Time-of-Day Application Impacts Effectiveness of Herbicides

Recently at a Respect the Rotation event in Mt. Olive, Wes Everman, Extension Weed Specialist with NC State University discussed the results of a study by NC State’s Dr. Allen York, and University of Georgia’s Dr. Stanley Culpepper showing that time-of-day application made a difference in the effectiveness of Liberty Link herbicide:

“They looked at both morning and evening applications. They found that if you made a Liberty application before sunrise, control was reduced on palmer amaranth. Also in the evening, if an application was made within an hour of sunset, also saw reduced control of palmer amaranth."

Everman explains that while the studies have yet to be completed, it appears that the time of day that the plant actively photosynthesizes combined with the mode of action in the herbicide have to be compatible:

“We still haven’t done the studies to fully investigate what is going on physiologically, but knowing the mode of action of the herbicide, and the plant biology, what we are figuring is that plant isn’t photosynthesizing actively early in the morning or late in the evening. It’s more of a midday active plant. Liberty only works on actively growing plants.”

Everman says this phenomenon appears in other chemicals as well:

“Dr. York observed where an application was made on a cloudy day and he saw reduced control compared to applications made on sunny days. So cloud cover, especially for the full day, could have an impact on your Liberty Activity.”

And it also appears that sunny days versus cloudy days, particularly several in a row, make a difference in effectiveness according to Everman:

“We have seen that with some other modes of action particularly the photo system inhibitors. Specifically metribuzin, which is a trade name of Sencor. Generally if you have several cloudy days in a row, assuming a spring application, you wont see the same level of control as you would where you have consecutive sunny days. “

Dr. Wes Everman, NC State Extension Weed Scientist.