Iowa State University

Cold Temperatures and Herbicide Performance   
by Bob Hartzler

June 25, 2004 -   Much of the state experienced record low temperatures last night.  This event occurs at a peak time for postemergence herbicide applications in soybean.  Research has documented that below normal temperatures during the week prior to application can adversely affect postemergence herbicide performance (see Variable Herbicide Performance).  In 2001 many farmers experienced waterhemp escapes in Roundup Ready soybean when glyphosate was applied following a period when temperatures dropped into the low 50's (approximately 10^º below normal).  Today's temperatures in the low 40's or high 30's have the potential to significantly reduce the activity of glyphosate and other herbicides applied during the remainder of June.

In a perfect world the best option would be to delay applications for a week to allow plants to fully recover from last night's cold temperatures.  However, weeds in many fields have reached the size where they are beginning to compete with the crop.  Thus, delays in application could result in significant economic losses.  In fields with large weeds (> 6" height) it probably is best to treat the fields and take what mother nature gives.  In Roundup Ready soybeans applicators have much more flexibility in rate selection than with conventional soybeans and herbicides.  Increasing the rate of glyphosate probably will help overcome the negative impacts of the recent cold temperatures.

It is likely that many farmers will encounter reduced weed control due to recent weather events.  The abundance of rainfall in most areas of Iowa may help counteract some of the negative impact of these temperatures, but problems should still be expected.  Fields treated within the next week should be closely monitored following application to document herbicide performance.  When retreatment is necessary, follow up applications should be delayed until weeds damaged by the initial application show signs of recovery and new growth.

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