Bleacher Herbicides Under Pressure: The Emerging Risk Of HPPD Resistance
SALLY REED AND DR. LARRY STECKEL
JACKSON, TENNESSEE
Pigweed problems are nothing new for growers across the region. With Palmer amaranth having confirmed resistance to five herbicide modes of action in Tennessee, control options continue to be limited. Historically, bleacher (Group 27 HPPD inhibitors) applications have provided sufficient control, especially in corn. In recent years, there has been growing concern for the level of control observed when using these products.
We evaluated field-level control of Palmer amaranth using bleacher (HPPD-inhibiting) herbicides and assessed potential resistance across multiple populations. Four populations spanning Jackson, Gibson, and Lauderdale Counties were treated with:
- topramezone
- mesotrione
- tembotrione
- tolpyralate
- topramezone + glufosinate (tank mix)
Weed size at application varied from 4 to 8 inches. No residual herbicide was applied.
Control varied widely by population, ranging from more than 80% to less than 30%, but the relative performance of each herbicide program followed the same trend across populations (Figure 1):
The photos are from a Lauderdale County population that represents the worst-case scenario observed for bleacher herbicide control in 2025. In this select population, the topramezone + glufosinate tank mix and tolpyralate provided the best control (the same trend observed across all populations) yet still did not achieve acceptable control levels. While this is an extreme case, it highlights a concerning trajectory for HPPD herbicide performance in Tennessee.
Thinking to the future, maintaining HPPD herbicide performance will depend on integrating them into a more robust weed management program. In corn, tank-mixing with atrazine remains an effective strategy, as atrazine is still providing good pigweed control in Tennessee. Tank mixes that include glufosinate can boost control, especially when targeting smaller pigweed. Residual herbicides should be layered to extend activity and reduce new emergence. Rotating to other effective modes of action and making timely applications will be crucial in keeping these products working. With this in mind, we encourage consistent scouting to catch escapes early, prevent seed return, and limit future resistance pressure. ∆
SALLY REED AND DR. LARRY STECKEL
UNIVERSITY OF TENNESSEE