High temperatures reduce the effectiveness of Group 27 herbicides

As the calendar shifts into the summer months and post-emergent herbicides are applied to late seeded crops, now is a good time to review some research on the effects of high temperatures on Group 27 herbicides. Research conducted a few years ago, led by Drs. Mithila Jugulam and Curt Thompson, focused on the control of Palmer’s pigweed with mesotrione (Callisto, others). However, the results they reported may explain some poor control observed following recent applications of group 27 herbicides such as topramezone (Impact, others) or pyrasulfotole (Huskie, others) in populations that l are thought to be susceptible to group 27 herbicides.

Research results show that when Callisto was applied at “low” temperatures (77 degrees F daytime and 59 degrees night), Palmer’s Pigweed was controlled by the lowest herbicide rate included in study (about 0.75 fl oz/A). However, when Callisto was applied at “high” temperatures (104 degrees F during the day and 86 degrees at night), Palmer’s Pigweed was not controlled by the highest rate of Callisto used in the study. (3 fl oz/A). These application rates do not necessarily correspond to field use rates, as lower herbicide rates are often used in greenhouse or growth chamber studies such as these to account for more ” tender” which are more easily controlled than plants grown under field conditions.

Research has described two key changes in Palmer’s amaranth that result in reduced control. First, Palmer’s amaranth metabolized mesotrione faster at higher temperatures. This means that the herbicide degrades faster when temperatures are high, making it less effective. Second, the HPPD enzyme, which is the target site of group 27 herbicides, had increased activity at high temperatures, allowing the plant to more easily overcome the effects of mesotrione.

For maximum effectiveness, Group 27 herbicides should be applied under the coolest conditions possible. It is very likely that other amaranth species such as spider mite and other group 27 herbicides including tembotrione (Laudis), tolpyralate (Shieldex), topramezone (Impact), Balance Flexx (isoxaflutole ) and pyrasulfatole (Huskie) will react similarly to Amarante Palmer in mesotrione.

Information provided by Sarah Lancaster, Extension Weed Management Specialist and Mithila Jugulam, K-State Weed Physiologist.

Stacy Campbell is the Agriculture and Natural Resources Officer for the Cottonwood Extension District. Email him at [email protected] or call Hays’ office at 785-628-9430.

Sara H. Byrd