Controlling Palmer Amaranth with Early-Post-Emergence Herbicides in Central Arizona Cotton System
Palmer amaranth (Amaranthus palmeri) remains one of the most problematic and yield-limiting weeds in cotton production systems across central Arizona. Its rapid growth rate, high fecundity, and widespread herbicide resistance make timely and effective management critical, particularly during early crop establishment. Early post-emergence (EPOST) (when Palmers are 1-4 inches tall) herbicide applications play a vital role in integrated weed management by targeting weeds at vulnerable growth stages and reducing early-season competition. This study evaluated the efficacy of selected EPOST herbicide programs, including combinations of glyphosate, acetochlor, diuron, pendimethalin, and pyroxasulfone, under field conditions at the Maricopa Agricultural Center to provide practical guidance for growers and stakeholders. Weed (Palmer amaranth) density was measured in a one-square-meter quadrant for each treatment, with four replications. More details on study design and methodology are in the related study Controlling Palmer Amaranth in Cotton with Post-emergence Herbicides (Mukherjee et al., 2026).
Results and discussions
All early post-emergence herbicide treatments numerically reduced Palmer amaranth (Amaranthus palmeri) infestation and improved visual control ratings relative to the untreated check across evaluation timings; however, because statistical comparisons among treatments were not observed in this study, the differences reported here should be interpreted as descriptive trends rather than confirmed treatment effects. Across all rating dates, the three-way tank mixture of glyphosate + acetochlor + diuron (PowerMax3 + Warrant + Direx 4L) produced the most consistent suppression of Palmer amaranth, a pattern plausibly attributable to the simultaneous delivery of foliar (systemic) and soil-residual activity across multiple complementary modes of action. In contrast, single-active-ingredient treatments, pendimethalin (Prowl H2O) and pyroxasulfone (Zidua SC), appeared to provide more variable control across timings, a pattern consistent with the documented sensitivity of soil-residual herbicide performance to weed density,soil moisture at and after application, and prevailing environmental conditions. These results are based on a one-year field trial and motivate a statistically powered follow-up trial in 2026 to permit formal inference on treatment differences and on the interaction between herbicide program and environmental drivers.
Variability in herbicide performance could be attributed to factors such as initial weed density, weed pressure gradient in the field, environmental conditions during application, and the precise timing of herbicide application relative to weed growth stage. Under high weed pressure, reliance on a single mode of action may be insufficient for sustained control (Beckie 2006, Powles et al., 2010), underscoring the importance of diversified herbicide programs. Sustained adoption of such integrated programs is essential to preserving the efficacy of remaining effective chemistries and to maintaining the long-term agronomic and economic viability of cotton production in Arizona.
Table 1: Details of Early-post emergence treatments in Cotton at Maricopa Agricultural Center, Maricopa, AZ
| Trade name | Active ingredients | Site of application group | Application rate | Application timing |
|---|---|---|---|---|
| PowerMax3 + Warrant + Direx 4L | Glyphosate, Acetochlor, Diuron | 9,15,7 | 30 fl oz/acre + 3 pts/acre + 32 fl oz/acre | Early post-emergence |
| Prowl H20 | Pendimethalin | 3 | 3 pints/acre | Early post-emergence |
| Zidua SC | Pyroxasulfone | 15 | 2 pts/acre | Early post-emergence |
Figure 1a. Visual comparison (in one square meter quadrant) between untreated check (UTC) and treated early post-emergence application with PowerMax+Warrant+Direx 4L at multiple days after application (DAA).
Avik Mukherjee
Figure 1b. Visual comparison (in one square meter quadrant) between untreated check (UTC) and treated early post-emergence application with Prowl H2O at multiple days after application (DAA).
Avik Mukherjee
Figure 1c. Visual comparison (in one square meter quadrant) between untreated check (UTC) and treated early post-emergence application with Zidua SC at multiple days after application (DAA).
Avik Mukherjee
Figure 2a. The efficacy trends of early post-emergence herbicides PowerMax+Warrant+Direx 4L at multiple days after application (DAA). Each black line represents the standard deviation of the sample mean.
Figure 2. The efficacy trends of early post-emergence herbicides Prowl H2O at multiple days after application (DAA). Each black line represents the standard deviation of the sample mean.
Figure 2c. The efficacy trends of early post-emergence herbicides Zidua SC at multiple days after application (DAA). Each black line represents the standard deviation of the sample mean.
Acknowledgements
We are grateful to Cotton Incorporated and the Arizona Cotton Growers Association for funding this project, and to industry stakeholders (BAYER, BASF, FMC, and CORTEVA) for their support in participating in this trial. We would also like to thank Arizona Pest Management Center and Benjamin Pierce 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
Beckie, H. J. (2006). Herbicide-resistant weeds: management tactics and practices. Weed technology, 20(3), 793-814.
Mukherjee, A; Singh, B; Norton, R, and Pierce, B (2026). Controlling Palmer Amaranth with Post-emergence Herbicides, The University of Arizona Cooperative Extension. (AZ2188)
Powles, S. B., & Yu, Q. (2010). Evolution in action: plants resistant to herbicides. Annual review of plant biology, 61(1), 317-347.