Most of the time adding a good surfactant if one is not already formulated in the product is adequate. Ammonium sulfate (AMS) is often recommended and may or may not improve performance of glyphosate. The advantage offered by AMS is often dependent on water quality. Spray water containing high amounts of calcium, potassium, magnesium, and iron can be antagonistic to salt formulated herbicides such as glyphosate, dicamba, and 2,4-D.

There is a complicated formula to determine exactly how much AMS is needed to overcome this antagonistic effect based on a complete water analysis. Different water qualities require different amounts, which explains why one farmer experiences great improvements by adding AMS while others may see no improvement at all. If you know your water contains high amounts of these elements, AMS should be used in burndown spray applications.

All liquid AMS formulations are also not the same, with different amounts of ammonium sulfate per gallon.

And finally the discussion gets around to herbicide antagonism. Weed control with herbicides is a biological system where the herbicide interferes with some life-requiring process within the plant. Even with the sophisticated biochemistry associated with the development of modern herbicides, there has never been nor will there ever be a perfect herbicide that controls all weeds and does not harm crops.

In order to broaden the spectrum and pick up weeds that one herbicide may miss, we often tank mix multiple herbicides. When two or more herbicides are mixed, three things can happen. The mixture can produce an additive effect where the weed control is roughly the sum of the control expected if the herbicides had been applied alone. Or, a synergistic effect can be noted where the control is better than the sum of the tank mixed herbicides applied alone. These are the anticipated or “hoped for” results.

Unfortunately, the third effect can be antagonism where the result is reduced control of certain weeds.

Herbicide antagonism is easy to see, but very difficult to predict and even harder to explain. Some antagonism is caused by one herbicide reducing the absorption of another; some is a result of reduced translocation once inside the plant; while others may simply alter target sites of the companion herbicide.

When we talk about antagonism, the graminicides such as Select, Fusilade, Ricestar, and Clincher are always the first to be mentioned because they are notoriously easy to antagonize, but they are not the only ones that require our attention. We often see control with glyphosate reduced when it is mixed with other herbicides used in burndown programs.

Command added to glyphosate reduces glyphosate efficacy when applied prior to planting rice and it is well-documented that dicamba used for horseweed control can reduce the activity of glyphosate.

Antagonism to glyphosate can most often be overcome by increasing rates. With the lower price of glyphosate, it is doubtful that anyone is shaving rates, but burndown tank mixes may require a slight rate bump.

So, with all this discussion, “What went wrong in 2011 burndown programs?” Probably all the above. I do think we can improve our herbicide performance with careful attention to application technology including volume, tip selection, and speed. Knowing the water quality used in spray solutions may help to make intelligent decisions about what additives to use and although we cannot change herbicide antagonistic interactions, we can keep the rates up to the higher recommended levels. As we have learned this spring, the most expensive herbicide application is the one that does not work.