Temperature and Herbicide Application Questions
Ideal temperatures for applying most POST herbicides are between 65 and 85 F. Weeds may be killed slowly below 60 F. Some herbicides may injure crops if applied above 85 F. Avoid applying volatile herbicides such as 2,4-D ester, MCPA ester and dicamba during hot weather, especially near susceptible broadleaf crops, shelterbelts, or farmsteads.
Temperatures following herbicide application determine crop safety or injury. Cold temperatures may influence crop safety and weed control from herbicides. Plants degrade herbicides by metabolism, but plant metabolism slows during cool or cold conditions, which extends the amount of time required to degrade herbicides in plants. Rapid degradation under warm conditions allows crop plants to escape herbicide injury. Herbicides may be sprayed following cold night-time temperatures if day-time temperatures warm to at least 60 degrees.
Wild oat is more sensitive to fenoxaprop (Puma) during cool rather than warm/hot conditions. Green and yellow foxtail are warm season grasses and may shutdown under cold conditions resulting in reduced control. Grass and broadleaf weeds are controlled more effectively when plants are actively growing. Cool or cold conditions at and following application of Achieve and products containing fenoxaprop give greater grass weed control but also may cause crop injury. Other ACCase herbicides of Assure II, Poast, Fusilade DX, and Select provide better grass control in warm weather when grasses are actively growing.
Cold temperatures, including freezing conditions following application of ALS herbicides, Sencor, and bromoxynil may increase crop injury of respective crops with little effect on weed control.
Basagran, Cobra, Flexstar, Liberty, paraquat, Stampede, and Ultra Blazer may not cause crop injury when cold temperatures follow application but less weed control may result.
2,4-D, dicamba, MCPA, Stinger, Starane, glyphosate (resistant crops) have adequate crop safety and provide similar weed control, but weed death is slowed when cold temperatures follow application.
Recommendations for applying products containing fenoxaprop, ALS herbicides, and Sencor is to delay application until daytime temperatures exceed 60 degrees F and after active plant growth resumes.
Adjuvants may also affect crop safety and weed control. Oil additives may increase risk of crop injury but may be necessary for greatest weed control. Refer to each herbicide label for specific information on adjuvant use during stress conditions. Use an oil additive if risk of crop injury is acceptable for those herbicides that allow use.