Can pre-plant herbicides ‘reach back’ to control weeds?

A widespread rain sweeping across Ontario this past weekend provided plenty of moisture to activate soil-applied, pre-plant herbicides in crop fields throughout the province.

With the timely rain, many growers are now asking: after activation do these herbicides have the capability to ‘reach back’ and control already-emerged weeds and grasses such as foxtail?

University of Guelph weed scientist Peter Sikkema says growers and agronomists have to be careful when expecting pre-emerge herbicides to control weeds and grasses that have already emerged.

During a Tuesday morning virtual agribusiness breakfast meeting of provincial agronomists, extension specialists, and crop retail representatives, Sikkema noted: “To the best of my knowledge, there’s only two herbicides that specifically claim ‘reach back’ activity on the label and they are Broadstrike, as well as Converge.”

Even with the reach back activity of Broadstrike and Converge, “all your stars have to align perfectly for them to pull down any emerged weeds,” adds Sikkema. He’s seen both products deliver reach back performance in his research plots “but that activating rain event has to occur at the exact right time, the weeds can’t be too big. And it’s the sensitive weed species that you will get that reach back activity on.”

In general, soil-applied products typically deliver between 50 to 80 percent control even in the absence of an activating rain. “They will still have some activity on weeds germinating after that activating rain event and they’ll provide some residual weed control, but most do not pull down emerged weeds with the exception of the two that I mentioned.

“I still think there’s value in that soil-applied herbicide, even if you didn’t get that activating rain event in the first  seven to 10 days,” says Sikkema. “However, in terms of emerged weeds, most of the time you will have to control them with a post-emergence herbicide.”