“There are opportunities where you don’t have to use glyphosate. It’s up to growers to do so,” says Christy Sprague, Michigan State University weed science specialist.

She was among a panel of speakers addressing growing weed resistance problems during the 2010 Commodity Classic (CC) last week in Anaheim, CA. It was part of numerous grower-oriented sessions, including one on insect resistance and the need for growers to follow refuge guidelines.

Not following refuge requirements for insecticide applications can possibly lead to corn rootworm and other insect resistance treatments, says Ken Ostlie, University of Minnesota entomologist.

Insect and weed resistance to chemical treatments should be on every grower’s mind and the CC presentations addressed the needs of farmers not to be complacent on resistance threats.

“You have to scout your fields to see what (weed) grows and not just rely on Roundup,” says Nilda Burgos, University of Arkansas entomologist, part of the weed-resistance panel. “There is no such thing as zero resistance, but the more diversified you are (in herbicide modes of action), the lower the risk for resistance problems.”

Paul Barchenger, a Hutchinson, MN, grower-consultant, encourages producers to “clean up the edges” of fields when spraying for weeds. “Perimeter management in a resistant field is important,” he says. “You need to clean up the edges to prevent those weeds from spreading.”

Sprague says there is a wide range of herbicides that can control weeds with resistance to glyphosate and other compounds. “Try to switch out your herbicide proteins where resistance is an issue,” she says. “Try to focus on including different wider types of herbicides.”

On the insect side, Ostlie says control of European corn borer remains strong through the use of Bt corn hybrids with the corn borer control traits. But corn rootworm problems are showing up in some fields where Bt corn has been planted.

“We’re seeing percent of control at about 75%,” says Ostlie, discussing Bt hybrids with the rootworm trait planted in parts of Minnesota. “With about 350,000 beetles/acre, if you’re getting 75% control, that’s a lot of survivors.”

The failure of some growers to use a typical 20% refuge between Bt-planted hybrids could be causing less control due to a developing resistance, he says.
Of more than 200 surveys taken of growers and consultants, he says only three out of four growers indicated they used a refuge. “If you don’t have a refuge, resistance can develop more rapidly,” says Ostlie.

Some new Bt hybrids require only a 5% refuge. However, Ostlie says some growers may feel that since the refuge is only 5%, then not using a refuge may not be as critical. “This could be damaging,” he stresses, because resistance can still develop.

Ostlie says problems with lodging on Bt corn systems using a four-row refuge between plants in a 16-row planting system puzzled growers, consultants and entomologists in recent years. “We’re not sure why it happened,” he says. “But we feel it’s just the tip of the iceberg.”