Central Arizona’s hot, dry growing conditions make weed management especially challenging and costly for cotton growers, who are already dealing with high irrigation and input expenses. When weeds are not effectively controlled early in the season, they can significantly reduce yields, increase production costs, and accelerate the development of herbicide resistance. Since the dicamba ban, growers have fewer herbicide options, making it even more important to use the remaining tools wisely. Approaches that combine herbicide performance testing, proper application timing, rotation, and mixing of different modes of action, along with improved spray technologies, are key to protecting yields and profitability. However, growers and Pest Control Advisors (PCA) still need clearer, locally tested information on how current herbicide programs perform under Central Arizona conditions, such as extreme summer heat, variable soil moisture, and long growing seasons. To address these needs, this study conducted a systematic evaluation of post-emergence herbicides in commercial cotton systems. The information from this study could provide science-based, region-specific recommendations to help growers manage Palmer amaranth (Amaranthus palmeri) more effectively, safeguard crop yields, and extend the useful lifespan of available chemistries, thereby supporting the long-term sustainability of cotton farming in Central Arizona. The objective of this field study was to evaluate the performance of selected commercially available post-emergence herbicides for controlling Palmer amaranth in cotton production systems
Figure 1a: Amaranthus Palmeri (Palmer Amaranth) seedling. Picture was taken at the study site, Field 2 (Border 99-101) at the Maricopa Agricultural Center, Maricopa, AZ.
Avik Mukherjee
Figure 1b: Female Amaranthus Palmeri (Palmer Amaranth). Picture were taken at the study site, Field 2 (Border 99-101) at the Maricopa Agricultural Center, Maricopa, AZ.
Avik Mukherjee
Figure 1c: Male Amaranthus Palmeri (Palmer Amaranth). Picture were taken at the study site, Field 2 (Border 99-101) at the Maricopa Agricultural Center, Maricopa, AZ.
Avik Mukherjee
Methodologies
A field study was conducted at the University of Arizona Maricopa Agricultural Center (33.0735° N, 111.9738° W) in 2025 to evaluate the efficacy of experimental and labeled herbicides, applied in selected combinations and rates, for controlling broadleaf weeds in cotton. The experiment was arranged in a randomized complete block design (RCBD) with four replications, including five post-emergence herbicidal treatments (Table 1) and an untreated control (UTC). Each plot measured 35 ft (10.67 m) × 13 ft 4 in (4.06 m), consisting of four cotton rows spaced 40 in (1.02 m) apart, with 10 ft (3.05 m) alleys and two buffer rows between replications.
Two cotton varieties, Dicamba-tolerant DP2414B3TXF (50,000 seeds/acre) and Enlist-trait PHY415W3FE, were planted at a rate of 50,000 seeds/acre on May 14 in Field 2 (Borders 99–101, total 2 acres). Post-emergence herbicides were applied on June 04 using a tractor-mounted sprayer outfitted with XR TeeJet 9502VS nozzles spaced 40 inches (1.02 m) apart and operating at 38 psi (262 kPa). A spray volume of 15 gallons per acre (140 L ha⁻¹) was used to apply herbicides. Weed density (plants m⁻²) as well as the cotton and Palmer amaranth heights (cm) were recorded at weekly intervals over four weeks following application. Biomass samples were collected 6 weeks after planting from one square meter (9 square feet) quadrats per plot, oven-dried at 80°C (176°F) for 72 hours, and weighed to obtain the dry weight. Although the trial included both pre- and post-emergence treatments, this article focuses on the post-emergence options.
Figure 2a: Palmer and cotton plant height (per sq. meter) data collected from each treatment plot at Maricopa Agricultural Center, University of Arizona
Figure 2b: Weed density (per sq. meter) data collected from each treatment plot at Maricopa Agricultural Center, University of Arizona.
Table 1: Herbicide combinations from BASF, Bayer, FMC, and Corteva used as treatments.
| Trade name | Active ingredients | Site of action group | Application rate | Application timing |
|---|---|---|---|---|
| Liberty Ultra® | glufosinate | 10 | 29 fl oz/acre | Post-emergence |
| PowerMax® + Warrant® + Glufosinate | glyphosate, acetochlor, glufosinate | 9, 15, 10 | 30 fl oz/acre + 3 pts/acre + 43 fl oz/acre | Post-emergence |
| Anthem Flex® | pyroxasulfone and carfentrazone | 15, 14 | 2.73 fl oz/acre | Post-emergence |
| Enlist One® | 2,4-D | 4 | 2 pts/acre | Post-emergence |
| Enlist Duo® | 2,4-D and glyphosate | 4, 9 | 4.75 pts/acre | Post-emergence |
Results and discussions
The field was heavily infested by Palmer amaranth, preventing other weeds from emerging in significant numbers. All post-emergence herbicide treatments provided noticeable control of Palmer amaranth compared to the untreated control (UTC) under prevailing field weed pressure. Notably, Enlist Duo consistently resulted in effective suppression of Palmer amaranth, maintaining robust control throughout four weeks following application. Additionally, all post-emergence herbicides resulted in reductions in Palmer amaranth biomass (40- 50%) and height (30-45%) compared to the untreated control 6 weeks after planting across both Dicambatolerant cotton. Whereas, Enlist herbicides worked significantly (at least 50%) well to reduce Palmer biomass and mean height as well. These results underscore the critical value of diversified herbicide programs and strategic choice of herbicides in Arizona cotton systems, especially given the challenges posed by resistant Palmer amaranth and the region’s arid conditions. Under high Palmer amaranth pressure, all post-emergence herbicide treatments significantly improved weed control relative to the untreated control, indicating their effectiveness even under severe infestation levels. Enlist Duo provided the most consistent and sustained suppression, maintaining high levels of control for up to four weeks after application, which highlights its robustness under Arizona field conditions. Across dicamba-tolerant cotton systems, postemergence herbicides reduced Palmer amaranth biomass by 40-50% and plant height by 30-45%, demonstrating meaningful suppression of weed growth rather than short-term injury alone. Enlist-based programs were particularly effective, achieving ≥50% reductions in both biomass and height, suggesting superior activity against aggressive Palmer amaranth populations. Overall, dualmode herbicide or mixed herbicide programs performed significantly better in controlling Palmer amaranth.
