(Wines&Vines) - Working with wheat farmers on Canada’s prairies, agronomist Ken Sapsford of the University of Saskatchewan is no stranger to herbicide resistance in grains.

The use of broad-spectrum herbicides such as glyphosate—best known as Monsanto’s Roundup, though it exists in 22 other formulations—is so widespread that Monsanto has developed Roundup Ready hybrids of some crops to facilitate its use.

But weeds have followed suit, with 28 species worldwide exhibiting resistance to the chemical. Roundup is simply no longer able to corral them.

Some of those species are common in British Columbia’s Okanagan Valley, where Sapsford and his wife bought a 2.5-acre vineyard in 2013.

Kochia (Kochia scoparia), Canada fleabane (Conyza canadensis), and both common and giant ragweed (Ambrosia artemisiifolia and Ambrosia trifida, respectively) are four resistant species resident in Canada; and of these, kochia and Canada Fleabane are present in the Okanagan.

While glyphosate resistance hasn’t yet been observed in local populations of the weeds, Sapsford said that’s no reason for complacence; in fact, the weeds are ideal candidates for developing glyphosate resistance.

Where there’s a problem

Typically found on open, bare ground, these resistant species adapt well to hardy conditions with low competitive pressure. Roundup weakens them, but there are no competitors in these environments that can leverage that weakness to overtake the weeds. And so the weeds thrive.

“Where glyphosate is applied as a pre-seed burndown and a crop is planted, the crop provides competition to any weeds that may survive the treatment and helps to control the weed,” Sapsford explained. But “when glyphosate is applied to an area to control all weeds and keep the ground bare, there is nothing to compete with any weed that may survive the treatment.”

This makes kochia and Canada fleabane especially problematic in vineyards, where they’re often found on bare soil within the rows. The canopy doesn’t shade them, giving them the light needed for photosynthesis in an environment with little competitive pressure. This is where Roundup comes into play, keeping the vineyard floor clean and reducing competition for resources such as water.