Effect of Rainfall on Glyphosate

Chris Boerboom, Ext. Weed Scientist

I received this question this week.  Although it may not have a clear answer, it is worth a short discussion. 

“I've sprayed glyphosate on some soybeans when I was not expecting rain for at least 3 hours. Unfortunately, a light rain began to fall about 20 to 30 minutes after a glyphosate treatment.  How can I tell if the herbicide is working or if I'll need to respray?”

To start, what is the definition of a light rain?  Is it a trace, 0.01 inch, 0.05 inches, or more?  For postemergence herbicides, I cannot find any reports on the volume of rain that it takes to wash glyphosate off leaves after a quick search of the literature.  Many research papers report on the effect of the time between herbicide application and rainfall on the level of weed control (ie rain fastness). In many of these cases, the researchers use 1 inch of simulated rain, much more than a light rain.  However, I think many people have had the experience of making a postemergence herbicide application only to be followed by a shower that developed out of nowhere.  Many times these applications are still successful in controlling the weeds.  If the leaf was not fully wet by the rain, I think it is likely that enough herbicide will remain to still be effective.  If the leaf was fully wet by the rain, then it is much harder to know how much herbicide may have been washed from the leaf.  We don’t know if the leaf just got to the point of being fully wet without losing herbicide or if it was wet plus being washed.  Of course, leaves of some weeds appear to be wetted easier than others and might be more at risk of a light rain.   

To tell if the glyphosate is working is tough because glyphosate is slow acting.  The first weeds that might show symptoms are likely to be sensitive to the lowest rates.  So, if you have rain and loose half of the glyphosate from sensitive plants like foxtails, you still might see symptoms on foxtails (turning off color) by 5 days after spraying and get full control of them.  However, if only half the glyphosate remains, you may not get control of the broadleaf weeds, which are more tolerant.  So, I would focus on checking for glyphosate activity on broadleaf weeds such as velvetleaf or lambsquarters after 5 days and see if they are starting to get wilty or turning off color.  This would be a good sign.  Unfortunately, this still does not tell us if they got a full dose of glyphosate or if a percentage was washed off.  If some herbicide was washed off, some weeds might recover.  In this case, if noticeable symptoms are not seen after 5 days, I would respray if the weeds are so large that you cannot wait any longer. 

This applicator needs to monitor this field because it rained soon after the glyphosate application.  However, all applicators are wise to monitor fields that were rained on even after longer intervals.  Regardless of formulation, glyphosate is not rapidly absorbed in the leaves of weeds and there is a risk of wash-off.  Last year we sprayed moderately sized lambsquarters with 32 oz/a Glystar Plus or 22 oz/a WeatherMax (both provide 0.75 lb ae/a glyphosate) and then washed the plants with a simulated rain at 0.5, 1, 2, and 4 hours after the applications. 

We had reduced lambsquarters control with both formulations even with a 4 hour interval between the application and the simulated rain.  The WeatherMax was slightly better than Glystar Plus at the intermediate times, but control was still less than either formulation without rain.  The reduction in control with rain should be less with smaller or more sensitive plants.  Still, it is worth a quick field check to make sure you are getting the weed control you need in cases when it has rained.

	Lambsquarters control (%)
Simulated rain	Glystar Plus	WeatherMax
0.5 hr	56	51
1 hr	41	61
2 hr	45	79
4 hr	69	81
No rain	93	96

Suspicious Common Lambsquarters Escapes Wanted

Chris Boerboom and Dave Stoltenberg, Agronomy Dept.

We are aware of many application or environmental reasons why common lambsquarters is not consistently controlled with glyphosate.  However, we have continuing concerns about the potential for common lambsquarters to become resistant to glyphosate in Wisconsin for many reasons that we have discussed previously.  We are interested in determining if there are lambsquarters populations in Wisconsin that starting to show resistance to glyphosate.  If you have common lambsquarters that has escaped control of a glyphosate application that does not seem to have a logical explanation, please let Chris Boerboom know at boerboom@wisc.edu.  If the case is compelling, we would like to arrange to collect a sample with mature seed for further testing.

Consider the following points in making your assessment.  The environmental and application factors that can lead to glyphosate performance problems should be ruled out. Some factors that may reduce glyphosate performance include:

- incorrect glyphosate rate for weed size or species

- rainfall prior to complete glyphosate absorption

- weather-stressed weeds (drought, cold, etc.)

- incomplete spray coverage of weeds below the canopy

- reduced glyphosate activity with early morning or late evening applications

- weeds emerging after the glyphosate application

If all of the following points exist, the potential for a true resistance case increases.

- no known application errors and correct glyphosate rate was added to the tank

- environment was favorable for glyphosate performance (unstressed plants)

- only one species escaped control (weed escapes may be localized in patches)

- weed species in question is typically controlled by glyphosate

- weed shows evidence of being sprayed (e.g. glyphosate symptoms)

- glyphosate has been used frequently in the past

Routine Herbicide Resistance Screening

Chris Boerboom, Ext. Weed Scientist

We have not established a service at the University of Wisconsin to conduct routine screening of weeds for herbicide resistance such as testing lambsquarters for atrazine resistance or foxtail for ALS resistance.  However, a screening service is available through Michigan State University’s Diagnostic Services.

Steven Gower, MSU, writes “Confirming herbicide-resistant weed populations is the first step of any resistance management program.  Verification will provide producers with the knowledge to implement the best possible management strategies, with the ultimate goal of preventing or limiting the spread of herbicide-resistant weeds. 

If herbicide resistance is suspected in any weed species, samples may be submitted to MSU Diagnostic Services for an herbicide resistance screen.  In most circumstances, a whole plant pot assay established from seed will be our standard test for herbicide resistance confirmation.  Mature, high quality seed or seedheads should be collected from suspicious plants in late summer or fall and submitted in a paper bag or envelope.  Do not seal plants or seed in plastic! 

Fees associated with herbicide-resistant weed testing are $75 per sample per herbicide site of action (i.e. ACCase inhibitors, ALS inhibitors, Photosynthesis inhibitors).  Each additional site of action is $30 per sample.”

Please contact Steven Gower (517-432-9693, sgower@msu.edu) with any questions regarding resistance confirmation or sample collection.  Samples can be mailed to:

Michigan State University

Diagnostic Services

101 Center for Integrated Plant Systems

East Lansing, MI  48824-1311