Pigweed is breaking through Tennessee cotton and soybean fields. Pre-applied herbicides, depending on field and weather conditions, likely allowing pigweed to bust through and now is the time to make some decisions.

According to a May 27 blog by Larry Steckel, University of Tennessee Extension weed specialist, residual control varies depending much on weather:

“A good example of this is a study where we are examining the residual performance of Brake F2 against standards like Cotoran, Caparol and Warrant. 

At Jackson, even the half rate of Brake F2 (8 oz/A) was still holding pigweed 21 days after planting. However, at the Milan location which received about 2 inches or more rain than Jackson the full rate of Brake F2 (16 oz/A) was pigweed infested by 18 days after application.  Warrant was actually providing the best residual control at the Milan location while at the drier Jackson location it has allowed some Palmer to emerge.

In soybean, herbicides like Valor applied at 2 oz/A have begun to allow pigweed to emerge about 14 days after application while others like Boundary or Authority XL were lasting about 17 days after emergence.

This time of year with good temperatures and moisture, Palmer amaranth can easily go from a seed no bigger than the period at the end of this sentence to a weed 8 inches tall.  As most are aware none of the PPO inhibiting herbicides (Flexstar, Cobra, Ultra Blazer, etc) will control a Palmer much over 3 inches tall. In my mind a 3-to 4-inch tall Palmer is a “rescue” treatment.  One has to use the max herbicide rate plus add MSO to have any chance to control it.

In these cases where Palmer has broken through the pre-applied herbicide, a follow-up PPO herbicide or Liberty application on newly emerged Palmer will be needed quickly. I know some producers are utilizing Liberty Link soybeans on their worst pigweed fields.

Liberty will control larger Palmer than Flexstar and Cobra.  However, in fields with overwhelming pressure, spraying small (<3 inches) Palmer as well as good coverage will be critical to obtaining good control with Liberty.  In short, be prepared to run the sprayers hard over the next week or two regardless of the herbicide trait technology.”