The size of the Palmer amaranth ranged from 6 to 14 inches tall, says Kichler. “In Macon County, the population was highly resistant while in Tift County, we had more of mixed population of glyphosate-resistant Palmer amaranth.”

At 20 days after treatment, when the percent of Palmer amaranth control was evaluated in Tift County, 2,4-D, Clarity and Liberty were the most effective while Roundup was the least effective. Roundup gave about 50-percent control, he says.

“All of the combinations were significantly better than Liberty or Roundup alone. Control increased when we combined 2,4-D with glyphosate. The population in Tift County did have some sensitivity to glyphosate. We had great control with the three-way combination of Roundup, Liberty and 2,4-D and also Roundup, Liberty and Clarity.”

In Macon County, at 22 days after treatment, control was terrible, he says, with 2,4-D and Clarity being the most effective treatments among the herbicides that were applied by themselves.

“But we still had less than 60 percent control. Roundup was the least effective at 30 percent control. When we looked at the combinations, control also was terrible with those. But all combinations were better than Liberty or Roundup applied by themselves. The Liberty plus Clarity, Roundup plus Liberty plus Clarity, and Roundup and Liberty tank-mixed with 2,4-D were the most effective in Macon County.

“Roundup and Liberty tank-mixed with 2,4-D was very effective in Tift County, but the pigweed got ahead of us in Macon County. Clarity in that tank-mix was very effective in Tift County with the sensitive population there.”

Looking at the percent of grass control 22 days after planting in Tift and Macon Counties, Kichler says Roundup applied alone was the most effective, but 2,4-D and Clarity was least effective, giving less than 20 percent control.

Evaluating the annual grasses with the tank-mix combinations, the Roundup combinations gave similar control to Roundup applied alone, he says.

“The Liberty plus 2,4-D or Liberty tank-mixed with Clarity gave us similar control compared to Liberty applied alone. But when we looked at the three-way combinations of Roundup, Liberty and Clarity, and Roundup, Liberty plus 2,4-D, control was increased significantly, and those two treatments gave us similar control as Roundup applied by itself. In Tift County, we had really good control with our Roundup tank-mixes – Roundup and Roundup plus 2,4-D.:

In summary, says Kichler, Roundup plus Liberty plus the auxin mixes were the most effective options.

“The control of large Palmer amaranth was good in Tift County, but the population was more sensitive to glyphosate. In Macon County, where we had full resistance to glyphosate, control was very poor. The auxin combinations improved grower flexibility to a point, but if you have good growing conditions – considering how Palmer amaranth grows – it might give you only a few days of flexibility.”

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