August 11, 2005

Giant ragweed still No. 1 on problematic hit parade

WEST LAFAYETTE, Ind. - Elvis and the Beatles never dominated the Billboard music charts the way a noxious plant has topped a list of Indiana's most annoying weeds.

Giant ragweed, a weed bothersome to farmers and hay fever sufferers, again ranked No. 1 on Purdue University's Top 10 Most Problematic Weeds list. The aggressive, pollen-producing weed has held the top spot for better than eight consecutive years.

The list is based on Purdue Extension surveys conducted every four years with farmers and other agricultural professionals. The most recent survey was done in 2004.

Other weeds on the list include lambsquarters, Canada thistle, cocklebur, velvetleaf, horseweed (marestail), waterhemp, burcucumber, chickweed and dandelion.

That giant ragweed continues to stand leaf and lobe above other unwanted vegetation is no surprise to Glenn Nice, Purdue Extension weed scientist. He calls Indiana "the giant ragweed national forest."

"As to why giant ragweed is such a problematic weed, part of it is that the weed's germination pattern has adapted to agriculture in the sense that, when compared to the wild type of giant ragweed, it appears germination occurs all season long," said Nice, who helped conduct the Purdue survey. "With giant ragweed, we often get it germinating midsummer and mid-cropping season and producing seed by the time we harvest, even though we seem to control it for most of the growing season."

Giant ragweed is a summer annual which, if left undisturbed, can grow as tall as 15 feet. While susceptible to certain herbicides, the weed has developed resistance to herbicides with aceto-lactase synthase (ALS) inhibitors. ALS inhibitors kill weeds by preventing them from producing essential amino acids necessary for growth.

"It would appear from the survey that we have not made any inroads in stopping giant ragweed," Nice said.

Lambsquarters and Canada thistle came in second and third on the latest weed list. The two weeds consistently rank in the top five and, like giant ragweed, are major headaches for farmers, Nice said.

"Generally, the first problem that we deal with is the competition issue," Nice said. "Giant ragweed, lambsquarters and Canada thistle all compete very well with our crops. They compete for nutrients, light, water and space. Weeds competing with our crops can reduce yields and can cause real problems if they get to be in high enough numbers.

"Another problem weeds can cause is quality of the harvest. Trash from giant ragweed and seed from weeds can end up in the yield. And, of course, when a farmer delivers this to the elevator they get docked for trash."

Four weeds showed up on the list for the first time or returned after failing to make the 2000 list: horseweed (No. 6), waterhemp (No. 7), chickweed (No. 9) and dandelion (No. 10).

Horseweed, chickweed and dandelion always have been difficult to control because they spread rapidly and often germinate at times when control methods are vulnerable, Nice said. In addition, some horseweed biotypes have developed resistance to glyphosate, the main ingredient in the popular herbicide Roundup.

Waterhemp is new to the list because the weed is new to Indiana. The summer annual weed migrated from the west and is now putting down roots in the Eastern Corn Belt, Nice said.

Noticeably absent from the current weed list are grasses, Nice said.

"In previous surveys, done as far back as 1972, grasses such as giant foxtail, fall panicum, Johnsongrass and shattercane often made the list," he said. "However, in 2000 we saw a drop-off in grasses. A lot of that is due to the adoption of the Roundup Ready system. Glyphosate generally does a very good job on annual and perennial grasses, so grasses are not perceived as being as large a problem as they were back in the '70s and '80s."

To read more about the Purdue survey, download "Indiana's Top Ten Most Problematic Weeds". For additional information about herbicides, log onto the 2005 Weed Control Guide for Ohio and Indiana or Purdue's Indiana Select-A-Herb Web site.

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Source: Glenn Nice, (765) 496-2121, gnice@purdue.edu

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
Agriculture News Page

Related Web site:
Purdue University Weed Science

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