Effect of cool temperatures on herbicide performance
by Bob Hartzler

Revised April 20, 2002 - The wide temperature fluctuations in the past couple weeks have brought on questions of burndown herbicide performance during cool temperatures.  This is similar to when cool temperatures visit us during postemergence application season.   This article addresses both situations.  As with most herbicide decisions, there is not a single answer that fits all situations. 

Herbicides perform best under ideal conditions when both the crop and weeds are actively growing.  When the environment reduces the vigor of the crop or the weed, undesirable consequenses can occur - either crop injury or poor weed control.  The most noticeable effect of cool temperatures on herbicide performance is a slower kill of weeds.  A herbicide kills a plant by disrupting some physiological process essential for growth.  Under cool temperatures, physiological processes slow down, thus the herbicide is slower acting.  In some situations the desired effect (dead weeds) will result - it just takes longer to get the job done.  In other situations, the slower activity of the herbicide will allow some weeds to survive, or the crop may be injured.

The simple solution would be to wait until more favorable temperatures arrive; however, delays in application could create problems if weeds are already at the optimum size for control when the cool temperatures arrive.  Factors to consider in whether to spray under these cool conditions or to wait until warmer conditions arrive include:
       -  susceptibility of target weeds to herbicide
       -  margin of crop safety to herbicide
       -  weed size and population

The likelihood of decreased weed control due to cool temperatures will vary depending upon the target weed and the herbicide and rate applied (Table 1).  Weed species highly susceptible to the herbicide are less likely to show a negative response to cool temperatures than less susceptible weed species.  For example, assume two soybean fields have similar weed infestations of giant foxtail and velvetleaf and both fields are sprayed during a period of unseasonably cool temperatures.  One field was Roundup Ready and treated with Roundup Ultra, whereas the other was treated with Pursuit.  The field treated with Pursuit would be more likely to have foxtail escape since foxtail is more tolerant of Pursuit than velvetleaf.  On the other hand, velvetleaf would be more likely to escape in the Roundup Ready field than giant foxtail, again due to differential tolerance of these species to Roundup.  Although it is best to delay applications, the potential for a negative response to temperature can be reduced by ensuring that optimum rates (full label) are used.

Undesirable crop responses are more likely to occur when using herbicides with lower margins of crop safety.  Certain herbicide labels have warnings on the label concerning increased risk of injury under cool conditions.  If plans are to use one of these products, the best decision would be to delay application until more favorable conditions occur or switch to a product with a greater margin of crop safety.  A partial list of products with cool temperature warnings includes Basis Gold, Accent Gold, Reflex, Lightning, Cobra, and Buctril.  Read all labels to determine if restrictions or warnings are present concerning use in cool temperatures.

Crop and weed size also should be considered when determining whether or not to spray.  A soybean field in  the first trifoliate stage with one inch foxtail provides greater flexibility in application timing than a corn field at the V4 stage with three to four inch giant foxtail.  In the corn situation, the foxtail may exceed the height for optimum control by the time temperatures increase, even with the reduced growth rates under the cool temperatures.

In summary, the ideal solution is to wait until better weather conditions arrive to treat fields.  However, if weed size or other situations dictate that fields be treated now, select the product that has the best margin of crop safety and is strong against the target species.  Keep in mind that reduced rate treatments are less likely to provide acceptable control under adverse conditions than when plants are actively growing.  Finally, the performance of the row-crop cultivator is not affected by temperature and would be a great choice under these conditions.

Table 1.  Effect of temperature on percent reduction (%) in dry weight of green foxtail and redroot pigweed provided by postemergence Accent applications.¹

¹Weed Technology. 1991.  Vol. 5: 92-96.  Nalewaja, Woznica and Manthey.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University