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52 W i n e s & V i n e s s e P T e M B e R 2 0 1 4 W orking with wheat farm- ers on Canada's prairies, agronomist Ken Saps- ford 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 Mon- santo'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 lon- ger 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 flea- bane (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 Highlights • The bare soil between vineyard rows can be enticing for weeds that are resistant to broad-spectrum herbicides. • Weeds use water that is needed by grapevines—especially during times of drought. • Rather than doubling the amount of a single herbicide used, experts recommend combining herbicides. Resistant Weeds Threaten Vineyards British Columbia vineyard owner and agronomist discusses herbicides By Peter Mitham I n late July John Roncoroni, a University of California Cooperative Extension weed science advisor for Napa County, held a field demon- stration about the efficacy of herbicides newly registered for vineyard application. He had laid out 25 treatment plots and rated the herbicides for weed control in vineyards. The trials demonstrated that some new herbicides are very effective, especially if a mix of more than one is ap- plied. But they also showed how ineffective some herbi- cides were against trouble- some, newly arrived weeds. The trial regimen The test was held in south Napa Valley near Carneros and focused on pre-emergent herbicides, though each ap- plication included Roundup systemic herbicide in case any weeds had germinated unseen, plus AMSPro surfac- tant to improve penetration. The plots had been cleared during the winter with glypho- sate (such as Roundup) and Shark (carfentrazone, a contact herbicide that is mixed with glyphosate to help control some of the tolerant weeds). The ap- plications were made in Febru- ary just before significant rains (8.85 inches within a week). The plots were 25 feet long by 6 feet wide under the Efficacy of weed killers in Napa Canada fleabane is among four herbicide-resistant species identified in Canada. G R A P E G R O W I N G