The drought of 2012 is shaping up to be one of those widespread tough ones no one forgets, like 1983 or 1988. Hopefully, we will get a good general rain here shortly. This drought has been the source of calls from frustrated folks who are still trying to control Palmer amaranth. The question has been asked should I discontinue weed control with the yield potential of my crops going down every day. When thinking about this question keep a couple of facts in mind.
First, clearly our crops are struggling for every pound of lint or bushel of soybeans they can produce with the drought and heat. However, since this drought is so widespread the commodity prices should be strong this fall which makes every additional pound of yield more valuable than in a typical growing season.
Second, Palmer amaranth is even more competitive in a hot and dry environment than other weeds. Palmer is native to the desert southwest, so hot and dry is right up its alley. The optimum temperature for photosynthesis in soybean is 86° while cotton is 91°. In contrast, Palmer amaranth will run at its optimum photosynthesis rate at 97° and stay near that high photosynthetic level up to temperatures as high as 114°. In other words, what we and most crops consider hot is right in Palmer amaranths wheel house. Palmer will magnify drought stress and reduce soybean and cotton yield in a drought more completely than other weeds. Therefore, if Palmer amaranth can be controlled it should be. The question then comes into play is the Palmer amaranth at a controllable stage. This question has to be asked and answered quickly as a controllable stage is just a few days in most cases.
Palmer amaranth at 3” or more in height cannot be effectively controlled with a PPO herbicide like Flexstar, Ultra Blazer or Cobra particularly when the weed is under drought stress. A repeat application will typically not control regrowth of Palmer from earlier PPO herbicide applications and can be futile waste. This is particularly true in fields with overwhelming Palmer amaranth numbers. However, a sequential application of a PPO herbicide will typically set the Palmer back some, which has some value in fields where the Palmer amaranth numbers are fewer. This value can be capitalized on if hand weeding can provide some supplemental control.
Liberty on the other hand can control larger Palmer than a PPO herbicide. However, we are experiencing some inconsistent Palmer control with Liberty in some instances due to the Palmer being hardened off by the weather. A sequential application of Liberty applied about 5 to 7 days after the initial application will typically provide control of first application survivors.
