Gone are the days when Alabama farmers saw glyphosate-resistant Palmer amaranth pigweed as a potential problem, restricted mostly to fields in neighboring states. It’s here, and it’s spreading, says Mike Patterson, Auburn University Extension weed scientist.

This past year, he says, it was confirmed in 20 counties in the state. In north Alabama, it has been found in Colbert, Lauderdale, Lawrence, Limestone, Madison, Ethowah, Calhoun and Cullman counties. In southeast Alabama, resistant Palmer pigweed has been confirmed in Barbour, Russell, Henry, Houston, Dale, Coffee, Covington, Pike and Geneva counties. It also has been found in the southwest counties of Baldwin, Escambia and Mobile.

There are critical points for growers to remember when attempting to control glyphosate-resistant Palmer pigweed in row crops, says Patterson. It’s important, he says, to plant into clean fields.

“Kill all weeds prior to planting, especially resistant weeds, using products that give residual control past the planting date,” he says. “And maintain constant residual control by using soil-active herbicides throughout the growing season.”

Growers should maintain post-harvest pigweed control, especially in early harvested crops like corn, by mowing, followed by 2,4-D or Gramoxone plus 2,4-D, he recommends. “Also, rotate crops and use herbicides with different modes of action in each crop,” says Patterson.

Turning specifically to controlling glyphosate-resistant Palmer amaranth in cotton production, growers must maintain residual control from burndown through layby, he says. Examples of such programs include the following:

• Diuron plus Gramoxone with crop oil concentrate (with Ignite if resistant horseweed) preplant foliar 15 days ahead of planting.

• Reflex (with Prowl, or diuron, or Staple) pre-emergence behind the planter. You must use Reflex with preplant foliar mix on fine-textured soils of the Tennessee Valley (and get one half inch of rain before planting cotton). If this is not activated by rain or irrigation before cotton emerges, then apply Staple before Palmer is 2 inches tall.

• Metolachlor (Dual Magnum, etc.) or Warrant with glyphosate (RRF cotton) or Ignite (Liberty-Link cotton) early postemergence (three to four-leaf cotton).

• Layby Pro or diuron (add 2 ounces of Valor) plus MSMA layby (these mixtures will kill a relatively large pigweed if the sprayer can cover it, but it must be kept off the cotton.

All of these residual herbicides must be activated by timely rainfall or overhead irrigation in order to work, says Patterson. These products will add between $30 (Reflex, Staple, Dual, Valor) and $40 (add Ignite burndown) per acre to your weed control costs above a system where two to three applications of glyphosate have worked in the past.