Lack Of Activating Rain On PRE Applied Herbicides

DR. LARRY STECKEL

JACKSON, TENN.
   Corn, soybean, cotton and grain sorghum are all being planted as fast as possible. In typical Tennessee weather tradition, we have gone from too wet to too dry in a very short period of time.  The rain forecasted for this week did not appear and the best chance for rain will likely be early next week.  There is no doubt we will need that rain!
   There have been a lot of herbicides applied PRE and many of them will sit a good 10 to 20 days before our next best chance to get a rain to activate them.  A common question has been “how long can those herbicides wait before a rain?”
   There is no one answer to this question as it really depends upon the herbicide and soil type. In general, herbicides like Dual Magnum and Outlook will likely last about 10 to 14 days without a rain before they play out. Zidua will last a little longer. PPO herbicides like Valor, Reflex and Authority can sit a good while without rain. The problem is that Palmer amaranth can become established in the meantime and even if the herbicides do become activated, those early emerging Palmer will likely escape.
   Herbicides like atrazine or Balance, which is in Corvus, can actually reach out and kill small Palmer amaranth after a rain. However, there are limits to this and once Palmer gets larger than 2” it becomes fairly tolerant to even this late activation.
   Scouting fields to ensure PRE applied herbicides are working is always important, but it will be even more so this year. Having boots in the field to determine the timing of which fields will need to be treated with that first POST shot will be crucial. With so much crop planted in such a small amount of time coupled with likely very spotty PRE activation will set us up to have many acres to spray POST all at once.  Knowing which fields that absolutely must be treated first will be very important! ∆
   DR. LARRY STECKEL: Extension Weed Specialist, University of Tennessee