By Frances Browne and Katelyn Price, Auburn University
Given its tolerance to herbicides, Palmer amaranth greater than 12 inches ranks among the most difficult weeds to control in row crops.
When it reaches a foot or more in height, the only consistent options are hand weeding, mechanical control, or Gramoxone (through a weed wiper or similar device). Every effort should be made to address Palmer pre-plant, at-plant, and early in the season.
Residual herbicides and early season, timely post treatments are critical to limit its emergence, establishment, and proliferation.
Trials were conducted in Alabama in 2018 and 2019 to compare rescue treatments for large, escaped Palmer amaranth. Treatments were applied in June in non-crop plots with high densities of Palmer amaranth 10-35 inches tall.
Other weeds present in high numbers were annual grasses, morningglories, sicklepod, and Florida beggarweed. Xtend and Enlist treatments were applied with TTI 11002 nozzles and Liberty with TT 11002 nozzles.
A NIS surfactant was used with all Liberty treatments. Weed control ratings were made 4 weeks after the initial treatment (WAIT) and biomass was collected 5 WAIT. Since treated Palmer can frequently recover from injury, biomass reduction is a better indicator of the efficacy of each program.
Control with the Xtend systems varied significantly by year, while results with the Enlist programs were similar over years.
Palmer amaranth was almost 15 inches tall at the initial application in 2018, and due to weather delays, almost 24 inches in 2019. A week’s delay in 2019 resulted in 9 inches in additional weed height and very poor control with Xtend programs.
Some of the treatments which worked very well in 2018 (ex. Xtendimax followed by Liberty 3 or 7 DAIT) did not perform in 2019.
This is a reminder for growers who hold unrealistic expectations that new technology will always bail them out. Enlist programs provided better Palmer control in this study due to the fact that Enlist One can be tank mixed Liberty, as allowed by the label. The combination of auxin plus Liberty improves control. Another observation seen in the pictures below is that grass control was superior in 2018 versus 2019.
Wet, moderate conditions prevailed in 2018, while extremely hot, dry conditions persisted in mid-May through late summer of 2019.
Control of Palmer amaranth over 12 inches tall is erratic. The sooner growers spray emerged Palmer, the better and more consistent control they achieve.
Differences in environmental conditions, weed size, etc. create differences in control across fields and years. Weeds surviving the rescue treatments should be pulled to eliminate seed production and harvest interference.
Dealing with Palmer amaranth before they come up is a lot easier than after they emerge. Many soil herbicides such as Valor, Reflex, Warrant, Dual Magnum, Outlook, and Brake provide excellent control in cotton if activated properly by rainfall or irrigation.
Old chemistry such as Cotoran, Diuron, Caparol and yellow herbicides can also provide residual control.
Rotations with corn, deep turning soil every 4 to 5 years, and utilizing cover crops are also viable methods to manage Palmer amaranth. Group 15 herbicides (Warrant, Dual Magnum and Outlook) also provide excellent control and can be sprayed over the top of cotton to provide residual Palmer amaranth and grass control further into crop season.
Combinations of Group 15 herbicides with dicamba (new formulation), Enlist One, or Liberty are good early post options to prevent mid-season escapes of Palmer amaranth and annual grasses.
These tank mixtures typically should be used when cotton is 2 to 3 leaves (about 3 to 4 weeks after planting) and in an overall control program that will hopefully minimize the need for rescue treatments. (Li with Frances Browne and Katelyn Price)