Managing Glyphosate-Resistant Palmer Amaranth
by Alan York
Resistance of Palmer amaranth to glyphosate was suspected in 2005.
Seed were collected from 290 fields. The cotton areas of the state were the focus, with heavy populations in the Central Coastal Plains, but little in the counties adjacent to the coast. Resistance was found in 49 fields in 11 counties. Most of the resistant populations required 2 to 4 times the rate of glyphosate for 50% fresh weight reduction (89 g/ha for susceptible). The most resistant population required a 22X rate. Resistance has been confirmed (heritability proven).
High level of grower awareness, a lot of worrying, and some action. Growers are considering options for control, including rotation of some fields traditionally in cotton to corn or soybean. The market now allows the rotation option. The benefit is that good control of Palmer amaranth may be obtained in corn or soybean, which will reduce the soil seedbank, and modes of action other than glyphosate can be used to help thwart the increase in resistance.
- Message to growers: 1) Be aware and detect resistance early; 2) Consider rotational crops; 3) Use multiple herbicide modes of action; 4) Use residual herbicides; preemergence (PRE) herbicides are a significant aid in resistance management; 5) Use techniques to reduce the soil seedbank; 6) Reduce selection pressure on ALS inhibitors.
Some physiology/ecology research; major focus on field-oriented management research.
- Salvage treatments: There is a need for control of Palmer amaranth plants that become larger than 2 inches. Pyrithiobac (Staple®), fluometuron (Cotoran® and others), and MSMA (several formulations) have little effectiveness at this stage of weed growth.
- Use of chloroacetamides: Use of metolachlor (Dual®) and others (alachlor, acetochlor) can help avoid getting into a salvage situation. In the Blacklands area of the state, where organic matter can range from 5 to 30%, producers do not use preemergence herbicides. And, although cotton tolerates Dual®, tolerance of alachlor and acetochlor is poor and is not labeled for use in cotton.
- Residual herbicides, especially preemergence herbicides: Other residual herbicides being evaluated are pyrithiobac (if no ALS resistance), fomesafen (Reflex®), prometryn (Caparol®), pendimethalin (Prowl® , Pendimax®), and two-way mixtures.
- LibertyLink® and Roundup Ready Flex®: Glufosinate (Ignite®) in LibertyLink® cotton is promising, but early timing is imperative. Use of preemergence herbicides and appropriate timing of glufosinate application is being investigated in these systems.
- No-till vs conventional tillage: Conventional and no-till systems are being compared for Palmer amaranth control, with control in no-till as good as or better than control in conventional tillage. – Dicamba (Clarity®)-resistant cotton: Is it a viable future option for controlling resistant Palmer amaranth?
- Ecology/physiology: Looking at emergence dynamics and some physiological processes.