Results from this year’s efficacy trial indicate that adherence to label-recommended best management practices is essential to achieve optimal herbicide performance. Programs that integrate herbicides with dual or multiple modes of action consistently improved weed control, resulting in greater reductions in weed biomass and plant height. Implementation of this approach could reduce the rate at which Palmer amaranth evolves resistance to multiple herbicides. In addition, application timing was a critical determinant of efficacy, as treatments applied to smaller, actively growing weeds provided superior control. Delayed applications allowed weeds to increase in size and canopy cover, which reduced spray coverage and consequently diminished herbicide effectiveness. In the desert southwest, where solar radiation is not limiting, atmospheric conditions, particularly relative humidity (RH), can strongly influence post-application herbicide performance. Previous studies have shown that glufosinate efficacy against Palmer amaranth and waterhemp is significantly greater under high RH conditions (~90%) compared with low RH (~35%), highlighting the importance of environmental factors at application in determining weed control outcomes. (Coetzer et al., 2001). In addition to relative humidity, previous research (Takano and Dayan 2021) recommends that glufosinate applications be made during daytime conditions, with a minimum of 3-4 hours available for product activation to ensure optimal herbicidal efficacy. In upcoming field trials, we plan to test multiple application rates and best practices for each herbicide.
Figure 3a: Visual comparison in one square meter quadrant between untreated check (UTC) and treated (post-emergence application) with Liberty at 5, 12, 19, and 24 days after treatment (DAT).
Figure 3b: Visual comparison in one square meter quadrant between untreated check (UTC) and treated (post-emergence application) with PowerMax+Warrant+Glufosinate at 5, 12, 19, and 24 days after treatment (DAT).
Figure 3c: Visual comparison in one square meter quadrant between untreated check (UTC) and treated (post-emergence application) with Anthem Flex at 5, 12, 19, and 24 days after treatment (DAT).
Figure 4a. The efficacy of post-emergence herbicide Liberty Ultra against Palmer amaranth at 5, 12, 19, and 24 days after treatment.
Figure 4b. The efficacy of post-emergence herbicide PowerMax+Warrant+Glufosinate against Palmer amaranth at 5, 12, 19, and 24 days after treatment.
Figure 4c. The efficacy of post-emergence herbicide Anthem Flex against Palmer amaranth at 5, 12, 19, and 24 days after treatment.
Figure 5a: Visual comparison in one square meter quadrant between untreated check (UTC) and treated (post-emergence application) with Enlist One at 5, 12, 19, and 24 days after application on Enlist trait cotton.
Figure 5b: Visual comparison in one square meter quadrant between untreated check (UTC) and treated (post-emergence application) with Enlist Duo at 5, 12, 19, and 24 days after application on Enlist trait cotton.
Figure 6a. The efficacy (%) of post-emergence herbicide Enlist One against Palmer amaranth at 5, 12, 19, and 24 days after treatment to Enlist trait cotton.
Figure 6b. The efficacy (%) of post-emergence herbicide Enlist Duo against Palmer amaranth at 5, 12, 19, and 24 days after treatment to Enlist trait cotton.
Figure 7: Percent (%) reduction in palmer amaranth biomass from a one square meter quadrant of each post-emergence herbicide-treated plot (% reductions compared to the control plot) in (a) Dicamba-tolerant variety and (b) Enlist-trait cotton.
Figure 8: Percent (%) reduction in Palmer amaranth height in each post-emergence herbicide-treated plot and control (UTC) in Dicamba-tolerant variety and Enlist-trait cotton.
Figure 9a: The Herbicide Efficacy Trial outcomes were shared with the stakeholders through multiple Cotton Tent Talks and workshops. Cotton Tent Talk sat Eloy, AZ in August 2025.
Figure 9a: The Herbicide Efficacy Trial outcomes were shared with the stakeholders through multiple Cotton Tent Talks and workshops. Cotton Tent Talk sat Casa Grande, AZ in September 2025.
Acknowledgement
We are grateful to Cotton Incorporated and Arizona Cotton Growers Association for funding this project and to the industry stakeholders for their support in participating in this trial. We would also like to thank Arizona Pest Management Center for assisting with herbicide applications.
Disclaimer
Any products, services, or organizations that are mentioned, shown, or indirectly implied in this publication do not imply endorsement by The University of Arizona.
References
Coetzer, E., Al-Khatib, K., & Loughin, T. M. (2001). Glufosinate efficacy, absorption, and translocation in amaranth as affected by relative humidity and temperature. Weed Science, 49(1), 8-13.
Takano, H. K., & Dayan, F. E. (2021). Biochemical basis for the time-of-day effect on glufosinate efficacy against Amaranthus palmeri. Plants, 10(10), 2021.