Weed control challenging as resistance spreads

Weed control in a high value crop such as wine grapes is still relatively inexpensive when the overall production system is considered, according to Karen Klonsky, agricultural and resource economics specialist, UC Davis.

“A recent analysis of wine grapes in Lake County showed the total cost of production at about $6,000 an acre; weed control was only $95 an acre,” she says.

One of the most perplexing problems threatening weed control, along with the corresponding economics, is the spread of resistant weed species. Glyphosate-resistant horseweed and fleabane continue to thwart grower efforts to eliminate or at least reduce competition with grapevines.

“Horseweed can produce from 100,000 to 200,000 seeds per plant per year,” says Scott Scheufele, graduate student with California State University, Fresno’s Plant Science Department.

In a recent presentation at the California Weed Science Society meeting on his research with glyphosate-resistance, he pointed out how easily the weed can spread and why it is so difficult to control.

“What makes horseweed control so difficult is its life cycle,” he says. “It has a 90 percent winter survival rate, and there are two germination points, one in the fall and the other in the spring.”

Nozzle type and volume will affect coverage and uptake of active material, according to Scheufele’s research. “Smaller droplets cover the plant material better, while larger droplets tend to splash off,” he says.

In a study looking at lower volumes vs. higher volumes, ranging from 10 gallons to 30 gallons per acre, the results showed lower volumes provided better control.

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