The effectiveness of glyphosate to control weeds is unquestioned, yet it’s still common to see less than desirable weed control in soybean fields where glyphosate was applied.  Chris Boerboom, UW-Extension Weed Scientist, has put together a nice little laundry list of factors that impact glyphosate’s effectiveness.  They are as follows: 

1.   Weed size:  Like most herbicides, glyphosate is most effective on small annual weeds.  It’s generally better to spray a little early than too late.

2.   Rate:  Larger weeds call for higher spray rates.  Check labels for the toughest weeds you are spraying and use the rate recommended for their size.

3.   Hard water:  Hard water antagonizes glyphosate’s activity.  This can be overcome by adding ammonium sulfate to the spray mixture before adding the glyphosate.

4.   Micro-nutrient fertilizers:  Ditto the issue and solution with hard water.

5.   Spray volume:  Lower spray volumes (e.g. 10 gal/A) have a general advantage over higher volumes.  More glyphosate is concentrated in each droplet.  Large volume applications may have an advantage  when weeds are large and the canopy is dense.

6.   Formulation:  Glyphosate is glyphosate, but not all products have the same amount of surfactant so check the label and add surfactant as directed.

7.   Weed tolerance:  Some weed species have more natural tolerance to glyphosate than others.  Grasses tend to be very susceptible while broadleaves can be more challenging (larger common lambsquarters for example).  Field horsetail and yellow nutsedge have natural tolerance.

8.   Rain:  It takes a few hours for glyphosate to be absorbed into plant leaves.  Rain during this period may reduce control of larger or hard to control weeds.

9.   Dew:  A heavy dew is thought to act similar to rain, especially with higher spray volumes.  A light dew may actually increase herbicide activity by slowing the drying of the droplets.

10.  Time of day:  Glyphosate activity is slightly reduced with early morning or late evening applications.  Again, the effect is more dramatic with larger weeds and reduced rates.

11.  Drought:  Drought-stressed weeds are not controlled easily by any herbicide, including glyphosate.      

12.  Dust:  Dust on leaves can inactivate glyphosate in spray droplets.  This can be caused by gravel roads or dust raised in tire tracks.