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Three Considerations for Burndown Herbicide Applications

Cold Temperatures and Burndown Herbicides

Field activities have started, seed has been delivered, and weeds are growing. But don’t get too anxious to get into the field says Bob Hartzler, Iowa State University Extension weed specialist. 

“A concern for many will be the effect of the widespread freeze on the performance of burndown herbicides,” says Hartzler. “Unfortunately, there is no simple blanket statement that can be made since the plant response will vary depending on weed species, weed size, and the herbicides used.”

There are several factors to consider before applying postemergence herbicides. Keep these three tips in mind as you plan your herbicide applications. 

1. Apply postemergence herbicides when weeds are actively growing. 
“A statement found on most postemergence herbicide labels is ‘Apply when weeds are actively growing.’ This is by far the most important consideration in determining whether to apply a postemergence product,” explains Hartzler. “Most weeds that emerge in March are adapted to subfreezing temperatures and will not be killed by frost.”

Even though those weeds won’t be killed by a frost, they will need time to recover. “Performance of herbicides will be reduced if applied too soon following a frost,” says Hartzler. 

How long should you wait to make an application? There’s no simple answer, says Hartzler. It depends on the weed species, severity of the frost, and weather conditions following the freeze. He advises growers to closely monitor the weeds for evidence of new growth.

2. Consider your environment.
The effectiveness of herbicides will depend partially on the environment. “Glyphosate relies on translocation within plants for good activity, and herbicide movement within plants is greatly slowed during cool periods,” says Hartzler. “The general recommendation is to avoid glyphosate applications when evening temperatures fall below 40°F; 2,4-D is somewhat more consistent than glyphosate during cool periods.”

Addition of 2,4-D LV ester can enhance burndown performance in certain situations, he adds. Burndown herbicides that interfere with photosynthesis are affected both by temperature and the intensity of sunlight during and after application. 

3. Know the weather conditions.
“Weather conditions prior to and following application have a strong influence on performance of early spring herbicide applications,” says Hartzler. “In some situations, the result will simply be a slower kill of target plants, but in other situations, control failures may occur.”

Try to avoid applications during periods of prolonged cool temperatures (<40°F. at night; <55°F. during the day), says Hartzler. 

“If applications must be made during marginal conditions, increasing rates of the herbicide and spray additives to maximum levels allowed on labels can enhance performance consistency,” suggests Hartzler. “Adjusting the sprayer or spray volume to achieve more uniform coverage of the target can also reduce variability in activity.”

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