By ANN HINCH
Assistant Editor

WEST LAFAYETTE, Ind. — Among rows of corn, perhaps the only redeeming feature of giant ragweed is to partially divert the destructive Japanese beetle from munching on the crop this time of year.

It’s a short reprieve; once the corn begins to silk, the bugs will likely start snacking on those tassels. And then the ragweed will truly be nothing more than a neighboring parasite to the crop.

Thousands of years after humans started planting, scientists with Purdue University are still trying to find just the right mixtures of herbicides and techniques to help farmers rescue their crop from competitive weeds. On June 26, Weed Science professors and research assistants squired farmers and agribusiness representatives around test corn and soybean fields in West Lafayette to learn more about weed control.

Giant ragweed is one of the most common weeds interfering with corn and soybean production in the eastern Corn Belt, and is highly prevalent throughout Purdue’s test plots. Other weeds under study, according to Dr. Bill Johnson, associate professor of weed science, are common lamb’s quarters, giant foxtail, redroot pigweed and velvet-leaf.

Purdue cultivates crops on at least two sites, 15 miles apart, because of varying terrain. The Agronomy Center for Research and Education (ACRE) is mostly flat prairie ground rich in organic material, according to Jay Young, site superintendent for the Throckmorton-Purdue Agricultural Center (TPAC).

Further south, TPAC’s rolling fields are more representative of most Indiana soil, giving lower yields.

Because of the number and variability of tests conducted, growers who want specific research information may wish to visit www.btny.purdue.edu/weed  science/Pubs.html or Purdue extension’s website at www.btny.purdue.edu/Pubs/ #weeds for study data.

Resistance

If weeds could be choked out consistently from the same herbicides year after year, there would be no need for further research once proper killers had been developed.

But just as bacteria can change to evade a proven antibiotic, so can weeds adapt to herbicides.

Johnson pointed out an infestation of ALS (acetolactate synthase)-resistant giant foxtail between rows of corn. ALS is a family of herbicides, he said, applied pre-emergence to control grassy weeds.

When it was introduced 15-17 years ago, “it was a huge deal,” he explained.

But repeated usage fails to kill the most resistant weeds and after awhile, they’re the ones that will seed and multiply. This is why a combination of herbicides is often used even if, as research associate Mike White pointed out, they act as “antagonists” to cancel out some of each other’s beneficial effects.

In contrast, Johnson said even after decades, weed resistance to the herbicide atrazine is quite low. Still, he said glyphosate (such as Roundup), a broad-spectrum herbicide, has replaced much of that.

Roundup research

Another big development for crops was the introduction by Monsanto of Roundup Ready soybean seeds in 1996, said Johnson; corn seeds containing the trait were later marketed for the 1998 growing season. The trait protects those crops from glyphosate – in some instances, perhaps too well.

White, who works for Dr. Thomas Bauman, professor of weed science, said on some plots at the ACRE site northwest of the Purdue campus, they are actually trying to find ways to kill Roundup Ready corn for two reasons.

First, there are times farmers want to replant early-planted corn badly damaged by cold weather. Second, sometimes during harvest, stray corn seed falls out of combines into plots intended for another crop, such as soybeans; what emerges later is known as volunteer corn.

In either case, farmers don’t want the unwanted plants robbing the intended crop of fertilizer and water, and need to kill them.
“I know when people drive by, they think we’ve sprayed the wrong thing on the wrong fields,” White said, of his small plots of sickly-looking corn plants.

Because corn is a grass, he said researchers are looking at ways to “heat up” grass herbicides such as Select, normally used around soybeans, to work on unwanted corn.

In Indiana, about 90 percent of all soybeans planted are Roundup Ready, according to grad student Andy Westhoven, who is studying biological distribution and management of giant ragweed and common lamb’s quarters among them.

Throughout the state, he has found 20 populations of ragweed in 14 counties which are tolerant of Roundup – that is, two to six times more resistant to the herbicide than normal. “It’s just the plant’s ability to survive an application,” he explained. He has also found seven populations of tolerant lamb’s quarters.

Application timing

The timing of herbicide application is as much a factor in how well it works as the type and volume being used, or how much rain falls in a growing season. Johnson said some farmers have come to rely upon the Roundup Ready technology and don’t always apply pre-emergent herbicides, or even post-emergent chemicals as early as they should.

“In the last 500 years of weed science, we’ve not found (that applying too late rather than earlier cuts down more weeds),” he said.

He said farmers’ dominant state of mind seems to be “clean out” fields in July and August. But, waiting too long allows weeds to suck up water, especially important in a dry spring like this one – according to Kevin Westerfield, Johnson’s research associate, the TPAC site received only three inches of rain from late April through late June.

Waiting too long also feeds weeds expensive fertilizer, such as nitrogen, which Johnson said isn’t available for corn until the following year after the weeds have decomposed.
“The longer your weeds go, the lower your yield gets,” Young agreed.