Situation:

Mississippi is just beginning to have problems with glyphosate-resistant Palmer amaranth. It is not a cotton-specific problem; rather it is locale-specific.

Distribution:

The resistant population is more prevalent in the north Delta and increases in occurrence closer to the levee, with the worst resistance inside the levee. Two populations near Tunica in 2007, the only weeds in the fields, exhibited the typical patchy appearance (approximately 15 by 15 feet). Each patch may have been from the seed of a single resistant plant that survived the previous year. The taxonomic classification of these plants is in doubt as some had characteristics of both Palmer amaranth and the waterhemp complex [What we refer to as the waterhemp complex or waterhemp may include tall waterhemp (A. tuberculatus), common waterhemp (A. rudis), or a cross between the two. [See Steckel article in Weed Technology, vol. 21 (2007), pp. 567-570.]

Impacts:

Observations from Mississippi work include:

• Palmer amaranth emergence: Palmer amaranth emerges later in the season than other pigweed species.
• Grower awareness: Grower awareness is increasing. The attitude of, “It’s not my problem until it’s my problem,” is expressed only to a limited extent.
• Management issues: In-season residual early- and mid-postemergence herbicides may be vital. In soybean, early harvest of early-maturity cultivars may allow Palmer amaranth to continue to grow and set seed after harvest, contributing to the seedbank and necessitating post-harvest control. Many producers count on glyphosate plus metolachlor (Sequence®) early postemergence for early in-season control with residual activity. Preplant options include flumioxazin (Valor®) or pendimethalin (Prowl®, others) or another dinitroaniline herbicide with a burndown herbicide. Many Mississippi producers are using flumioxazin (Valor®) in the fall and early burndown (30 to 45 days before planting).

Research

• Mechanism of resistance.
• Monitoring and confirmation of suspected resistant populations.
• Management strategies: A “group strategy” for management recommendations is being developed in Mississippi to limit the spread of resistant Palmer amaranth. Of seed collected from 30 biotypes, resistance is suspected in four populations, and all plants from two populations (one inside the levee) appear to be highly resistant, with no control of any plants within these two populations.