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Jim Meade / Iowa City
Posts: 2,415
Registered: ‎04-30-2010
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Rain Needed For Herbicide Activation

Here is a 2007 ISU study which cites a 1997 UI site.  The chart tells it all.

Bottom line - 1" for full control of foxtails.  1/2" gives 85-90% control.  Up to 1/4" shows about 50% control.

So, it looks like 1/2" gives most of the results but for full results we need an inch. 

http://www.weeds.iastate.edu/mgmt/qtr00-1/dryweather.htm