Soybean growers in the far North are used to battling drought, floods, weeds and the effects of iron chlorosis. But they don't expect nasty skirmishes with their herbicides, too.

Last year, some postemergent herbicides - applied to soybeans grown in high-salt soils and already hurt by iron chlorosis - added enough stress to knock down yields as much as 20 bu/acre. So shows recent North Dakota State University (NDSU) research.

That research was suggested by Jerry Nordick of Rothsay, MN, after he'd applied the labeled rate of a herbicide on chlorotic soybeans a few years ago. After applying that tank, he refilled and sprayed a lower rate of the product on the rest of that field.

The yield off the area with the stronger rate was about half that of the lower concentration, depending on the amount of chlorotic stress. That's when Nordick volunteered to host one of the field research sites.

"I noticed different crop reactions to different rates and under different growing conditions, especially when the beans were the most yellow and chlorotic," says Nordick, who grows 500 acres of the crop.

The study, on the effects of herbicides on iron chlorosis-affected soybeans, has just completed a second year."We had as much as 20 bu difference between the herbicides that were more injurious and those that were easier on the plants," says Dave Franzen, NDSU extension soil scientist. The accompanying table, listing yield effects of 10 herbicides applied to different fields with iron chlorosis, shows 1998 data that he and NDSU weed specialist Richard Zollinger collected.

"Our preliminary 1999 data supports that," adds Franzen. Last year the researchers also included Roundup Ready soybeans in their study. "Preliminary data shows that Roundup wasn't any better than some of the other herbicides."

In the 1998 study, low-salt fields treated with Pursuit and Raptor tended to be the highest yielding, but yields dropped to the bottom of the pack when those herbicides were applied to beans on high-salt soils. Fields sprayed with Galaxy tended to be top-yielding.

"That's rather unexpected, because Basagran, which people consider safer than Galaxy, tended to be in the middle all the time." He theorizes that there was a higher rate of Basagran in the Basagran treatment, while Galaxy consists of reduced rates of both Basagran and Blazer.

FirstRate rated highly at sites with heavier-textured soils, but lower on coarser soils. It also was the only treatment where yields were not affected by differences in salt and carbonate levels.

A big point to the research is that growers need to know which fields can support soybeans, Franzen says.

Franzen's advice to Northern growers: If your salt level's 1 mmho/cm or over, be cautious when managing beans. If soybeans are planted there, expect lower yields, especially if you have to apply a postemergent herbicide. Another option to using herbicides on already stressed beans? Hire people to walk weedy fields, he suggests.

"Historically, soybeans have been grown in the southeastern three counties in the state, where people know where they can grow soybeans and where they can't," he adds. "But now we seed soybeans all the way up the Red River Valley, all the way up to Canada."

Those acres grow nice crops of wheat, barley and sunflowers, but not necessarily soybeans, he says. "People are just putting them on anything, and some areas have relatively high levels of salts. Some varieties don't seem to be able to take it."

Some high-salt areas of Minnesota, Franzen adds, were hard-hit with kochia this past year. So growers used a Raptor-Cobra combination that not only killed the weeds, but may have seriously reduced soybean yields.

"People were using them out of ignorance of the possible yield effects. We just want them to know that maybe soybeans aren't for them until they get these weeds under control. And if you have salts and have to use certain herbicides, expect it to really affect your stands and plant health." ?