Steward herbicides to slow resistance in orchards and vineyards

By Vicky Boyd

Editor 

Until a few years ago, applying glyphosate several times during the growing season to chemically mow orchard and vineyard middles and to keep plant rows clean was standard practice.

After all, the herbicide was safer for groundwater supplies than some of the other products, and it was relatively inexpensive.

But the discovery of pockets of two glyphosate-resistant weeds has caused consultants and growers of permanent crops to change their thinking.

“I’m concerned about where this is going to go in the future,” says Gerry Rominger, an almond grower near Arbuckle, Calif., who has glyphosate-resistant ryegrass. “Maybe there will be some other weed species where Roundup doesn’t work.”

A couple of years ago, Rominger says he began to notice scattered patches of ryegrass that weren’t controlled by glyphosate.

Thinking it might be an application error, he instructed his employees to slow down when spraying. He also had them retreat the areas with higher rates.

“There’s a lot now, and it’s to the point where Roundup is totally ineffective on it,” Rominger says.

The resistant ryegrass is confined mostly to the perimeters of his orchards, and Rominger blames it on repeated glyphosate use.

To control the weed, he tankmixes Poast, a grass herbicide from BASF Corp., with glyphosate. He also has to add a crop oil to enhance the Poast activity, and the two additional products increase his grass-control costs.

Use different weapons to battle weeds

Even if you currently don’t have a glyphosate-resistance problem, herbicide manufacturers recommend following a resistance-management plan.

“I think it’s just a matter of time until the grower will encounter a resistant biotype,” says Chuck Foresman, a technical brand manager with Syngenta Crop Protection in Greensboro, N.C. “The thing I recommend to people is they plan ahead—they need to have a management plan in place and use other modes of action so they don’t get surprised by populations of weeds that are very difficult to control.”

Regardless of the weed or herbicide, experts recommend following sound herbicide resistance management practices.

One of the keys is to avoid repeated applications of the same chemistry. Instead, growers should rotate or tankmix chemicals with different modes of action.

The Herbicide Resistance Action Committee has assigned chemical families different reference numbers. For example, glyphosate belongs to Group 9.

Foresman says manufacturers have tried to simplify choices by printing the group number on pesticide labels.

“If they’re applying a Group 9 herbicide over and over and over again, then they should start thinking about switching to that up,” he says. “They should start thinking about going to another herbicide, like Gramoxone Inteon, that’s in another group and provides another mode of action.”

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