°F.
Some “Fop” ACCase herbicides (Puma) are more effective during cold/cool temperatures and are much less effective when grass weeds are drought stressed. Other ACCase herbicides, such as Assure II*, Poast, and Select* control grasses best in warm weather when grasses are actively growing. ALS grass herbicides in wheat generally provide more consistent and greater grass control in warm, dry conditions compared with cool, wet conditions. Cool or cold conditions at or following application of ACCase herbicides and significant rainfall shortly after Achieve application may increase injury to wheat. Wild oat is a cool season grass but green and yellow foxtail are warm season grasses which may stop growing under cold conditions, resulting in poor control. Weeds are controlled most effectively when plants are actively growing.
Cold temperatures and freezing conditions following application of ALS herbicides, Buctril*, and Sencor* may increase crop injury with little effect on weed control. Delay applying fenoxaprop, ALS herbicides, and Sencor* until daytime temperatures exceed 60°F and after active plant growth resumes.
Basagran*, Cobra, Flexstar, Ignite/Liberty , paraquat* (Gramoxone), Reflex, and Ultra Blazer are less likely to cause crop injury when cold temperatures follow application but less weed control may result.
2,4-D, MCPA, Banvel*, Starane*, Stinger*, and Roundup* (resistant crops) have adequate crop safety and provide similar weed control across a wide range of temperatures, but weed death is slowed when cold temperatures follow application.
* Or generic equivalent