ISSUE 6   June 17, 2010


Have had reports of normal X rates of herbicides, especially applications with glyphosate, giving less weed control than expected when applied during cool/cold temperatures. Paragraph 16 on page 70 of the weed guide gives some information on glyphosate activity when applied in cool or cold conditions.

"16. Weed control from glyphosate applied during or after cold weather may be the same as from application in warm weather but the end result (weed control) will take longer. Ideal temperatures for applying POST herbicides are between 65 and 85 F. Speed of kill will be slower during cold weather also. Use higher rates to overcome reduced control from cold temperatures before or after application. Cold weather is a stress to plants. Weeds with low level resistance may not be controlled whether in good or adverse conditions. Proper timing of glyphosate application is critical for adequate weed control. Glyphosate applied during cold weather and to large weeds will result in less weed control. AMS enhances weed control and can partially overcome reduced control of stressed plants."

Data from research conducted by Dr. Jeff Stachler in 2009 (unpublished) give more insight on temperature affect. His data show that wide temperature fluctuations before and after application will decrease herbicide activity (plant response) as compared when cool/cold temperature exist before and after application. Final weed control was mostly affected when cool/cold conditions existed before and after applications but when temperatures varied widely the few days before and after application then control was greatly reduced. More research is needed as there are many variables at play here but wide temperature fluctuations may explain some situations where weed control was poor.



For those that attended the Wild World of Weeds Workshop in January, Dr. Carrie Laboski gave our invited speaker presentation on N competition by weeds. Her results showing the amount of N taken up by weeds are startling. The data she presented reinforces good weed management not only in corn but any crop. Several have requested her presentation and below gives the web site where her presentation can be found. It has been posted on the website:

It's the first presentation under teaching materials.

Some conclusions include:

  • Economic optimum N rate (EONR) increases as weed control decreases.
  • Additional N required to attain the same yield as using a PRE herbicide (no or little weed infestation):
  • Additional N needed was greater than weed biomass N
  • With no weed control, weed + corn N uptake was greater than corn N uptake with no weeds present
  • At current N prices:


    Weed scientists in the NC region have developed a publication series called, "The Glyphosate, Weeds, and Crop Series". We have just printed another publication called, "Biology and Management of Common Ragweed". As this weed has become a greater problem in row crops, and has shown resistance to glyphosate, ALS herbicides, and PPO inhibiting herbicides, this information can be helpful to use every tool to control it.

    The publication is free and can be ordered from our NDSU Ext. Distribution Center

    The publication is also available on the web:

    Rich Zollinger
    NDSU Extension Weed Specialist



    Scout Roundup Ready sugarbeet fields 14 to 18 days after the initial glyphosate application to determine the effectiveness of the herbicide treatment. If weeds survived the initial application, determine why they survived. Once surviving plants have grown 1 to 3" taller/larger compared to when they were sprayed, apply glyphosate at the maximum legal amount available. The maximum amount of glyphosate that may be applied to Roundup Ready sugarbeet in a single application prior to the 8-leaf sugarbeet stage is 1.125 pounds acid equivalent per acre (lbs ae/A). Consult the Glyphosate Formulations Labeled for RR Sugarbeet in 2010 publication found at the website, or page 71 of the 2010 North Dakota Weed Control Guide for additional information about product rates of various glyphosate formulations. The total maximum amount of glyphosate that may be applied prior to the 8-leaf sugarbeet stage is 1.96 lb ae/A. The maximum amount of glyphosate that may be applied in a single application after the 8-leaf sugarbeet stage is 0.75 lb ae/A. If weeds surviving the initial glyphosate application can be controlled by other sugarbeet herbicides, mix the appropriate herbicides with glyphosate in the second application. Include an adjuvant that maximizes the activity of the tank-mix partner. If lambsquarters is surviving the initial glyphosate application, include a good quality nonionic surfactant (NIS) with all glyphosate formulations, unless prohibited. If all weeds were controlled by the initial glyphosate application and newly emerged weeds are present, apply glyphosate approximately 21 days after the initial application or when newly emerged weeds reach 2 to 3 inches in height.

    Jeff Stachler
    Agronomist - Weed Science

    NDSU Crop and Pest Report Home buttonTop of Page buttonTable of Contents buttonPrevious buttonNext button