The dry conditions that we experienced (in Tennessee) last week and that continue through this week are not activating pre applied herbicides. Reports of Palmer amaranth already one to two inches tall with emerging crops appears to be common. How to proceed from here on weed management depends upon the crop as well as the herbicide tolerant trait in the crop.
In RR or conventional soybeans a number of folks are weighing how to manage the emerged Palmer amaranth. The uncertainty comes from the fact that the PRE applied herbicide is still present and will activate with a rain. If the PRE applied herbicide is a Valor or Authority based product and Prefix (fomesafen and Dual Mag) is used now to control the Palmer some soybean injury would be likely if fomesafen and Valor or Authority activate all at once. With this in mind it would be best to try to remove the existing Palmer amaranth with Ultra Blazer or Cobra and come back later with Prefix.
A few are considering applying Gramoxone now and replanting. This would be the best option if Palmer amaranth is near the 3” range as neither Ultra Blazer, Cobra or fomesafen (Prefix, Flexstar, Rhythm etc) will likely control Palmer that large.
In LL soybeans Liberty can control Palmer amaranth even up to 4” tall in one shot. Remember for best results to apply the Liberty from midmorning until about an hour before sundown. For larger Palmer amaranth a sequential application of Liberty applied about 7 days later may be needed for complete control.
There are some good options in corn for this problem. All the common premixes like Halex GT, Capreno and RealmQ tankmixed with atrazine will control Palmer. Once corn gets about 12” tall consider using 4 ozs/A of Status in place of atrazine. Our studies over the years would indicate that at least two modes of action effective on Palmer are needed to control it in corn once Palmer gets 6” tall.
In grain sorghum if Palmer is less than 4” tall the best option would be 2 lbs of atrazine and crop oil. Another good option would be Huskie tankmixed with either atrazine or 8 oz of dicamba. Remember do not apply anything other than Dual to grain sorghum until it is showing 3 true leaves. At that time it has its feet under it well enough to handle herbicide applications. If POST herbicides are applied prior to 3 lf grain sorghum stand loss may occur.
In cotton, Liberty will need to be applied fairly quickly to control the emerged Palmer amaranth. Tankmix in Dual Magnum with the Liberty in cotton that is a Glytol/LL or Roundup Xtend. For WideStrike cotton it is safest if the Liberty is applied first and then followed with Dual Magnum.