Published on Monday, September 21, 2020
Beck’s Practical Farm Research (PFR)® data shows the importance of using pre-emerge herbicides with multiple effective SOAs in order to lay the foundation for a successful herbicide program. The question is whether the pre-emerge herbicide will last until canopy closure. Once we reach canopy closure, the limited light makes it difficult for new flushes of weeds to emerge.
A pre-emerge herbicide is like a blanket, a blanket that reduces new flushes of weeds. Once the pre-emerge herbicide begins to break, the blanket starts to get holes allowing new flushes of weeds. We typically see pre-emerge herbicides break roughly four weeks after application. Therefore, achieving canopy closure prior to the pre-emerge breaking aids in weed suppression. Many post-emergence herbicides do not provide residual activity and although they may control emerged weeds, these herbicides will provide no protection against future weed flushes. Making an in-season residual application key for season-long weed control.
In a year such as 2020, many pre-emerge herbicides broke before canopy closure because of the slow soybean growth, making the use of an in-season residual crucial. When using a Group 15 in-season residual such as Warrant®, we’ve seen up to an 8%, 7%, and 4% increase in waterhemp control when Warrant was applied after utilizing a one, two, and three “effective” SOAs pre-emerge herbicide, respectively. We see less response to in-season residuals when three “effective” SOAs are used over one or two SOAs. In heavier weed pressure, in-season residuals are still recommended for extra protection, even if you are utilizing three “effective” SOAs in your pre-emerge herbicide. Group 15 premixes, such as Warrant® Ultra and Prefix®, contain a Group 15, as well as fomesafen, a Group 14 herbicide. Group 15 herbicides do not have post-emergence activity but the addition of a Group 14, such as fomesafen, will help increase post-emergence control of glyphosate-resistant (GR) waterhemp. Some waterhemp populations have been documented to be resistant to Group 14 post-emergence herbicides such as fomesafen, Cobra®, etc.
Weed management takes a full-season approach that utilizes an effective pre-emerge herbicide, effective post-emergence trip, and in-season residuals. No matter which trait platform you select, we must reduce the amount of late-season escapes, in return reducing the soil seed bank. Reducing the soil seed bank will set us up for success for years to come.
Author: Joe Bolte
Categories: Agronomy, Agronomy Talk