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Some areas of Alabama have been experiencing temperatures in the 30's this last week and forecasters say that more cold weather is to come. This is also the time to be applying burndown herbicides to prepare for planting corn, soybean, and cotton. This isn't something new for Alabama as we can often have temperatures in the 70's during the day and the 30's at night. A concern for many growers is how their burndown herbicides will work with near-freezing temperatures at night and cooler days.
The statement found on most postemergence herbicide labels is to apply when "weeds are actively growing". However, freezing or very cold temperatures don't kill the plant but it does take them time to recover. Herbicide performance after a frost will be reduced if applied after a frost. Watching the plants is the best way to determine if the plants have recovered and are ready to be treated with a burndown.
Any herbicide that translocates within the plant will be greatly slowed down during cool temperatures. An example of a herbicide that translocates is glyphosate. The general recommendation is to avoid applying glyphosate when night temperatures fall below 40°F. Herbicides such as dicamba and 2,4-D work better than others during this time of cooler temperatures. The herbicides used in burndowns that are light-activated such as Sharpen, atrazine, and Gramoxone will not be adversely affected by the cooler weather although they would work more consistently if night-time temperatures were warmer (in the 50's). This is because they interfere with photosynthesis and are affected by temperature and the amount of sunlight the day of application and the days following.
Herbicide applications are strongly affected by weather conditions including temperature. Herbicide labels will advise on the proper temperatures to apply in order to get a consistent kill of the target weeds. As the Sharpen label states, "Burndown activity may be slowed or reduced under cloudy and/or foggy or cooler weather conditions, or when weeds are growing in drought or other stress conditions". The "other" stress conditions can refer to low nighttime temperatures when the weeds are recovering. It is best to avoid any herbicide applications during times in which nighttime temperatures will be less than 40°F and daytime temperatures are less than 55°F. However, in Alabama we often have the low nighttime temperatures followed by higher daytime temperatures. You can expect to have slower activity and may not see as consistent control depending on the product you use. Sometimes increasing the herbicide rate or spray additives can enhance performance. Always read the label and stay within the label rates.
Joyce A. Tredaway
Crop, Soil, and Environmental Sciences, Auburn University
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