Herbicide-resistant weed species typically are found on open-bare ground. Photo source: Cornell Cooperative Extension

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.

Glyphosate doesn’t impact the vine as long as the spray doesn’t touch the leaves, Sapsford said, leading grapegrowers to apply it at rates of as much as five to seven liters (1,800 to 2,500 grams acid equivalent) per acre.

By comparison, grain growers apply glyphosate at rates closer to 1 litre (360 grams acid equivalent of glyphosate) per acre.

Bigger isn’t always better
But just because a regular dose of glyphosate does a good job doesn’t mean a stronger dose will do a better job, Sapsford said. Rather, it ultimately inures the weed to the herbicide.

“The higher the rate of application, and the more frequent it is, the sooner resistant weed strains are likely to emerge,” he said. “I’ve seen it on the prairies, and it’s only a matter of time before grapegrowers are facing resistant strains of these weeds.”

Kochia responds by producing more of the protein that Roundup attacks, frustrating the action of the herbicide, while Canada fleabane increases efforts to isolate the glyphosate, preventing it from moving to where it can take effect.

“Plants have developed different resistance mechanisms such as reduced translocation, isolation of (the) herbicide and gene amplification to develop resistance to glyphosate,” Sapsford explained.

A smarter approach to weed control is combining herbicides to broaden the challenges weeds face. Rather than battling one kind of poison, a species will face two herbicides with different modes of action. The double-headed challenge makes it harder for the weed to adapt to either one successfully.

“Glyphosate has made it very easy to control the weeds under the vine. But easy is not always the answer, good management is required to avoid resistance,” Sapsford said.

Anecdotal evidence
Sapsford’s vineyard manager, Pierre Levesque of Earlco Vineyards Ltd. in Penticton, B.C., says his work for Sapsford has heightened his attention to signs of emerging glyphosate resistance among weeds in the approximately 130 acres of vineyard he oversees in the southern Okanagan, primarily along the Naramata Bench.

“I have a feeling that people are using too high a dosage of glyphosate,” he said, acknowledging that even he has been using too much—2.5% versus a recommended rate of 1.5%.

“But I know my neighbors are talking about 5% and 7%. That’s deadly.”

Levesque is now combining herbicides from different groups.

Glyphosate, a Group 9 herbicide inhibits amino acid synthesis, but Levesque combines it with Chateau, a moisture-activated herbicide belonging to Group 14, which inhibits a chlorophyll enzyme. It kills the seeds of weeds, preventing them from returning after mowing.

“It’s a different mechanism, and a different active ingredient,” he said. “That’s definitely made a difference. It’s minimized the amount of weeds on your second round.”

Earlco has also invested in a Fischer GmbH mower that mows within the rows, between the vines, and is able to apply a focussed spray to the area around the trunks to ensure thorough, and targeted, suppression of weeds.

But if glyphosate resistance is becoming an issue of greater concern, Brad Hanson, cooperative extension weed specialist for vineyards and orchards in the Department of Plant Sciences at UC Davis, told Wines & Vines that it hasn’t become one that’s taking an economic toll on West Coast growers.

Rather, it stands to increase management headaches and costs more than anything else.

“Is this a huge problem in the vineyard system? Probably not,” he said. “It’s an inconvenience—and certainly a greater expense, but for the most part it’s not a crop failure situation like it could be in cotton or corn or wheat.”

Fighting for water
But with competition for water a significant issue in California right now, weed control is attracting greater interest.

“Any weeds on the vineyard floor are using water that’s not going to be available to the crop,” he said. “Growers who are very, very concerned about water are saying, ‘Y’know what, maybe I should kill all of that with tillage or herbicide applications because I can’t afford to have that weed using water.”

Knowing how control weeds without making them stronger makes it important that growers follow good management practices when they go about conserving water.

Back in the Okanagan, Sapsford will continue work towards that, discussing his findings with local agronomists. He spoke at a professional development event this past March, and will also address a workshop at the University of British Columbia’s campus in the Okanagan in September.