When’s the best time of day to spray herbicides? Morning, noon, or night?

University of Guelph weed scientist Dr. Peter Sikkema says that every herbicide has a time-of-day effect but generally speaking, growers get the best weed control at 3:00 pm in the afternoon. That’s Sikkema’s conclusion based on time-of-day efficacy trials he conducted using 13 active ingredients.

Sikkema’s team of research assistants and graduate students put in some long days to test each active ingredient, spraying them in three-hour intervals beginning at 6:00 am through to midnight. He reports that the efficacy of every herbicide tends to follow an inverted U-shaped curve with poorest control coming at 6:00 am and midnight.

The degree of the curve varies greatly for different chemistry. The curve is practically flat for glyphosate, he notes. “Farmers know they can spray glyphosate anytime and they are going to get good weed control.”

A product like Liberty (glufosinate) has a much steeper curve to its time-of-day effect. On this episode of RealAgriculture Corn School, Sikkema notes that spraying Liberty at noon compared to midnight can increase yield by up to 50 bu/ac.

While mid-day spraying does offer the best control, it does present some risks. Research also reveals that weed control and crop injury tend to follow the same line. “When you get the best weed control at 3:00 pm, you also have the greatest crop injury,” Sikkema adds.

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