A big question this spring is how does one control Palmer amaranth in Xtend soybean with no dicamba option?  The fundamentals of leaning heavily on herbicides that provide good residual control of pigweed is the answer. Starting in about 2019, the most consistent strategy to control Palmer amaranth has been overlaying residuals.  That was because POST options started becoming less reliable due to the evolution of dicamba/2,4-D resistance stacked onto pigweed already resistant to glyphosate, PPO and ALS- inhibiting herbicides.

Liberty is the last best single POST option but it must be applied to Palmer less than 4” tall.  That sounds easy on paper but logistically getting every field sprayed before that fast growing weed exceeds 4” is very difficult, if not impossible. If the pigweed is large, then two Liberty applications about 7 days apart is really the only “rescue” treatment.

However, there are many effective residual herbicides that can provide good control of most Palmer amaranth in Tennessee.  They center around using a premix that is based on either metolachlor, pyroxasulfone, acetochlor, or dimethenamid applied PRE.  Those herbicides should not be used alone PRE, but rather should be coupled with some combination of 1 or 2 of the following herbicides to provide the most consistent pigweed control: metribuzin, flumioxazin, fomesafen, saflufenacil, or sulfentrazone. Those PRE-applied tankmixtures then should be followed by either metolachlor, pyroxasulfone, or acetochlor applied early POST.

This strategy should put a lot less pressure on Liberty as it only has to control a small amount of pigweed in a given field.

There are unfortunately two drawbacks to this strategy.  First, is that a timely rain or irrigation will be needed to activate residual herbicides. The other problem to this plan is in fields where pigweed has developed metabolic type resistance, which may shorten the length of residual activity. In either case, those herbicides will still provide some control though POST Liberty applications will likely need to be even timelier.