An agronomist at North Dakota State University says the timing of residual herbicide application is critical in controlling resistant water hemp.
Tom Peters said the effects of a residual generally last 30 to 40 days, and water hemp in the Upper Midwest typically emerges around the end of May.
“So if you’re planting soybeans early—let’s say you’re planting about the first of May—you’re giving away 30 days out of the 40 days of soil-applied herbicide life, before the herbicide will even start to work for you.”
He said it’s different for corn because the plant canopies rapidly, taking away sunlight weeds need to germinate.
But for soybean and sugarbeet growers, Peters recommends a concept called lay-by.
“Lay-by is using that same soil-applied herbicide, but instead of applying it at planting time, you delay the application of the lay-by herbicide until either the sugarbeet or soybeans have germinated and emerged.”
In a normal year that would be around the middle of May for most growers.
“That will give time for the herbicide to be activated by rainfall, so it will be available to work for you by the end of May.”
Peters said the beauty of the lay-by system is that growers have potentially 40 days of weed control starting closer to the time water hemp germinates, making the residual herbicide more effective.
Source: www.brownfieldagnews.com