"Every time the river comes out, we get a new flush of weeds," says Jerry Parker, Lauderdale County, TN, extension agent. "We get weed seeds from every state upstream in the Mississippi River system."

Lauderdale County, one of five Tennessee counties bordering the river, has the largest land area not protected by man-made levees. In four of the past five years, the Mississippi has put water over bottomland fields.

"Until a few years ago, we inherited cockleburs and johnsongrass; they were our big weed problems," says Rob Reviere, who rotates soybeans, cotton and corn in fields adjacent to the river. "Now Palmer pigweed is the toughest weed we deal with. And if you don't get it before the weed is six inches tall, there isn't much that will handle Palmer pigweed except Roundup."

Palmer pigweed is a close relative to common (redroot) pigweed, but is more resilient and harder to control, says Parker.

"Several other postemergence herbicides will take care of redroot pigweed, especially if it's small," he adds. "But they don't do much for Palmer pigweed, unless all conditions are ideal.

"If you miss a few plants, you'll have trouble keeping Palmer pigweed contained," Parker continues. "Several farmers here crop hill land as well as river-bottom land. We're seeing weeds transferred from one region to the other, primarily by harvesting equipment."

Applying Roundup to Roundup Ready soybeans may be a good way to control Palmer pigweed, says Parker. But it might be more expensive than a conventional program. And a 1997 test revealed that not all Roundup Ready varieties yield as well as conventional ones.

Last year, he compared 16 varieties of Roundup Ready soybeans from six seed companies in the same field. Group V conventional and Roundup Ready varieties tied at 40 bu/acre. However, Group IV conventional soybeans yielded 43 bu, while Group IV Roundup Ready beans made 35.

"If a grower can find Roundup Ready beans with nematode and root disease resistance, that has yield potential equal to a good conventional variety, I'd say Roundup Ready is a workable technology," he says.

"Roundup lets us address serious weed problems with a minimum of chemical input, but you need to put the economic pencil to it."

Reviere is more enthusiastic.

"I have heard that Roundup Ready beans earn $30 per acre less than conventional soybeans, but that's not the case here in the river bottom, where Palmer pigweed can get six feet tall," says Reviere. "Two years ago, I had pigweed so big in one field that it knocked belts off my combine."

He applies 1 1/2 pints of Roundup, then usually comes back with another pint to get late-germinating pigweed.